ViewDragHelper源码解析

分析版本：support-v4-23.3.0

ViewDragHelper 封装了许多手势拖动操作，在某些条件下使用 ViewDragHelper 开发可提高效率。

ViewDragHelper

大家应该都知道 DrawerLayout 吧，该控件有一个手势操作就是将 Menu 从屏幕边缘位置滑动出来，而这部分的手势操作是交给 ViewDragHelper 去处理的。ViewDragHelper 并不是第一个用于分析手势处理的类，Gesturedetector 也是，但是在和拖动相关的手势分析方面 ViewDragHelper 更胜一筹。

特点

• ViewDragHelper 可以检测到是否触及到边缘
• ViewDragHelper 并不是直接作用于要被拖动的 View，而是使其控制的视图容器中的子 View 可以被拖动
• ViewDragHelper.Callback 是连接 ViewDragHelper 与 View 之间的桥梁
• ViewDragHelper 的本质其实是分析 onInterceptTouchEvent 和 onTouchEvent 的 MotionEvent 参数，然后根据分析的结果去改变一个容器中被拖动子 View 的位置

使用

public class VDHLayout extends LinearLayout{
    private ViewDragHelper mDragger;

    public VDHLayout(Context context, AttributeSet attrs){
        super(context, attrs);
        //第二个参数就是滑动灵敏度的意思
        mDragger = ViewDragHelper.create(this, 1.0f, new ViewDragHelper.Callback(){
            //这个地方实际上函数返回值为true就代表可以滑动 为false 则不能滑动
            @Override
            public boolean tryCaptureView(View child, int pointerId){
                return true;
            }

            //这个地方实际上left就代表 你将要移动到的位置的坐标。返回值就是最终确定的移动的位置。
            //我们要让view滑动的范围在我们的layout之内
            //实际上就是判断如果这个坐标在layout之内 那我们就返回这个坐标值。
            //如果这个坐标在layout的边界处 那我们就只能返回边界的坐标给他。不能让他超出这个范围
            //除此之外就是如果你的layout设置了padding的话，也可以让子view的活动范围在padding之内的.
            @Override
            public int clampViewPositionHorizontal(View child, int left, int dx){
                return left;
            }

            @Override
            public int clampViewPositionVertical(View child, int top, int dy){
                return top;
            }
        });
    }

   @Override
    public boolean onInterceptTouchEvent(MotionEvent event){
        return mDragger.shouldInterceptTouchEvent(event);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event){
        mDragger.processTouchEvent(event);
        return true;
    }
}

onInterceptTouchEvent 中通过使用 mDragger.shouldInterceptTouchEvent(event) 来决定我们是否应该拦截当前的事件。onTouchEvent 中通过 mDragger.processTouchEvent(event) 处理事件。

源码分析

ViewDragHelper

从创建开始：

public class ViewDragHelper {
    private final ViewGroup mParentView;
    private final Callback mCallback;

    private int mEdgeSize;//边缘距离，当在边缘的这个距离内算触发边缘
    private int mTouchSlop;//最小移动距离
    private float mMaxVelocity;//最大移动速度
    private float mMinVelocity;//最小移动速度
    private ScrollerCompat mScroller;

    /**     * Factory method to create a new ViewDragHelper.     *     * @param forParent Parent view to monitor     * @param cb Callback to provide information and receive events     * @return a new ViewDragHelper instance     */
     * Factory method to create a new ViewDragHelper.
     *
     * @param forParent Parent view to monitor
     * @param cb Callback to provide information and receive events
     * @return a new ViewDragHelper instance
     */
    public static ViewDragHelper create(ViewGroup forParent, Callback cb) {
        return new ViewDragHelper(forParent.getContext(), forParent, cb);
    }  
  
    /**     * Factory method to create a new ViewDragHelper.     *     * @param forParent Parent view to monitor     * @param sensitivity Multiplier for how sensitive the helper should be about detecting     *                    the start of a drag. Larger values are more sensitive. 1.0f is normal.     * @param cb Callback to provide information and receive events     * @return a new ViewDragHelper instance     */
     * Factory method to create a new ViewDragHelper.
     *
     * @param forParent Parent view to monitor
     * @param sensitivity Multiplier for how sensitive the helper should be about detecting
     *                    the start of a drag. Larger values are more sensitive. 1.0f is normal.
     * @param cb Callback to provide information and receive events
     * @return a new ViewDragHelper instance
     */
    public static ViewDragHelper create(ViewGroup forParent, float sensitivity, Callback cb) {
        final ViewDragHelper helper = create(forParent, cb);
        helper.mTouchSlop = (int) (helper.mTouchSlop * (1 / sensitivity));
        return helper;
    }

    /**     * Apps should use ViewDragHelper.create() to get a new instance.     * This will allow VDH to use internal compatibility implementations for different     * platform versions.     *     * @param context Context to initialize config-dependent params from     * @param forParent Parent view to monitor     */
     * Apps should use ViewDragHelper.create() to get a new instance.
     * This will allow VDH to use internal compatibility implementations for different
     * platform versions.
     *
     * @param context Context to initialize config-dependent params from
     * @param forParent Parent view to monitor
     */
    private ViewDragHelper(Context context, ViewGroup forParent, Callback cb) {
        if (forParent == null) {
            throw new IllegalArgumentException("Parent view may not be null");
        }
        if (cb == null) {
            throw new IllegalArgumentException("Callback may not be null");
        }

        mParentView = forParent;
        mCallback = cb;

        final ViewConfiguration vc = ViewConfiguration.get(context);
        final float density = context.getResources().getDisplayMetrics().density;
        mEdgeSize = (int) (EDGE_SIZE * density + 0.5f);

        mTouchSlop = vc.getScaledTouchSlop();
        mMaxVelocity = vc.getScaledMaximumFlingVelocity();
        mMinVelocity = vc.getScaledMinimumFlingVelocity();
        mScroller = ScrollerCompat.create(context, sInterpolator);
    }
  
    /**     * Interpolator defining the animation curve for mScroller     */
     * Interpolator defining the animation curve for mScroller
     */
    private static final Interpolator sInterpolator = new Interpolator() {
        public float getInterpolation(float t) {
            t -= 1.0f;
            return t * t * t * t * t + 1.0f;
        }
    };
}

这个构造函数是私有的，也是仅有的构造函数，所以外部只能通过 create() 工厂方法来创建 ViewDragHelper 实例了。 这里要求了我们传递的自定义 ViewGroup 和回调对象不能为空，否则会直接抛出异常中断程序。在这里也初始化了一些触摸滑动需要的参考值和辅助类。

• mParentView 和 mCallback 分别保存传递过来的对应参数
• ViewConfiguration 类里定义了 View 相关的一系列时间、大小、距离等常量
• mEdgeSize 表示边缘触摸的范围。例如 mEdgeSize 为 20dp 并且用户注册监听了左侧边缘触摸时，触摸点的 x 坐标小于 mParentView.getLeft() + mEdgeSize 时（即触摸点在容器左边界往右 20dp 内）就算做是左侧的边缘触摸
• mTouchSlop 是一个很小的距离值，只有在前后两次触摸点的距离超过 mTouchSlop 的值时，我们才把这两次触摸算作是『滑动』，我们只在此时进行滑动处理，否则任何微小的距离的变化我们都要处理的话会显得太频繁，如果处理过程又比较复杂耗时就会使界面产生卡顿
• mMaxVelocity 、mMinVelocity 是 fling 时的最大、最小速率，单位是像素每秒
• mScroller 是 View 滚动的辅助类
• 还有个 sensitivity ，是用来调节 mTouchSlop 的值。sensitivity 越大，mTouchSlop 越小，对滑动的检测就越敏感。 例如 sensitivity 为 1 时，前后触摸点距离超过 20dp 才进行滑动处理，现在 sensitivity 为 2 的话，前后触摸点距离超过 10dp 就进行处理了

Callback

public class ViewDragHelper {
  /**     * A Callback is used as a communication channel with the ViewDragHelper back to the     * parent view using it. <code>on*</code>methods are invoked on siginficant events and several     * accessor methods are expected to provide the ViewDragHelper with more information     * about the state of the parent view upon request. The callback also makes decisions     * governing the range and draggability of child views.     */
     * A Callback is used as a communication channel with the ViewDragHelper back to the
     * parent view using it. <code>on*</code>methods are invoked on siginficant events and several
     * accessor methods are expected to provide the ViewDragHelper with more information
     * about the state of the parent view upon request. The callback also makes decisions
     * governing the range and draggability of child views.
     */
    public static abstract class Callback {
        /**         * Called when the drag state changes. See the <code>STATE_*</code> constants         * for more information.         *         * @param state The new drag state         *         * @see #STATE_IDLE         * @see #STATE_DRAGGING         * @see #STATE_SETTLING         */
         * Called when the drag state changes. See the <code>STATE_*</code> constants
         * for more information.
         *
         * @param state The new drag state
         *
         * @see #STATE_IDLE
         * @see #STATE_DRAGGING
         * @see #STATE_SETTLING
         */
        public void onViewDragStateChanged(int state) {}

        /**         * Called when the captured view's position changes as the result of a drag or settle.         *         * @param changedView View whose position changed         * @param left New X coordinate of the left edge of the view         * @param top New Y coordinate of the top edge of the view         * @param dx Change in X position from the last call         * @param dy Change in Y position from the last call         */
         * Called when the captured view's position changes as the result of a drag or settle.
         *
         * @param changedView View whose position changed
         * @param left New X coordinate of the left edge of the view
         * @param top New Y coordinate of the top edge of the view
         * @param dx Change in X position from the last call
         * @param dy Change in Y position from the last call
         */
        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {}

        /**         * Called when a child view is captured for dragging or settling. The ID of the pointer         * currently dragging the captured view is supplied. If activePointerId is         * identified as {@link #INVALID_POINTER} the capture is programmatic instead of         * pointer-initiated.         *         * @param capturedChild Child view that was captured         * @param activePointerId Pointer id tracking the child capture         */
         * Called when a child view is captured for dragging or settling. The ID of the pointer
         * currently dragging the captured view is supplied. If activePointerId is
         * identified as {@link #INVALID_POINTER} the capture is programmatic instead of
         * pointer-initiated.
         *
         * @param capturedChild Child view that was captured
         * @param activePointerId Pointer id tracking the child capture
         */
        public void onViewCaptured(View capturedChild, int activePointerId) {}

        /**         * Called when the child view is no longer being actively dragged.         * The fling velocity is also supplied, if relevant. The velocity values may         * be clamped to system minimums or maximums.         *         * <p>Calling code may decide to fling or otherwise release the view to let it         * settle into place. It should do so using {@link #settleCapturedViewAt(int, int)}         * or {@link #flingCapturedView(int, int, int, int)}. If the Callback invokes         * one of these methods, the ViewDragHelper will enter {@link #STATE_SETTLING}         * and the view capture will not fully end until it comes to a complete stop.         * If neither of these methods is invoked before <code>onViewReleased</code> returns,         * the view will stop in place and the ViewDragHelper will return to         * {@link #STATE_IDLE}.</p>         *         * @param releasedChild The captured child view now being released         * @param xvel X velocity of the pointer as it left the screen in pixels per second.         * @param yvel Y velocity of the pointer as it left the screen in pixels per second.         */
         * Called when the child view is no longer being actively dragged.
         * The fling velocity is also supplied, if relevant. The velocity values may
         * be clamped to system minimums or maximums.
         *
         * <p>Calling code may decide to fling or otherwise release the view to let it
         * settle into place. It should do so using {@link #settleCapturedViewAt(int, int)}
         * or {@link #flingCapturedView(int, int, int, int)}. If the Callback invokes
         * one of these methods, the ViewDragHelper will enter {@link #STATE_SETTLING}
         * and the view capture will not fully end until it comes to a complete stop.
         * If neither of these methods is invoked before <code>onViewReleased</code> returns,
         * the view will stop in place and the ViewDragHelper will return to
         * {@link #STATE_IDLE}.</p>
         *
         * @param releasedChild The captured child view now being released
         * @param xvel X velocity of the pointer as it left the screen in pixels per second.
         * @param yvel Y velocity of the pointer as it left the screen in pixels per second.
         */
        public void onViewReleased(View releasedChild, float xvel, float yvel) {}

        /**         * Called when one of the subscribed edges in the parent view has been touched         * by the user while no child view is currently captured.         *         * @param edgeFlags A combination of edge flags describing the edge(s) currently touched         * @param pointerId ID of the pointer touching the described edge(s)         * @see #EDGE_LEFT         * @see #EDGE_TOP         * @see #EDGE_RIGHT         * @see #EDGE_BOTTOM         */
         * Called when one of the subscribed edges in the parent view has been touched
         * by the user while no child view is currently captured.
         *
         * @param edgeFlags A combination of edge flags describing the edge(s) currently touched
         * @param pointerId ID of the pointer touching the described edge(s)
         * @see #EDGE_LEFT
         * @see #EDGE_TOP
         * @see #EDGE_RIGHT
         * @see #EDGE_BOTTOM
         */
        public void onEdgeTouched(int edgeFlags, int pointerId) {}

        /**         * Called when the given edge may become locked. This can happen if an edge drag         * was preliminarily rejected before beginning, but after {@link #onEdgeTouched(int, int)}         * was called. This method should return true to lock this edge or false to leave it         * unlocked. The default behavior is to leave edges unlocked.         *         * @param edgeFlags A combination of edge flags describing the edge(s) locked         * @return true to lock the edge, false to leave it unlocked         */
         * Called when the given edge may become locked. This can happen if an edge drag
         * was preliminarily rejected before beginning, but after {@link #onEdgeTouched(int, int)}
         * was called. This method should return true to lock this edge or false to leave it
         * unlocked. The default behavior is to leave edges unlocked.
         *
         * @param edgeFlags A combination of edge flags describing the edge(s) locked
         * @return true to lock the edge, false to leave it unlocked
         */
        public boolean onEdgeLock(int edgeFlags) {
            return false;
        }

        /**         * Called when the user has started a deliberate drag away from one         * of the subscribed edges in the parent view while no child view is currently captured.         *         * @param edgeFlags A combination of edge flags describing the edge(s) dragged         * @param pointerId ID of the pointer touching the described edge(s)         * @see #EDGE_LEFT         * @see #EDGE_TOP         * @see #EDGE_RIGHT         * @see #EDGE_BOTTOM         */
         * Called when the user has started a deliberate drag away from one
         * of the subscribed edges in the parent view while no child view is currently captured.
         *
         * @param edgeFlags A combination of edge flags describing the edge(s) dragged
         * @param pointerId ID of the pointer touching the described edge(s)
         * @see #EDGE_LEFT
         * @see #EDGE_TOP
         * @see #EDGE_RIGHT
         * @see #EDGE_BOTTOM
         */
        public void onEdgeDragStarted(int edgeFlags, int pointerId) {}

        /**         * Called to determine the Z-order of child views.         *         * @param index the ordered position to query for         * @return index of the view that should be ordered at position <code>index</code>         */
         * Called to determine the Z-order of child views.
         *
         * @param index the ordered position to query for
         * @return index of the view that should be ordered at position <code>index</code>
         */
        public int getOrderedChildIndex(int index) {
            return index;
        }

        /**         * Return the magnitude of a draggable child view's horizontal range of motion in pixels.         * This method should return 0 for views that cannot move horizontally.         *         * @param child Child view to check         * @return range of horizontal motion in pixels         */
         * Return the magnitude of a draggable child view's horizontal range of motion in pixels.
         * This method should return 0 for views that cannot move horizontally.
         *
         * @param child Child view to check
         * @return range of horizontal motion in pixels
         */
        public int getViewHorizontalDragRange(View child) {
            return 0;
        }

        /**         * Return the magnitude of a draggable child view's vertical range of motion in pixels.         * This method should return 0 for views that cannot move vertically.         *         * @param child Child view to check         * @return range of vertical motion in pixels         */
         * Return the magnitude of a draggable child view's vertical range of motion in pixels.
         * This method should return 0 for views that cannot move vertically.
         *
         * @param child Child view to check
         * @return range of vertical motion in pixels
         */
        public int getViewVerticalDragRange(View child) {
            return 0;
        }

        /**         * Called when the user's input indicates that they want to capture the given child view         * with the pointer indicated by pointerId. The callback should return true if the user         * is permitted to drag the given view with the indicated pointer.         *         * <p>ViewDragHelper may call this method multiple times for the same view even if         * the view is already captured; this indicates that a new pointer is trying to take         * control of the view.</p>         *         * <p>If this method returns true, a call to {@link #onViewCaptured(android.view.View, int)}         * will follow if the capture is successful.</p>         *         * @param child Child the user is attempting to capture         * @param pointerId ID of the pointer attempting the capture         * @return true if capture should be allowed, false otherwise         */
         * Called when the user's input indicates that they want to capture the given child view
         * with the pointer indicated by pointerId. The callback should return true if the user
         * is permitted to drag the given view with the indicated pointer.
         *
         * <p>ViewDragHelper may call this method multiple times for the same view even if
         * the view is already captured; this indicates that a new pointer is trying to take
         * control of the view.</p>
         *
         * <p>If this method returns true, a call to {@link #onViewCaptured(android.view.View, int)}
         * will follow if the capture is successful.</p>
         *
         * @param child Child the user is attempting to capture
         * @param pointerId ID of the pointer attempting the capture
         * @return true if capture should be allowed, false otherwise
         */
        public abstract boolean tryCaptureView(View child, int pointerId);

        /**         * Restrict the motion of the dragged child view along the horizontal axis.         * The default implementation does not allow horizontal motion; the extending         * class must override this method and provide the desired clamping.         *         *         * @param child Child view being dragged         * @param left Attempted motion along the X axis         * @param dx Proposed change in position for left         * @return The new clamped position for left         */
         * Restrict the motion of the dragged child view along the horizontal axis.
         * The default implementation does not allow horizontal motion; the extending
         * class must override this method and provide the desired clamping.
         *
         *
         * @param child Child view being dragged
         * @param left Attempted motion along the X axis
         * @param dx Proposed change in position for left
         * @return The new clamped position for left
         */
        public int clampViewPositionHorizontal(View child, int left, int dx) {
            return 0;
        }

        /**         * Restrict the motion of the dragged child view along the vertical axis.         * The default implementation does not allow vertical motion; the extending         * class must override this method and provide the desired clamping.         *         *         * @param child Child view being dragged         * @param top Attempted motion along the Y axis         * @param dy Proposed change in position for top         * @return The new clamped position for top         */
         * Restrict the motion of the dragged child view along the vertical axis.
         * The default implementation does not allow vertical motion; the extending
         * class must override this method and provide the desired clamping.
         *
         *
         * @param child Child view being dragged
         * @param top Attempted motion along the Y axis
         * @param dy Proposed change in position for top
         * @return The new clamped position for top
         */
        public int clampViewPositionVertical(View child, int top, int dy) {
            return 0;
        }
    }
}

• onViewCaptured （当 captureView 被捕获时回调）
• tryCaptureView （是否需要 capture 这个 View）
• clampViewPositionHorizontal （横向移动的时候回调）
• clampViewPositionVertical （纵向移动的时候回调）
• onViewDragStateChanged （当ViewDragHelper状态发生变化时回调（IDLE,DRAGGING,SETTING[自动滚动时]））
• onViewPositionChanged （当 captureView 的位置发生改变时回调）
• onEdgeTouched （当触摸到边界时回调）
• onEdgeLock （true 的时候会锁住当前的边界，false 则 unLock ）
• onEdgeDragStarted （边界拖动开始的时候回调）
• getOrderedChildIndex （改变同一个坐标（ x , y ）去寻找 captureView 位置的方法）
• getViewHorizontalDragRange （最大横滑动的滑动距离）
• getViewVerticalDragRange （最大纵向滑动的距离）
• onViewReleased （当 captureView 被释放的时候回调）

shouldInterceptTouchEvent

在 onInterceptTouchEvent 中的处理：

public class ViewDragHelper {
    /**     * A null/invalid pointer ID.     */
     * A null/invalid pointer ID.
     */
    public static final int INVALID_POINTER = -1;
  
/**     * Edge flag indicating that the left edge should be affected.     */
     * Edge flag indicating that the left edge should be affected.
     */
    public static final int EDGE_LEFT = 1 << 0;

    /**     * Edge flag indicating that the right edge should be affected.     */
     * Edge flag indicating that the right edge should be affected.
     */
    public static final int EDGE_RIGHT = 1 << 1;

    /**     * Edge flag indicating that the top edge should be affected.     */
     * Edge flag indicating that the top edge should be affected.
     */
    public static final int EDGE_TOP = 1 << 2;

    /**     * Edge flag indicating that the bottom edge should be affected.     */
     * Edge flag indicating that the bottom edge should be affected.
     */
    public static final int EDGE_BOTTOM = 1 << 3;

    /**     * Edge flag set indicating all edges should be affected.     */
     * Edge flag set indicating all edges should be affected.
     */
    public static final int EDGE_ALL = EDGE_LEFT | EDGE_TOP | EDGE_RIGHT | EDGE_BOTTOM;
  
    /**     * A view is not currently being dragged or animating as a result of a fling/snap.     */
     * A view is not currently being dragged or animating as a result of a fling/snap.
     */
    public static final int STATE_IDLE = 0;//静止空闲状态

    /**     * A view is currently being dragged. The position is currently changing as a result     * of user input or simulated user input.     */
     * A view is currently being dragged. The position is currently changing as a result
     * of user input or simulated user input.
     */
    public static final int STATE_DRAGGING = 1;//正在被拖动

    /**     * A view is currently settling into place as a result of a fling or     * predefined non-interactive motion.     */
     * A view is currently settling into place as a result of a fling or
     * predefined non-interactive motion.
     */
    public static final int STATE_SETTLING = 2;//正在安置状态中（用户并没有交互操作），就是自动滚动的过程中
  
    private int mActivePointerId = INVALID_POINTER;
  
    // Current drag state; idle, dragging or settling
    private int mDragState;//当前drag状态
    private View mCapturedView;//正在被drag的view

    // Last known position/pointer tracking
    private int mActivePointerId = INVALID_POINTER;
    private float[] mInitialMotionX;//一开始的X坐标，数组代表多手指的情况，index就是手指id
    private float[] mInitialMotionY;//一开始的Y坐标，数组代表多手指的情况，index就是手指id
    private float[] mLastMotionX;//随时更新的的X坐标，数组代表多手指的情况，index就是手指id
    private float[] mLastMotionY;//随时更新的的Y坐标，数组代表多手指的情况，index就是手指id
    private int[] mInitialEdgesTouched;//刚触摸的时候触摸的边缘位置
    private int[] mEdgeDragsInProgress;//记录边缘拖动
    private int[] mEdgeDragsLocked;//记录锁定的边缘
    private int mPointersDown;
  
    private VelocityTracker mVelocityTracker;
    private int mTrackingEdges;

    /**     * Check if this event as provided to the parent view's onInterceptTouchEvent should     * cause the parent to intercept the touch event stream.     *     * @param ev MotionEvent provided to onInterceptTouchEvent     * @return true if the parent view should return true from onInterceptTouchEvent     */
     * Check if this event as provided to the parent view's onInterceptTouchEvent should
     * cause the parent to intercept the touch event stream.
     *
     * @param ev MotionEvent provided to onInterceptTouchEvent
     * @return true if the parent view should return true from onInterceptTouchEvent
     */
    public boolean shouldInterceptTouchEvent(MotionEvent ev) {
        final int action = MotionEventCompat.getActionMasked(ev);//当前action
        final int actionIndex = MotionEventCompat.getActionIndex(ev);//当前actionIndex

        if (action == MotionEvent.ACTION_DOWN) {
            // Reset things for a new event stream, just in case we didn't get
            // the whole previous stream.
            cancel();//归位
        }

        //初始化mVelocityTracker并添加事件
        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        switch (action) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();
                final int pointerId = MotionEventCompat.getPointerId(ev, 0);//手指id
                saveInitialMotion(x, y, pointerId);//保存手指id对应的信息

                final View toCapture = findTopChildUnder((int) x, (int) y);//找到该坐标上的最上面的那个view

                // Catch a settling view if possible.
                if (toCapture == mCapturedView && mDragState == STATE_SETTLING) {//释放后正在进行动画的时候马上又去drag
                    tryCaptureViewForDrag(toCapture, pointerId);//去drag
                }

                final int edgesTouched = mInitialEdgesTouched[pointerId];//得到当前边缘触摸
                if ((edgesTouched & mTrackingEdges) != 0) {//mTrackingEdges为是否开启某边缘的触摸drag
                    mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);//回调
                }
                break;
            }

            case MotionEventCompat.ACTION_POINTER_DOWN: {//多手指
                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);
                final float x = MotionEventCompat.getX(ev, actionIndex);
                final float y = MotionEventCompat.getY(ev, actionIndex);

                saveInitialMotion(x, y, pointerId);//保存手指id对应的信息

                // A ViewDragHelper can only manipulate one view at a time.
                if (mDragState == STATE_IDLE) {//如果当前状态是STATE_IDLE
                    final int edgesTouched = mInitialEdgesTouched[pointerId];//得到当前边缘触摸
                    if ((edgesTouched & mTrackingEdges) != 0) {//边缘是否能够滑动
                        mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);
                    }
                } else if (mDragState == STATE_SETTLING) {//手指放开，返回的状态
                    // Catch a settling view if possible.
                    final View toCapture = findTopChildUnder((int) x, (int) y);
                    if (toCapture == mCapturedView) {
                        tryCaptureViewForDrag(toCapture, pointerId);//去drag
                    }
                }
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                if (mInitialMotionX == null || mInitialMotionY == null) break;

                // First to cross a touch slop over a draggable view wins. Also report edge drags.
                final int pointerCount = MotionEventCompat.getPointerCount(ev);//得到当前手指个数
                for (int i = 0; i < pointerCount; i++) {
                    final int pointerId = MotionEventCompat.getPointerId(ev, i);//得到id

                    // If pointer is invalid then skip the ACTION_MOVE.
                    if (!isValidPointerForActionMove(pointerId)) continue;

                    final float x = MotionEventCompat.getX(ev, i);//得到X
                    final float y = MotionEventCompat.getY(ev, i);//得到Y
                    final float dx = x - mInitialMotionX[pointerId];//算出移动的X方向距离
                    final float dy = y - mInitialMotionY[pointerId];//算出移动的Y方向距离

                    final View toCapture = findTopChildUnder((int) x, (int) y);
                    final boolean pastSlop = toCapture != null && checkTouchSlop(toCapture, dx, dy);//是否可以滑动
                    if (pastSlop) {
                        // check the callback's
                        // getView[Horizontal|Vertical]DragRange methods to know
                        // if you can move at all along an axis, then see if it
                        // would clamp to the same value. If you can't move at
                        // all in every dimension with a nonzero range, bail.
                        final int oldLeft = toCapture.getLeft();
                        final int targetLeft = oldLeft + (int) dx;
                        final int newLeft = mCallback.clampViewPositionHorizontal(toCapture,
                                targetLeft, (int) dx);//通过回调得到新的left的值
                        final int oldTop = toCapture.getTop();
                        final int targetTop = oldTop + (int) dy;
                        final int newTop = mCallback.clampViewPositionVertical(toCapture, targetTop,//通过回调得到新的top的值
                                (int) dy);
                        final int horizontalDragRange = mCallback.getViewHorizontalDragRange(
                                toCapture);//通过回调得到横向拖动的范围
                        final int verticalDragRange = mCallback.getViewVerticalDragRange(toCapture);//通过回调得到纵向拖动的范围
                        if ((horizontalDragRange == 0 || horizontalDragRange > 0
                                && newLeft == oldLeft) && (verticalDragRange == 0
                                || verticalDragRange > 0 && newTop == oldTop)) {
                            break;
                        }
                    }
                    reportNewEdgeDrags(dx, dy, pointerId);//经过计算之后回调结果出去
                    if (mDragState == STATE_DRAGGING) {
                        // Callback might have started an edge drag
                        break;
                    }

                    if (pastSlop && tryCaptureViewForDrag(toCapture, pointerId)) {
                        break;
                    }
                }
                saveLastMotion(ev);
                break;
            }

            case MotionEventCompat.ACTION_POINTER_UP: {
                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);
                clearMotionHistory(pointerId);//清除数据
                break;
            }

            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL: {
                cancel();//清除数据
                break;
            }
        }

        return mDragState == STATE_DRAGGING;//如果是dragging状态，就拦截手势事件
    }
  
    /**     * The result of a call to this method is equivalent to     * {@link #processTouchEvent(android.view.MotionEvent)} receiving an ACTION_CANCEL event.     */
     * The result of a call to this method is equivalent to
     * {@link #processTouchEvent(android.view.MotionEvent)} receiving an ACTION_CANCEL event.
     */
    public void cancel() {
        mActivePointerId = INVALID_POINTER;
        clearMotionHistory();

        if (mVelocityTracker != null) {
            mVelocityTracker.recycle();
            mVelocityTracker = null;
        }
    }
  
    private void clearMotionHistory() {
        if (mInitialMotionX == null) {
            return;
        }
        Arrays.fill(mInitialMotionX, 0);
        Arrays.fill(mInitialMotionY, 0);
        Arrays.fill(mLastMotionX, 0);
        Arrays.fill(mLastMotionY, 0);
        Arrays.fill(mInitialEdgesTouched, 0);
        Arrays.fill(mEdgeDragsInProgress, 0);
        Arrays.fill(mEdgeDragsLocked, 0);
        mPointersDown = 0;
    }
  
    private void saveInitialMotion(float x, float y, int pointerId) {
        ensureMotionHistorySizeForId(pointerId);
        mInitialMotionX[pointerId] = mLastMotionX[pointerId] = x;//赋最开始的值
        mInitialMotionY[pointerId] = mLastMotionY[pointerId] = y;//赋最开始的值
        mInitialEdgesTouched[pointerId] = getEdgesTouched((int) x, (int) y);//赋值边缘值
        mPointersDown |= 1 << pointerId;
    }
  
    private void ensureMotionHistorySizeForId(int pointerId) {
        if (mInitialMotionX == null || mInitialMotionX.length <= pointerId) {
            float[] imx = new float[pointerId + 1];
            float[] imy = new float[pointerId + 1];
            float[] lmx = new float[pointerId + 1];
            float[] lmy = new float[pointerId + 1];
            int[] iit = new int[pointerId + 1];
            int[] edip = new int[pointerId + 1];
            int[] edl = new int[pointerId + 1];

            if (mInitialMotionX != null) {
                System.arraycopy(mInitialMotionX, 0, imx, 0, mInitialMotionX.length);
                System.arraycopy(mInitialMotionY, 0, imy, 0, mInitialMotionY.length);
                System.arraycopy(mLastMotionX, 0, lmx, 0, mLastMotionX.length);
                System.arraycopy(mLastMotionY, 0, lmy, 0, mLastMotionY.length);
                System.arraycopy(mInitialEdgesTouched, 0, iit, 0, mInitialEdgesTouched.length);
                System.arraycopy(mEdgeDragsInProgress, 0, edip, 0, mEdgeDragsInProgress.length);
                System.arraycopy(mEdgeDragsLocked, 0, edl, 0, mEdgeDragsLocked.length);
            }

            mInitialMotionX = imx;
            mInitialMotionY = imy;
            mLastMotionX = lmx;
            mLastMotionY = lmy;
            mInitialEdgesTouched = iit;
            mEdgeDragsInProgress = edip;
            mEdgeDragsLocked = edl;
        }
    }
  
    /**     * 判断是否在边缘，且返回是哪里的边缘     */
     * 判断是否在边缘，且返回是哪里的边缘
     */
    private int getEdgesTouched(int x, int y) {
        int result = 0;

        if (x < mParentView.getLeft() + mEdgeSize) result |= EDGE_LEFT;
        if (y < mParentView.getTop() + mEdgeSize) result |= EDGE_TOP;
        if (x > mParentView.getRight() - mEdgeSize) result |= EDGE_RIGHT;
        if (y > mParentView.getBottom() - mEdgeSize) result |= EDGE_BOTTOM;

        return result;
    }
  
    /**     * Find the topmost child under the given point within the parent view's coordinate system.     * The child order is determined using {@link Callback#getOrderedChildIndex(int)}.     * 找到该坐标上的最上面的那个view     *     * @param x X position to test in the parent's coordinate system     * @param y Y position to test in the parent's coordinate system     * @return The topmost child view under (x, y) or null if none found.     */
     * Find the topmost child under the given point within the parent view's coordinate system.
     * The child order is determined using {@link Callback#getOrderedChildIndex(int)}.
     * 找到该坐标上的最上面的那个view
     *
     * @param x X position to test in the parent's coordinate system
     * @param y Y position to test in the parent's coordinate system
     * @return The topmost child view under (x, y) or null if none found.
     */
    public View findTopChildUnder(int x, int y) {
        final int childCount = mParentView.getChildCount();
        for (int i = childCount - 1; i >= 0; i--) {
            final View child = mParentView.getChildAt(mCallback.getOrderedChildIndex(i));
            if (x >= child.getLeft() && x < child.getRight() &&
                    y >= child.getTop() && y < child.getBottom()) {
                return child;
            }
        }
        return null;
    }
  
    /**     * Attempt to capture the view with the given pointer ID. The callback will be involved.     * This will put us into the "dragging" state. If we've already captured this view with     * this pointer this method will immediately return true without consulting the callback.     *     * @param toCapture View to capture     * @param pointerId Pointer to capture with     * @return true if capture was successful     */
     * Attempt to capture the view with the given pointer ID. The callback will be involved.
     * This will put us into the "dragging" state. If we've already captured this view with
     * this pointer this method will immediately return true without consulting the callback.
     *
     * @param toCapture View to capture
     * @param pointerId Pointer to capture with
     * @return true if capture was successful
     */
    boolean tryCaptureViewForDrag(View toCapture, int pointerId) {
        if (toCapture == mCapturedView && mActivePointerId == pointerId) {//当前有正在被drag的View且是同一个且还是同一个手指
            // Already done!
            return true;
        }
        if (toCapture != null && mCallback.tryCaptureView(toCapture, pointerId)) {//回调返回TRUE，表示要drag这个toCapture这个View
            mActivePointerId = pointerId;
            captureChildView(toCapture, pointerId);
            return true;
        }
        return false;
    }
  
    /**     * Capture a specific child view for dragging within the parent. The callback will be notified     * but {@link Callback#tryCaptureView(android.view.View, int)} will not be asked permission to     * capture this view.     *     * @param childView Child view to capture     * @param activePointerId ID of the pointer that is dragging the captured child view     */
     * Capture a specific child view for dragging within the parent. The callback will be notified
     * but {@link Callback#tryCaptureView(android.view.View, int)} will not be asked permission to
     * capture this view.
     *
     * @param childView Child view to capture
     * @param activePointerId ID of the pointer that is dragging the captured child view
     */
    public void captureChildView(View childView, int activePointerId) {
        if (childView.getParent() != mParentView) {
            throw new IllegalArgumentException("captureChildView: parameter must be a descendant " +
                    "of the ViewDragHelper's tracked parent view (" + mParentView + ")");
        }

        mCapturedView = childView;//记录mCaptureView
        mActivePointerId = activePointerId;//记录手指id
        mCallback.onViewCaptured(childView, activePointerId);//回调
        setDragState(STATE_DRAGGING);//设置drag状态
    }
  
    void setDragState(int state) {
        mParentView.removeCallbacks(mSetIdleRunnable);
        if (mDragState != state) {
            mDragState = state;//设置新的状态
            mCallback.onViewDragStateChanged(state);//回调
            if (mDragState == STATE_IDLE) {
                mCapturedView = null;
            }
        }
    }
  
    private final Runnable mSetIdleRunnable = new Runnable() {
        public void run() {
            setDragState(STATE_IDLE);
        }
    };
  
    private boolean isValidPointerForActionMove(int pointerId) {
        if (!isPointerDown(pointerId)) {
            Log.e(TAG, "Ignoring pointerId=" + pointerId + " because ACTION_DOWN was not received "
                    + "for this pointer before ACTION_MOVE. It likely happened because "
                    + " ViewDragHelper did not receive all the events in the event stream.");
            return false;
        }
        return true;
    }
  
    public boolean isPointerDown(int pointerId) {
        return (mPointersDown & 1 << pointerId) != 0;
    }
  
    /**     * Check if we've crossed a reasonable touch slop for the given child view.     * If the child cannot be dragged along the horizontal or vertical axis, motion     * along that axis will not count toward the slop check.     * 检查手指移动的距离有没有超过触发处理移动事件的最短距离     *      * @param child Child to check     * @param dx Motion since initial position along X axis     * @param dy Motion since initial position along Y axis     * @return true if the touch slop has been crossed     */
     * Check if we've crossed a reasonable touch slop for the given child view.
     * If the child cannot be dragged along the horizontal or vertical axis, motion
     * along that axis will not count toward the slop check.
     * 检查手指移动的距离有没有超过触发处理移动事件的最短距离
     * 
     * @param child Child to check
     * @param dx Motion since initial position along X axis
     * @param dy Motion since initial position along Y axis
     * @return true if the touch slop has been crossed
     */
    private boolean checkTouchSlop(View child, float dx, float dy) {
        if (child == null) {
            return false;
        }
        final boolean checkHorizontal = mCallback.getViewHorizontalDragRange(child) > 0;
        final boolean checkVertical = mCallback.getViewVerticalDragRange(child) > 0;

        if (checkHorizontal && checkVertical) {
            return dx * dx + dy * dy > mTouchSlop * mTouchSlop;
        } else if (checkHorizontal) {
            return Math.abs(dx) > mTouchSlop;
        } else if (checkVertical) {
            return Math.abs(dy) > mTouchSlop;
        }
        return false;
    }
  
    /**     * 对四个边缘都做了一次检查，检查是否在某些边缘产生拖动了，如果有拖动，就将有拖动的边缘记录在mEdgeDragsInProgress中，再调用Callback的onEdgeDragStarted(int edgeFlags, int pointerId)通知某个边缘开始产生拖动了。虽然reportNewEdgeDrags()会被调用很多次     */
     * 对四个边缘都做了一次检查，检查是否在某些边缘产生拖动了，如果有拖动，就将有拖动的边缘记录在mEdgeDragsInProgress中，再调用Callback的onEdgeDragStarted(int edgeFlags, int pointerId)通知某个边缘开始产生拖动了。虽然reportNewEdgeDrags()会被调用很多次
     */
    private void reportNewEdgeDrags(float dx, float dy, int pointerId) {
        int dragsStarted = 0;
        if (checkNewEdgeDrag(dx, dy, pointerId, EDGE_LEFT)) {
            dragsStarted |= EDGE_LEFT;
        }
        if (checkNewEdgeDrag(dy, dx, pointerId, EDGE_TOP)) {
            dragsStarted |= EDGE_TOP;
        }
        if (checkNewEdgeDrag(dx, dy, pointerId, EDGE_RIGHT)) {
            dragsStarted |= EDGE_RIGHT;
        }
        if (checkNewEdgeDrag(dy, dx, pointerId, EDGE_BOTTOM)) {
            dragsStarted |= EDGE_BOTTOM;
        }

        if (dragsStarted != 0) {
            mEdgeDragsInProgress[pointerId] |= dragsStarted;
            mCallback.onEdgeDragStarted(dragsStarted, pointerId);
        }
    }
  
    /**     * 返回true表示在指定的边缘开始产生拖动了     */
     * 返回true表示在指定的边缘开始产生拖动了
     */
    private boolean checkNewEdgeDrag(float delta, float odelta, int pointerId, int edge) {
        final float absDelta = Math.abs(delta);//主要监测的方向上的变化
        final float absODelta = Math.abs(odelta);//另外一个方向上的变化
      
      	//1.如果ACTION_DOWN发生时没有触摸到边缘，或者触摸到的边缘不是指定的edge，就直接返回false
      	//2.mTrackingEdges是由setEdgeTrackingEnabled(int edgeFlags)设置的
      	//3.锁定的边缘
		//4.mEdgeDragsInProgress是保存已发生过拖动事件的边缘的
        //5.检查本次移动的距离是不是太小了，太小就不处理了
        if ((mInitialEdgesTouched[pointerId] & edge) != edge  || (mTrackingEdges & edge) == 0 ||
                (mEdgeDragsLocked[pointerId] & edge) == edge ||
                (mEdgeDragsInProgress[pointerId] & edge) == edge ||
                (absDelta <= mTouchSlop && absODelta <= mTouchSlop)) {
            return false;
        }
        if (absDelta < absODelta * 0.5f && mCallback.onEdgeLock(edge)) {//次要方向上移动的距离是否远超过主要方向上移动的距离
            mEdgeDragsLocked[pointerId] |= edge;
            return false;
        }
        return (mEdgeDragsInProgress[pointerId] & edge) == 0 && absDelta > mTouchSlop;
    }
  
    private void saveLastMotion(MotionEvent ev) {
        final int pointerCount = MotionEventCompat.getPointerCount(ev);
        for (int i = 0; i < pointerCount; i++) {
            final int pointerId = MotionEventCompat.getPointerId(ev, i);
            final float x = MotionEventCompat.getX(ev, i);
            final float y = MotionEventCompat.getY(ev, i);
            mLastMotionX[pointerId] = x;
            mLastMotionY[pointerId] = y;
        }
    }
}

findTopChildUnder() 方法是获得 View，如果在同一个位置有两个子 View 重叠，想要让下层的子 View 被选中， 那么就要实现 Callback 里的 getOrderedChildIndex(int index) 方法来改变查找子View的顺序；例如 topView（ 上层View ）的 index 是 4， bottomView（下层View）的 index 是 3，按照正常的遍历查找方式（ getOrderedChildIndex() 默认直接返回 index ），会选择到 topView ， 要想让 bottomView 被选中就得这么写：

public int getOrderedChildIndex(int index) {
    int indexTop = mParentView.indexOfChild(topView);
    int indexBottom = mParentView.indexOfChild(bottomView);
    if (index == indexTop) {
        return indexBottom;
    }
    return index;
}

在 shouldInterceptTouchEvent() 方法中，一开始进入的是 ACTION_DOWN ，mCapturedView 默认为 null ，所以一开始会进到 tryCaptureViewForDrag() 里面，当条件 tryCaptureView() 满足的时候会回调 onViewCaptured() 出去，同时设置状态为 STATE_DRAGGING ，之后再回调 onEdgeTouched() 。ACTION_DOWN 部分处理完了，跳过 switch 语句块，剩下的代码就只有 return mDragState == STATE_DRAGGING; ，此时如果状态为 dragging 的话，就反会的 true ，那么表示手势事件被拦截了；在 ACTION_DOWN 部分没有对 mDragState 进行赋值，其默认值为 STATE_IDLE ，所以此处返回 false ，此时会将手势事件传递给子 View 。如果子 View 没有消耗手势事件，那么会进到 ViewDragHelper#processTouchEvent() 中；如果子 View 消耗了手势事件，那么父 View 的 onTouchEvent() 收不到 ACTION_DOWN 事件，不过只要子 View 没有调用过 requestDisallowInterceptTouchEvent(true)，父 View 的 onInterceptTouchEvent() 的 ACTION_MOVE 部分还是会执行的， 如果在此时返回了 true 拦截了ACTION_MOVE 事件，processTouchEvent() 里的 ACTION_MOVE 部分也就会正常执行，拖动也就没问题了。

如果有多个手指触摸到屏幕上了，对每个触摸点都检查一下，看当前触摸的地方是否需要捕获某个 View。这里先用 findTopChildUnder(int x, int y) 寻找触摸点处的子 View，再用 checkTouchSlop(View child, float dx, float dy) 检查当前触摸点到 ACTION_DOWN 触摸点的距离是否达到了 mTouchSlop，达到了才会去捕获 View。

processTouchEvent

public class ViewDragHelper {

    /**     * Process a touch event received by the parent view. This method will dispatch callback events     * as needed before returning. The parent view's onTouchEvent implementation should call this.     *     * @param ev The touch event received by the parent view     */
     * Process a touch event received by the parent view. This method will dispatch callback events
     * as needed before returning. The parent view's onTouchEvent implementation should call this.
     *
     * @param ev The touch event received by the parent view
     */
    public void processTouchEvent(MotionEvent ev) {
        final int action = MotionEventCompat.getActionMasked(ev);//当前action
        final int actionIndex = MotionEventCompat.getActionIndex(ev);//当前actionIndex

        if (action == MotionEvent.ACTION_DOWN) {
            // Reset things for a new event stream, just in case we didn't get
            // the whole previous stream.
            cancel();//归位
        }
        //初始化mVelocityTracker并添加事件
        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        switch (action) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();
                final int pointerId = MotionEventCompat.getPointerId(ev, 0);//手指id
                final View toCapture = findTopChildUnder((int) x, (int) y);

                saveInitialMotion(x, y, pointerId);//保存手指id对应的信息

                // Since the parent is already directly processing this touch event,
                // there is no reason to delay for a slop before dragging.
                // Start immediately if possible.
                tryCaptureViewForDrag(toCapture, pointerId);//去drag

                final int edgesTouched = mInitialEdgesTouched[pointerId];//得到当前边缘触摸
                if ((edgesTouched & mTrackingEdges) != 0) {//mTrackingEdges为是否开启某边缘的触摸drag
                    mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);//回调
                }
                break;
            }

            case MotionEventCompat.ACTION_POINTER_DOWN: {//多手指
                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);
                final float x = MotionEventCompat.getX(ev, actionIndex);
                final float y = MotionEventCompat.getY(ev, actionIndex);

                saveInitialMotion(x, y, pointerId);;//保存手指id对应的信息

                // A ViewDragHelper can only manipulate one view at a time.
                if (mDragState == STATE_IDLE) {
                    // If we're idle we can do anything! Treat it like a normal down event.

                    final View toCapture = findTopChildUnder((int) x, (int) y);//找到View
                    tryCaptureViewForDrag(toCapture, pointerId);//回调

                    final int edgesTouched = mInitialEdgesTouched[pointerId];//得到边缘
                    if ((edgesTouched & mTrackingEdges) != 0) {//是否能够触发边缘drag
                        mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);//回调
                    }
                } else if (isCapturedViewUnder((int) x, (int) y)) {
                    // We're still tracking a captured view. If the same view is under this
                    // point, we'll swap to controlling it with this pointer instead.
                    // (This will still work if we're "catching" a settling view.)

                    tryCaptureViewForDrag(mCapturedView, pointerId);
                }
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                if (mDragState == STATE_DRAGGING) {//正在拖动
                    // If pointer is invalid then skip the ACTION_MOVE.
                    if (!isValidPointerForActionMove(mActivePointerId)) break;

                    final int index = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
                    final float x = MotionEventCompat.getX(ev, index);//得到X
                    final float y = MotionEventCompat.getY(ev, index);//得到Y
                    final int idx = (int) (x - mLastMotionX[mActivePointerId]);//算出移动的X方向距离
                    final int idy = (int) (y - mLastMotionY[mActivePointerId]);//算出移动的Y方向距离

                    dragTo(mCapturedView.getLeft() + idx, mCapturedView.getTop() + idy, idx, idy);//移动

                    saveLastMotion(ev);
                } else {
                    // Check to see if any pointer is now over a draggable view.
                    final int pointerCount = MotionEventCompat.getPointerCount(ev);
                    for (int i = 0; i < pointerCount; i++) {
                        final int pointerId = MotionEventCompat.getPointerId(ev, i);

                        // If pointer is invalid then skip the ACTION_MOVE.
                        if (!isValidPointerForActionMove(pointerId)) continue;

                        final float x = MotionEventCompat.getX(ev, i);
                        final float y = MotionEventCompat.getY(ev, i);
                        final float dx = x - mInitialMotionX[pointerId];
                        final float dy = y - mInitialMotionY[pointerId];

                        reportNewEdgeDrags(dx, dy, pointerId);//经过计算之后回调结果出去
                        if (mDragState == STATE_DRAGGING) {
                            // Callback might have started an edge drag.
                            break;
                        }

                        final View toCapture = findTopChildUnder((int) x, (int) y);
                        if (checkTouchSlop(toCapture, dx, dy) &&
                                tryCaptureViewForDrag(toCapture, pointerId)) {
                            break;
                        }
                    }
                    saveLastMotion(ev);
                }
                break;
            }

            case MotionEventCompat.ACTION_POINTER_UP: {
                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);
                if (mDragState == STATE_DRAGGING && pointerId == mActivePointerId) {
                    // Try to find another pointer that's still holding on to the captured view.
                    int newActivePointer = INVALID_POINTER;
                    final int pointerCount = MotionEventCompat.getPointerCount(ev);
                    for (int i = 0; i < pointerCount; i++) {
                        final int id = MotionEventCompat.getPointerId(ev, i);
                        if (id == mActivePointerId) {
                            // This one's going away, skip.
                            continue;
                        }

                        final float x = MotionEventCompat.getX(ev, i);
                        final float y = MotionEventCompat.getY(ev, i);
                        if (findTopChildUnder((int) x, (int) y) == mCapturedView &&
                                tryCaptureViewForDrag(mCapturedView, id)) {
                            newActivePointer = mActivePointerId;//更新手指id
                            break;
                        }
                    }

                    if (newActivePointer == INVALID_POINTER) {
                        // We didn't find another pointer still touching the view, release it.
                        releaseViewForPointerUp();//释放
                    }
                }
                clearMotionHistory(pointerId);//清除数据
                break;
            }

            case MotionEvent.ACTION_UP: {
                if (mDragState == STATE_DRAGGING) {
                    releaseViewForPointerUp();//释放
                }
                cancel();
                break;
            }

            case MotionEvent.ACTION_CANCEL: {
                if (mDragState == STATE_DRAGGING) {
                    dispatchViewReleased(0, 0);//释放
                }
                cancel();
                break;
            }
        }
    }
  
    private void dragTo(int left, int top, int dx, int dy) {、、参数dx和dy是前后两次ACTION_MOVE移动的距离
        int clampedX = left;
        int clampedY = top;
        final int oldLeft = mCapturedView.getLeft();
        final int oldTop = mCapturedView.getTop();
        if (dx != 0) {
            clampedX = mCallback.clampViewPositionHorizontal(mCapturedView, left, dx);//回调
            ViewCompat.offsetLeftAndRight(mCapturedView, clampedX - oldLeft);//移动
        }
        if (dy != 0) {
            clampedY = mCallback.clampViewPositionVertical(mCapturedView, top, dy);//回调
            ViewCompat.offsetTopAndBottom(mCapturedView, clampedY - oldTop);//移动
        }

        if (dx != 0 || dy != 0) {
            final int clampedDx = clampedX - oldLeft;
            final int clampedDy = clampedY - oldTop;
            mCallback.onViewPositionChanged(mCapturedView, clampedX, clampedY,
                    clampedDx, clampedDy);//回调
        }
    }
  
    private void releaseViewForPointerUp() {
        mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);
        final float xvel = clampMag(
                VelocityTrackerCompat.getXVelocity(mVelocityTracker, mActivePointerId),
                mMinVelocity, mMaxVelocity);
        final float yvel = clampMag(
                VelocityTrackerCompat.getYVelocity(mVelocityTracker, mActivePointerId),
                mMinVelocity, mMaxVelocity);
        dispatchViewReleased(xvel, yvel);
    }
  
    /**     * Clamp the magnitude of value for absMin and absMax.     * If the value is below the minimum, it will be clamped to zero.     * If the value is above the maximum, it will be clamped to the maximum.     *     * @param value Value to clamp     * @param absMin Absolute value of the minimum significant value to return     * @param absMax Absolute value of the maximum value to return     * @return The clamped value with the same sign as <code>value</code>     */
     * Clamp the magnitude of value for absMin and absMax.
     * If the value is below the minimum, it will be clamped to zero.
     * If the value is above the maximum, it will be clamped to the maximum.
     *
     * @param value Value to clamp
     * @param absMin Absolute value of the minimum significant value to return
     * @param absMax Absolute value of the maximum value to return
     * @return The clamped value with the same sign as <code>value</code>
     */
    private float clampMag(float value, float absMin, float absMax) {
        final float absValue = Math.abs(value);
        if (absValue < absMin) return 0;
        if (absValue > absMax) return value > 0 ? absMax : -absMax;
        return value;
    }
  
    /**     * Like all callback events this must happen on the UI thread, but release     * involves some extra semantics. During a release (mReleaseInProgress)     * is the only time it is valid to call {@link #settleCapturedViewAt(int, int)}     * or {@link #flingCapturedView(int, int, int, int)}.     */
     * Like all callback events this must happen on the UI thread, but release
     * involves some extra semantics. During a release (mReleaseInProgress)
     * is the only time it is valid to call {@link #settleCapturedViewAt(int, int)}
     * or {@link #flingCapturedView(int, int, int, int)}.
     */
    private void dispatchViewReleased(float xvel, float yvel) {
        mReleaseInProgress = true;
        mCallback.onViewReleased(mCapturedView, xvel, yvel);
        mReleaseInProgress = false;

        if (mDragState == STATE_DRAGGING) {
            // onViewReleased didn't call a method that would have changed this. Go idle.
            setDragState(STATE_IDLE);
        }
    }

}

ACTION_DOWN 与 shouldInterceptTouchEvent 中的差不多。当进入到 ACTION_MOVE 的时候，先判断 mDragState 是否为 STATE_DRAGGING ，而唯一调用 setDragState(STATE_DRAGGING) 的地方就是 tryCaptureViewForDrag() 了， 刚才在 ACTION_DOWN 里调用过 tryCaptureViewForDrag() 。当为 STATE_DRAGGING 的时候，开始不停的调用 dragTo() 对 mCaptureView 进行真正拖动了。而 dragTo() 通过调用 offsetLeftAndRight() 和 offsetTopAndBottom() 来完成对 mCapturedView 移动。在 dispatchViewReleased() 的注释里面有介绍了两个方法，settleCapturedViewAt(int, int) 和 flingCapturedView(int, int, int, int) ：

settleCapturedViewAt && flingCapturedView

public class ViewDragHelper {
  
    /**     * Settle the captured view at the given (left, top) position.     * The appropriate velocity from prior motion will be taken into account.     * If this method returns true, the caller should invoke {@link #continueSettling(boolean)}     * on each subsequent frame to continue the motion until it returns false. If this method     * returns false there is no further work to do to complete the movement.     *     * @param finalLeft Settled left edge position for the captured view     * @param finalTop Settled top edge position for the captured view     * @return true if animation should continue through {@link #continueSettling(boolean)} calls     */
     * Settle the captured view at the given (left, top) position.
     * The appropriate velocity from prior motion will be taken into account.
     * If this method returns true, the caller should invoke {@link #continueSettling(boolean)}
     * on each subsequent frame to continue the motion until it returns false. If this method
     * returns false there is no further work to do to complete the movement.
     *
     * @param finalLeft Settled left edge position for the captured view
     * @param finalTop Settled top edge position for the captured view
     * @return true if animation should continue through {@link #continueSettling(boolean)} calls
     */
    public boolean settleCapturedViewAt(int finalLeft, int finalTop) {
        if (!mReleaseInProgress) {
            throw new IllegalStateException("Cannot settleCapturedViewAt outside of a call to " +
                    "Callback#onViewReleased");
        }

        return forceSettleCapturedViewAt(finalLeft, finalTop,
                (int) VelocityTrackerCompat.getXVelocity(mVelocityTracker, mActivePointerId),
                (int) VelocityTrackerCompat.getYVelocity(mVelocityTracker, mActivePointerId));
    }
  
    /**     * Settle the captured view at the given (left, top) position.     *     * @param finalLeft Target left position for the captured view     * @param finalTop Target top position for the captured view     * @param xvel Horizontal velocity     * @param yvel Vertical velocity     * @return true if animation should continue through {@link #continueSettling(boolean)} calls     */
     * Settle the captured view at the given (left, top) position.
     *
     * @param finalLeft Target left position for the captured view
     * @param finalTop Target top position for the captured view
     * @param xvel Horizontal velocity
     * @param yvel Vertical velocity
     * @return true if animation should continue through {@link #continueSettling(boolean)} calls
     */
    private boolean forceSettleCapturedViewAt(int finalLeft, int finalTop, int xvel, int yvel) {
        final int startLeft = mCapturedView.getLeft();
        final int startTop = mCapturedView.getTop();
        final int dx = finalLeft - startLeft;
        final int dy = finalTop - startTop;

        if (dx == 0 && dy == 0) {
            // Nothing to do. Send callbacks, be done.
            mScroller.abortAnimation();
            setDragState(STATE_IDLE);
            return false;
        }

        final int duration = computeSettleDuration(mCapturedView, dx, dy, xvel, yvel);
        mScroller.startScroll(startLeft, startTop, dx, dy, duration);//靠Scroll类完成

        setDragState(STATE_SETTLING);
        return true;
    }
  
    private int computeSettleDuration(View child, int dx, int dy, int xvel, int yvel) {
        xvel = clampMag(xvel, (int) mMinVelocity, (int) mMaxVelocity);
        yvel = clampMag(yvel, (int) mMinVelocity, (int) mMaxVelocity);
        final int absDx = Math.abs(dx);
        final int absDy = Math.abs(dy);
        final int absXVel = Math.abs(xvel);
        final int absYVel = Math.abs(yvel);
        final int addedVel = absXVel + absYVel;
        final int addedDistance = absDx + absDy;

        final float xweight = xvel != 0 ? (float) absXVel / addedVel :
                (float) absDx / addedDistance;
        final float yweight = yvel != 0 ? (float) absYVel / addedVel :
                (float) absDy / addedDistance;

        int xduration = computeAxisDuration(dx, xvel, mCallback.getViewHorizontalDragRange(child));
        int yduration = computeAxisDuration(dy, yvel, mCallback.getViewVerticalDragRange(child));

        return (int) (xduration * xweight + yduration * yweight);
    }

    private int computeAxisDuration(int delta, int velocity, int motionRange) {
        if (delta == 0) {
            return 0;
        }
        //如果给定的速率velocity不为0，就通过距离除以速率来算出时间
        final int width = mParentView.getWidth();
        final int halfWidth = width / 2;
        final float distanceRatio = Math.min(1f, (float) Math.abs(delta) / width);
        final float distance = halfWidth + halfWidth *
                distanceInfluenceForSnapDuration(distanceRatio);

        int duration;
        velocity = Math.abs(velocity);
        if (velocity > 0) {
            duration = 4 * Math.round(1000 * Math.abs(distance / velocity));
        } else {
            final float range = (float) Math.abs(delta) / motionRange;
            duration = (int) ((range + 1) * BASE_SETTLE_DURATION);
        }
        return Math.min(duration, MAX_SETTLE_DURATION);
    }

    /**     * Clamp the magnitude of value for absMin and absMax.     * If the value is below the minimum, it will be clamped to zero.     * If the value is above the maximum, it will be clamped to the maximum.     * 确保参数中给定的速率在正常范围之内     *     * @param value Value to clamp     * @param absMin Absolute value of the minimum significant value to return     * @param absMax Absolute value of the maximum value to return     * @return The clamped value with the same sign as <code>value</code>     */
     * Clamp the magnitude of value for absMin and absMax.
     * If the value is below the minimum, it will be clamped to zero.
     * If the value is above the maximum, it will be clamped to the maximum.
     * 确保参数中给定的速率在正常范围之内
     *
     * @param value Value to clamp
     * @param absMin Absolute value of the minimum significant value to return
     * @param absMax Absolute value of the maximum value to return
     * @return The clamped value with the same sign as <code>value</code>
     */
    private int clampMag(int value, int absMin, int absMax) {
        final int absValue = Math.abs(value);
        if (absValue < absMin) return 0;
        if (absValue > absMax) return value > 0 ? absMax : -absMax;
        return value;
    }

    /**     * Settle the captured view based on standard free-moving fling behavior.     * The caller should invoke {@link #continueSettling(boolean)} on each subsequent frame     * to continue the motion until it returns false.     *     * @param minLeft Minimum X position for the view's left edge     * @param minTop Minimum Y position for the view's top edge     * @param maxLeft Maximum X position for the view's left edge     * @param maxTop Maximum Y position for the view's top edge     */
     * Settle the captured view based on standard free-moving fling behavior.
     * The caller should invoke {@link #continueSettling(boolean)} on each subsequent frame
     * to continue the motion until it returns false.
     *
     * @param minLeft Minimum X position for the view's left edge
     * @param minTop Minimum Y position for the view's top edge
     * @param maxLeft Maximum X position for the view's left edge
     * @param maxTop Maximum Y position for the view's top edge
     */
    public void flingCapturedView(int minLeft, int minTop, int maxLeft, int maxTop) {
        if (!mReleaseInProgress) {
            throw new IllegalStateException("Cannot flingCapturedView outside of a call to " +
                    "Callback#onViewReleased");
        }

        mScroller.fling(mCapturedView.getLeft(), mCapturedView.getTop(),
                (int) VelocityTrackerCompat.getXVelocity(mVelocityTracker, mActivePointerId),
                (int) VelocityTrackerCompat.getYVelocity(mVelocityTracker, mActivePointerId),
                minLeft, maxLeft, minTop, maxTop);

        setDragState(STATE_SETTLING);
    }
}

这两个方法里一开始都会判断 mReleaseInProgress 为 false ，如果为 false 就会抛一个 IllegalStateException 异常， 而 mReleaseInProgress 唯一为true的时候就是在 dispatchViewReleased() 里调用 onViewReleased() 的时候。settleCapturedViewAt(int finalLeft, int finalTop) 以松手前的滑动速度为初速动，让捕获到的 View 自动滚动到指定位置。只能在 Callback 的 onViewReleased() 中调用。flingCapturedView(int minLeft, int minTop, int maxLeft, int maxTop) 以松手前的滑动速度为初速动，让捕获到的 View 在指定范围内 fling ，也只能在 Callback 的 onViewReleased() 中调用。

smoothSlideViewTo

ViewDragHelper 还有一个移动 View 的方法是 smoothSlideViewTo(View child, int finalLeft, int finalTop)：

public class ViewDragHelper {
    /**     * Animate the view <code>child</code> to the given (left, top) position.     * If this method returns true, the caller should invoke {@link #continueSettling(boolean)}     * on each subsequent frame to continue the motion until it returns false. If this method     * returns false there is no further work to do to complete the movement.     *     * <p>This operation does not count as a capture event, though {@link #getCapturedView()}     * will still report the sliding view while the slide is in progress.</p>     *     * @param child Child view to capture and animate     * @param finalLeft Final left position of child     * @param finalTop Final top position of child     * @return true if animation should continue through {@link #continueSettling(boolean)} calls     */
     * Animate the view <code>child</code> to the given (left, top) position.
     * If this method returns true, the caller should invoke {@link #continueSettling(boolean)}
     * on each subsequent frame to continue the motion until it returns false. If this method
     * returns false there is no further work to do to complete the movement.
     *
     * <p>This operation does not count as a capture event, though {@link #getCapturedView()}
     * will still report the sliding view while the slide is in progress.</p>
     *
     * @param child Child view to capture and animate
     * @param finalLeft Final left position of child
     * @param finalTop Final top position of child
     * @return true if animation should continue through {@link #continueSettling(boolean)} calls
     */
    public boolean smoothSlideViewTo(View child, int finalLeft, int finalTop) {
        mCapturedView = child;
        mActivePointerId = INVALID_POINTER;

        boolean continueSliding = forceSettleCapturedViewAt(finalLeft, finalTop, 0, 0);
        if (!continueSliding && mDragState == STATE_IDLE && mCapturedView != null) {
            // If we're in an IDLE state to begin with and aren't moving anywhere, we
            // end up having a non-null capturedView with an IDLE dragState
            mCapturedView = null;
        }

        return continueSliding;
    }
}

可以看到它不受 mReleaseInProgress 的限制，所以可以在任何地方调用，效果和 settleCapturedViewAt() 类似，因为它们最终都调用了 forceSettleCapturedViewAt() 来启动自动滚动，区别在于 settleCapturedViewAt() 会以最后松手前的滑动速率为初速度将 View 滚动到最终位置，而 smoothSlideViewTo() 滚动的初速度是0。 smoothSlideViewTo(View child, int finalLeft, int finalTop) 指定某个 View 自动滚动到指定的位置，初速度为 0 ，可在任何地方调用。