分析版本：android-23

Animation 和 Animator 是 Android 上实现动画的两种不同方式，本文是对这两种动画进行流程上的解析，其分析方向主要是 Listener 是怎么回调的，是怎么不断的调用自己的，是间隔多少时间调用自己的。

Animation

从 startAnimation 开始：

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mView.startAnimation(animation);

进入到 View 里面：

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public class View implements Drawable.Callback, KeyEvent.Callback, AccessibilityEventSource {

    public void startAnimation(Animation animation) {
        animation.setStartTime(Animation.START_ON_FIRST_FRAME);//START_ON_FIRST_FRAME == -1
        setAnimation(animation);
        invalidateParentCaches();
        invalidate(true);
    } 
  
    protected Animation mCurrentAnimation = null;
  
    public void setAnimation(Animation animation) {
        mCurrentAnimation = animation;

        if (animation != null) {
            // If the screen is off assume the animation start time is now instead of
            // the next frame we draw. Keeping the START_ON_FIRST_FRAME start time
            // would cause the animation to start when the screen turns back on
            if (mAttachInfo != null && mAttachInfo.mDisplayState == Display.STATE_OFF
                    && animation.getStartTime() == Animation.START_ON_FIRST_FRAME) {//START_ON_FIRST_FRAME == -1
                animation.setStartTime(AnimationUtils.currentAnimationTimeMillis());
            }
            animation.reset();
        }
    }
  
    protected void invalidateParentCaches() {
        if (mParent instanceof View) {
            ((View) mParent).mPrivateFlags |= PFLAG_INVALIDATED;
        }
    }
  
    void invalidate(boolean invalidateCache) {
        invalidateInternal(0, 0, mRight - mLeft, mBottom - mTop, invalidateCache, true);
    }
}

设置 Animation 变量并设置时间等，刷新父视图绘画缓存，刷新自己：

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public class View implements Drawable.Callback, KeyEvent.Callback, AccessibilityEventSource {
    void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache,
            boolean fullInvalidate) {
        if (mGhostView != null) {
            mGhostView.invalidate(true);
            return;
        }

        if (skipInvalidate()) {
            return;
        }

        if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)) == (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)
                || (invalidateCache && (mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == PFLAG_DRAWING_CACHE_VALID)
                || (mPrivateFlags & PFLAG_INVALIDATED) != PFLAG_INVALIDATED
                || (fullInvalidate && isOpaque() != mLastIsOpaque)) {
            if (fullInvalidate) {
                mLastIsOpaque = isOpaque();
                mPrivateFlags &= ~PFLAG_DRAWN;
            }

            mPrivateFlags |= PFLAG_DIRTY;

            if (invalidateCache) {
                mPrivateFlags |= PFLAG_INVALIDATED;
                mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
            }

            // Propagate the damage rectangle to the parent view.
            final AttachInfo ai = mAttachInfo;
            final ViewParent p = mParent;
            if (p != null && ai != null && l < r && t < b) {
                final Rect damage = ai.mTmpInvalRect;
                damage.set(l, t, r, b);
                p.invalidateChild(this, damage);//主要是这里
            }

            // Damage the entire projection receiver, if necessary.
            if (mBackground != null && mBackground.isProjected()) {
                final View receiver = getProjectionReceiver();
                if (receiver != null) {
                    receiver.damageInParent();
                }
            }

            // Damage the entire IsolatedZVolume receiving this view's shadow.
            if (isHardwareAccelerated() && getZ() != 0) {
                damageShadowReceiver();
            }
        }
    }
}

上面最重要的是 p.invalidateChild(this, damage); 那么进入到 ViewGroup 中：

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public abstract class ViewGroup extends View implements ViewParent, ViewManager {

    public final void invalidateChild(View child, final Rect dirty) {
        ViewParent parent = this;

        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            // If the child is drawing an animation, we want to copy this flag onto
            // ourselves and the parent to make sure the invalidate request goes
            // through
            final boolean drawAnimation = (child.mPrivateFlags & PFLAG_DRAW_ANIMATION)
                    == PFLAG_DRAW_ANIMATION;

            // Check whether the child that requests the invalidate is fully opaque
            // Views being animated or transformed are not considered opaque because we may
            // be invalidating their old position and need the parent to paint behind them.
            Matrix childMatrix = child.getMatrix();
            final boolean isOpaque = child.isOpaque() && !drawAnimation &&
                    child.getAnimation() == null && childMatrix.isIdentity();
            // Mark the child as dirty, using the appropriate flag
            // Make sure we do not set both flags at the same time
            int opaqueFlag = isOpaque ? PFLAG_DIRTY_OPAQUE : PFLAG_DIRTY;

            if (child.mLayerType != LAYER_TYPE_NONE) {
                mPrivateFlags |= PFLAG_INVALIDATED;
                mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
            }

            final int[] location = attachInfo.mInvalidateChildLocation;
            location[CHILD_LEFT_INDEX] = child.mLeft;
            location[CHILD_TOP_INDEX] = child.mTop;
            if (!childMatrix.isIdentity() ||
                    (mGroupFlags & ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS) != 0) {
                RectF boundingRect = attachInfo.mTmpTransformRect;
                boundingRect.set(dirty);
                Matrix transformMatrix;
                if ((mGroupFlags & ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS) != 0) {
                    Transformation t = attachInfo.mTmpTransformation;
                    boolean transformed = getChildStaticTransformation(child, t);
                    if (transformed) {
                        transformMatrix = attachInfo.mTmpMatrix;
                        transformMatrix.set(t.getMatrix());
                        if (!childMatrix.isIdentity()) {
                            transformMatrix.preConcat(childMatrix);
                        }
                    } else {
                        transformMatrix = childMatrix;
                    }
                } else {
                    transformMatrix = childMatrix;
                }
                transformMatrix.mapRect(boundingRect);
                dirty.set((int) (boundingRect.left - 0.5f),
                        (int) (boundingRect.top - 0.5f),
                        (int) (boundingRect.right + 0.5f),
                        (int) (boundingRect.bottom + 0.5f));
            }

            do {
                View view = null;
                if (parent instanceof View) {
                    view = (View) parent;
                }

                if (drawAnimation) {//设置Animation标志参数
                    if (view != null) {
                        view.mPrivateFlags |= PFLAG_DRAW_ANIMATION;
                    } else if (parent instanceof ViewRootImpl) {
                        ((ViewRootImpl) parent).mIsAnimating = true;
                    }
                }

                // If the parent is dirty opaque or not dirty, mark it dirty with the opaque
                // flag coming from the child that initiated the invalidate
                if (view != null) {
                    if ((view.mViewFlags & FADING_EDGE_MASK) != 0 &&
                            view.getSolidColor() == 0) {
                        opaqueFlag = PFLAG_DIRTY;
                    }
                    if ((view.mPrivateFlags & PFLAG_DIRTY_MASK) != PFLAG_DIRTY) {
                        view.mPrivateFlags = (view.mPrivateFlags & ~PFLAG_DIRTY_MASK) | opaqueFlag;
                    }
                }

                parent = parent.invalidateChildInParent(location, dirty);
                if (view != null) {
                    // Account for transform on current parent
                    Matrix m = view.getMatrix();
                    if (!m.isIdentity()) {
                        RectF boundingRect = attachInfo.mTmpTransformRect;
                        boundingRect.set(dirty);
                        m.mapRect(boundingRect);
                        dirty.set((int) (boundingRect.left - 0.5f),
                                (int) (boundingRect.top - 0.5f),
                                (int) (boundingRect.right + 0.5f),
                                (int) (boundingRect.bottom + 0.5f));
                    }
                }
            } while (parent != null);
        }
    }
}

通过 parent = parent.invalidateChildInParent(location, dirty); 进入到 ViewRootImpl 中：

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public final class ViewRootImpl implements ViewParent, View.AttachInfo.Callbacks, HardwareRenderer.HardwareDrawCallbacks {
    @Override
    public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
        checkThread();
        if (DEBUG_DRAW) Log.v(TAG, "Invalidate child: " + dirty);

        if (dirty == null) {//视图没脏
            invalidate();
            return null;
        } else if (dirty.isEmpty() && !mIsAnimating) {//不是动画的操作
            return null;
        }

        if (mCurScrollY != 0 || mTranslator != null) {
            mTempRect.set(dirty);
            dirty = mTempRect;
            if (mCurScrollY != 0) {
                dirty.offset(0, -mCurScrollY);
            }
            if (mTranslator != null) {
                mTranslator.translateRectInAppWindowToScreen(dirty);
            }
            if (mAttachInfo.mScalingRequired) {
                dirty.inset(-1, -1);
            }
        }

        invalidateRectOnScreen(dirty);//主要是这里

        return null;
    }
  
    private void invalidateRectOnScreen(Rect dirty) {
        final Rect localDirty = mDirty;
        if (!localDirty.isEmpty() && !localDirty.contains(dirty)) {
            mAttachInfo.mSetIgnoreDirtyState = true;
            mAttachInfo.mIgnoreDirtyState = true;
        }

        // Add the new dirty rect to the current one
        localDirty.union(dirty.left, dirty.top, dirty.right, dirty.bottom);
        // Intersect with the bounds of the window to skip
        // updates that lie outside of the visible region
        final float appScale = mAttachInfo.mApplicationScale;
        final boolean intersected = localDirty.intersect(0, 0,
                (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
        if (!intersected) {
            localDirty.setEmpty();
        }
        if (!mWillDrawSoon && (intersected || mIsAnimating)) {
            scheduleTraversals();//主要是这里
        }
    }
  
    void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();//阻塞的Barrier，『同步分割栏』
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);//记住CALLBACK_TRAVERSAL
            if (!mUnbufferedInputDispatch) {
                scheduleConsumeBatchedInput();
            }
            notifyRendererOfFramePending();
            pokeDrawLockIfNeeded();
        }
    }
}

先记住比较重要的信息，Choreographer.CALLBACK_TRAVERSAL，然后比较重要的 Choreographer 来了：

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public final class Choreographer {
    // Enable/disable vsync for animations and drawing.
    private static final boolean USE_VSYNC = SystemProperties.getBoolean(
            "debug.choreographer.vsync", true);
  
    // The display event receiver can only be accessed by the looper thread to which
    // it is attached.  We take care to ensure that we post message to the looper
    // if appropriate when interacting with the display event receiver.
    private final FrameDisplayEventReceiver mDisplayEventReceiver;
  
    /**
     * Posts a callback to run on the next frame.
     * <p>
     * The callback runs once then is automatically removed.
     * </p>
     *
     * @param callbackType The callback type.
     * @param action The callback action to run during the next frame.
     * @param token The callback token, or null if none.
     *
     * @see #removeCallbacks
     * @hide
     */
    public void postCallback(int callbackType, Runnable action, Object token) {
        postCallbackDelayed(callbackType, action, token, 0);
    }
  
    /**
     * Posts a callback to run on the next frame after the specified delay.
     * <p>
     * The callback runs once then is automatically removed.
     * </p>
     *
     * @param callbackType The callback type.
     * @param action The callback action to run during the next frame after the specified delay.
     * @param token The callback token, or null if none.
     * @param delayMillis The delay time in milliseconds.
     *
     * @see #removeCallback
     * @hide
     */
    public void postCallbackDelayed(int callbackType,
            Runnable action, Object token, long delayMillis) {
        if (action == null) {
            throw new IllegalArgumentException("action must not be null");
        }
        if (callbackType < 0 || callbackType > CALLBACK_LAST) {
            throw new IllegalArgumentException("callbackType is invalid");
        }

        postCallbackDelayedInternal(callbackType, action, token, delayMillis);
    }
  
    private void postCallbackDelayedInternal(int callbackType,
            Object action, Object token, long delayMillis) {
        if (DEBUG_FRAMES) {
            Log.d(TAG, "PostCallback: type=" + callbackType
                    + ", action=" + action + ", token=" + token
                    + ", delayMillis=" + delayMillis);
        }

        synchronized (mLock) {
            final long now = SystemClock.uptimeMillis();
            final long dueTime = now + delayMillis;
            mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);//将Runnable封装后放到mCallbackQueues中

            if (dueTime <= now) {
                scheduleFrameLocked(now);
            } else {
                Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
                msg.arg1 = callbackType;
                msg.setAsynchronous(true);
                mHandler.sendMessageAtTime(msg, dueTime);
            }
        }
    }
  
    private void scheduleFrameLocked(long now) {
        if (!mFrameScheduled) {
            mFrameScheduled = true;
            if (USE_VSYNC) {//使用SYNC信号形式
                if (DEBUG_FRAMES) {
                    Log.d(TAG, "Scheduling next frame on vsync.");
                }

                // If running on the Looper thread, then schedule the vsync immediately,
                // otherwise post a message to schedule the vsync from the UI thread
                // as soon as possible.
                if (isRunningOnLooperThreadLocked()) {//true
                    scheduleVsyncLocked();
                } else {
                    Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
                    msg.setAsynchronous(true);
                    mHandler.sendMessageAtFrontOfQueue(msg);
                }
            } else {
                final long nextFrameTime = Math.max(
                        mLastFrameTimeNanos / TimeUtils.NANOS_PER_MS + sFrameDelay, now);
                if (DEBUG_FRAMES) {
                    Log.d(TAG, "Scheduling next frame in " + (nextFrameTime - now) + " ms.");
                }
                Message msg = mHandler.obtainMessage(MSG_DO_FRAME);
                msg.setAsynchronous(true);
                mHandler.sendMessageAtTime(msg, nextFrameTime);
            }
        }
    }
  
    private boolean isRunningOnLooperThreadLocked() {
        return Looper.myLooper() == mLooper;
    }
  
    private void scheduleVsyncLocked() {
        mDisplayEventReceiver.scheduleVsync();
    }
}

最终调用 scheduleVsyncLocke() 方法，而这个方法里面是通过 mDisplayEventReceiver 调用 scheduleVsync()，该方法最终执行的是一个 native 方法，当 SYNC 信号返回时候，会回调 DisplayEventReceiver#onVsync(timestampNanos, builtInDisplayId, frame):

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public final class Choreographer {
    private final class FrameDisplayEventReceiver extends DisplayEventReceiver
            implements Runnable {
        private boolean mHavePendingVsync;
        private long mTimestampNanos;
        private int mFrame;

        public FrameDisplayEventReceiver(Looper looper) {
            super(looper);
        }

        @Override
        public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
            // Ignore vsync from secondary display.
            // This can be problematic because the call to scheduleVsync() is a one-shot.
            // We need to ensure that we will still receive the vsync from the primary
            // display which is the one we really care about.  Ideally we should schedule
            // vsync for a particular display.
            // At this time Surface Flinger won't send us vsyncs for secondary displays
            // but that could change in the future so let's log a message to help us remember
            // that we need to fix this.
            if (builtInDisplayId != SurfaceControl.BUILT_IN_DISPLAY_ID_MAIN) {
                Log.d(TAG, "Received vsync from secondary display, but we don't support "
                        + "this case yet.  Choreographer needs a way to explicitly request "
                        + "vsync for a specific display to ensure it doesn't lose track "
                        + "of its scheduled vsync.");
                scheduleVsync();
                return;
            }

            // Post the vsync event to the Handler.
            // The idea is to prevent incoming vsync events from completely starving
            // the message queue.  If there are no messages in the queue with timestamps
            // earlier than the frame time, then the vsync event will be processed immediately.
            // Otherwise, messages that predate the vsync event will be handled first.
            long now = System.nanoTime();
            if (timestampNanos > now) {
                Log.w(TAG, "Frame time is " + ((timestampNanos - now) * 0.000001f)
                        + " ms in the future!  Check that graphics HAL is generating vsync "
                        + "timestamps using the correct timebase.");
                timestampNanos = now;
            }

            if (mHavePendingVsync) {
                Log.w(TAG, "Already have a pending vsync event.  There should only be "
                        + "one at a time.");
            } else {
                mHavePendingVsync = true;
            }

            mTimestampNanos = timestampNanos;
            mFrame = frame;
            Message msg = Message.obtain(mHandler, this);//第二个参数是Runnable
            msg.setAsynchronous(true);
            mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
        }

        @Override
        public void run() {
            mHavePendingVsync = false;
            doFrame(mTimestampNanos, mFrame);
        }
    }
}

通过 Handler 发送一个异步的消息，然后就自己的 run() 方法中，执行 doFrame():

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public final class Choreographer {
    void doFrame(long frameTimeNanos, int frame) {
        final long startNanos;
        synchronized (mLock) {
            if (!mFrameScheduled) {
                return; // no work to do
            }

            if (DEBUG_JANK && mDebugPrintNextFrameTimeDelta) {
                mDebugPrintNextFrameTimeDelta = false;
                Log.d(TAG, "Frame time delta: "
                        + ((frameTimeNanos - mLastFrameTimeNanos) * 0.000001f) + " ms");
            }

            long intendedFrameTimeNanos = frameTimeNanos;
            startNanos = System.nanoTime();
            final long jitterNanos = startNanos - frameTimeNanos;
            if (jitterNanos >= mFrameIntervalNanos) {
                final long skippedFrames = jitterNanos / mFrameIntervalNanos;
                if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
                    Log.i(TAG, "Skipped " + skippedFrames + " frames!  "
                            + "The application may be doing too much work on its main thread.");
                }
                final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
                if (DEBUG_JANK) {
                    Log.d(TAG, "Missed vsync by " + (jitterNanos * 0.000001f) + " ms "
                            + "which is more than the frame interval of "
                            + (mFrameIntervalNanos * 0.000001f) + " ms!  "
                            + "Skipping " + skippedFrames + " frames and setting frame "
                            + "time to " + (lastFrameOffset * 0.000001f) + " ms in the past.");
                }
                frameTimeNanos = startNanos - lastFrameOffset;
            }

            if (frameTimeNanos < mLastFrameTimeNanos) {
                if (DEBUG_JANK) {
                    Log.d(TAG, "Frame time appears to be going backwards.  May be due to a "
                            + "previously skipped frame.  Waiting for next vsync.");
                }
                scheduleVsyncLocked();
                return;
            }

            mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
            mFrameScheduled = false;
            mLastFrameTimeNanos = frameTimeNanos;
        }

        try {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");

            mFrameInfo.markInputHandlingStart();
            doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);//输入

            mFrameInfo.markAnimationsStart();
            doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);//动画

            mFrameInfo.markPerformTraversalsStart();
            doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);//主要关心这个

            doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }

        if (DEBUG_FRAMES) {
            final long endNanos = System.nanoTime();
            Log.d(TAG, "Frame " + frame + ": Finished, took "
                    + (endNanos - startNanos) * 0.000001f + " ms, latency "
                    + (startNanos - frameTimeNanos) * 0.000001f + " ms.");
        }
    }
  
    void doCallbacks(int callbackType, long frameTimeNanos) {
        CallbackRecord callbacks;
        synchronized (mLock) {
            // We use "now" to determine when callbacks become due because it's possible
            // for earlier processing phases in a frame to post callbacks that should run
            // in a following phase, such as an input event that causes an animation to start.
            final long now = System.nanoTime();
            callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
                    now / TimeUtils.NANOS_PER_MS);//拿到之前传入的runnable，在该方法中还处理了丢帧的情况
            if (callbacks == null) {
                return;
            }
            mCallbacksRunning = true;

            // Update the frame time if necessary when committing the frame.
            // We only update the frame time if we are more than 2 frames late reaching
            // the commit phase.  This ensures that the frame time which is observed by the
            // callbacks will always increase from one frame to the next and never repeat.
            // We never want the next frame's starting frame time to end up being less than
            // or equal to the previous frame's commit frame time.  Keep in mind that the
            // next frame has most likely already been scheduled by now so we play it
            // safe by ensuring the commit time is always at least one frame behind.
            if (callbackType == Choreographer.CALLBACK_COMMIT) {
                final long jitterNanos = now - frameTimeNanos;
                Trace.traceCounter(Trace.TRACE_TAG_VIEW, "jitterNanos", (int) jitterNanos);
                if (jitterNanos >= 2 * mFrameIntervalNanos) {
                    final long lastFrameOffset = jitterNanos % mFrameIntervalNanos
                            + mFrameIntervalNanos;
                    if (DEBUG_JANK) {
                        Log.d(TAG, "Commit callback delayed by " + (jitterNanos * 0.000001f)
                                + " ms which is more than twice the frame interval of "
                                + (mFrameIntervalNanos * 0.000001f) + " ms!  "
                                + "Setting frame time to " + (lastFrameOffset * 0.000001f)
                                + " ms in the past.");
                        mDebugPrintNextFrameTimeDelta = true;
                    }
                    frameTimeNanos = now - lastFrameOffset;
                    mLastFrameTimeNanos = frameTimeNanos;
                }
            }
        }
        try {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, CALLBACK_TRACE_TITLES[callbackType]);
            for (CallbackRecord c = callbacks; c != null; c = c.next) {
                if (DEBUG_FRAMES) {
                    Log.d(TAG, "RunCallback: type=" + callbackType
                            + ", action=" + c.action + ", token=" + c.token
                            + ", latencyMillis=" + (SystemClock.uptimeMillis() - c.dueTime));
                }
                c.run(frameTimeNanos);//执行run方法
            }
        } finally {
            synchronized (mLock) {
                mCallbacksRunning = false;
                do {
                    final CallbackRecord next = callbacks.next;
                    recycleCallbackLocked(callbacks);//回收
                    callbacks = next;
                } while (callbacks != null);
            }
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }
}

在 c.run(frameTimeNanos) 中执行了之前通过 Choreographer 传送的 Runnable，回到 ViewRootImpl ：

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public final class ViewRootImpl implements ViewParent, View.AttachInfo.Callbacks, HardwareRenderer.HardwareDrawCallbacks {
    final class TraversalRunnable implements Runnable {
        @Override
        public void run() {
            doTraversal();
        }
    }
    final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
  
    void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);//取消阻塞

            if (mProfile) {
                Debug.startMethodTracing("ViewAncestor");
            }

            performTraversals();//View的measue，layout，draw的开始的地方

            if (mProfile) {
                Debug.stopMethodTracing();
                mProfile = false;
            }
        }
    }
}

直接看 ViewGroup 的 View.draw(Canvas, ViewGroup, long):

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public abstract class ViewGroup extends View implements ViewParent, ViewManager {
    /**
     * {@inheritDoc}
     */
    @Override
    protected void dispatchDraw(Canvas canvas) {
        boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
        final int childrenCount = mChildrenCount;
        final View[] children = mChildren;
        int flags = mGroupFlags;
        //动画开始
        if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {
            final boolean buildCache = !isHardwareAccelerated();
            for (int i = 0; i < childrenCount; i++) {
                final View child = children[i];
                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
                    final LayoutParams params = child.getLayoutParams();
                    attachLayoutAnimationParameters(child, params, i, childrenCount);
                    bindLayoutAnimation(child);
                }
            }

            final LayoutAnimationController controller = mLayoutAnimationController;
            if (controller.willOverlap()) {
                mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;
            }

            controller.start();

            mGroupFlags &= ~FLAG_RUN_ANIMATION;
            mGroupFlags &= ~FLAG_ANIMATION_DONE;

            if (mAnimationListener != null) {
                mAnimationListener.onAnimationStart(controller.getAnimation());
            }
        }

        int clipSaveCount = 0;
        final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
        if (clipToPadding) {
            clipSaveCount = canvas.save();
            canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
                    mScrollX + mRight - mLeft - mPaddingRight,
                    mScrollY + mBottom - mTop - mPaddingBottom);
        }

        // We will draw our child's animation, let's reset the flag
        mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;
        mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;

        boolean more = false;
        final long drawingTime = getDrawingTime();//获得当前的绘画时间  

        if (usingRenderNodeProperties) canvas.insertReorderBarrier();
        final int transientCount = mTransientIndices == null ? 0 : mTransientIndices.size();
        int transientIndex = transientCount != 0 ? 0 : -1;
        // Only use the preordered list if not HW accelerated, since the HW pipeline will do the
        // draw reordering internally
        final ArrayList<View> preorderedList = usingRenderNodeProperties
                ? null : buildOrderedChildList();
        final boolean customOrder = preorderedList == null
                && isChildrenDrawingOrderEnabled();
        for (int i = 0; i < childrenCount; i++) {
            while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
                final View transientChild = mTransientViews.get(transientIndex);
                if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                        transientChild.getAnimation() != null) {
                    more |= drawChild(canvas, transientChild, drawingTime);//遍历Child，分发draw事件
                }
                transientIndex++;
                if (transientIndex >= transientCount) {
                    transientIndex = -1;
                }
            }
            int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;
            final View child = (preorderedList == null)
                    ? children[childIndex] : preorderedList.get(childIndex);
            if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
                more |= drawChild(canvas, child, drawingTime);
            }
        }
        while (transientIndex >= 0) {
            // there may be additional transient views after the normal views
            final View transientChild = mTransientViews.get(transientIndex);
            if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                    transientChild.getAnimation() != null) {
                more |= drawChild(canvas, transientChild, drawingTime);
            }
            transientIndex++;
            if (transientIndex >= transientCount) {
                break;
            }
        }
        if (preorderedList != null) preorderedList.clear();

        // Draw any disappearing views that have animations
        if (mDisappearingChildren != null) {
            final ArrayList<View> disappearingChildren = mDisappearingChildren;
            final int disappearingCount = disappearingChildren.size() - 1;
            // Go backwards -- we may delete as animations finish
            for (int i = disappearingCount; i >= 0; i--) {
                final View child = disappearingChildren.get(i);
                more |= drawChild(canvas, child, drawingTime);
            }
        }
        if (usingRenderNodeProperties) canvas.insertInorderBarrier();

        if (debugDraw()) {
            onDebugDraw(canvas);
        }

        if (clipToPadding) {
            canvas.restoreToCount(clipSaveCount);
        }

        // mGroupFlags might have been updated by drawChild()
        flags = mGroupFlags;
        //判断动画是否结束     
        if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {
            invalidate(true);
        }

        if ((flags & FLAG_ANIMATION_DONE) == 0 && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == 0 &&
                mLayoutAnimationController.isDone() && !more) {
            // We want to erase the drawing cache and notify the listener after the
            // next frame is drawn because one extra invalidate() is caused by
            // drawChild() after the animation is over
            mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;
            final Runnable end = new Runnable() {
               public void run() {
                   notifyAnimationListener();//通知动画结束监听者  
               }
            };
            post(end);
        }
    }
  
    private void notifyAnimationListener() {
        mGroupFlags &= ~FLAG_NOTIFY_ANIMATION_LISTENER;
        mGroupFlags |= FLAG_ANIMATION_DONE;

        if (mAnimationListener != null) {
           final Runnable end = new Runnable() {
               public void run() {
                   mAnimationListener.onAnimationEnd(mLayoutAnimationController.getAnimation());
               }
           };
           post(end);
        }

        invalidate(true);
    }
  
    /**
     * Draw one child of this View Group. This method is responsible for getting
     * the canvas in the right state. This includes clipping, translating so
     * that the child's scrolled origin is at 0, 0, and applying any animation
     * transformations.
     *
     * @param canvas The canvas on which to draw the child
     * @param child Who to draw
     * @param drawingTime The time at which draw is occurring
     * @return True if an invalidate() was issued
     */
    protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        return child.draw(canvas, this, drawingTime);
    }
}

接下来看 View 的 draw(Canvas canvas, ViewGroup parent, long drawingTime) :

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public class View implements Drawable.Callback, KeyEvent.Callback, AccessibilityEventSource {
    /**
     * This method is called by ViewGroup.drawChild() to have each child view draw itself.
     *
     * This is where the View specializes rendering behavior based on layer type,
     * and hardware acceleration.
     */
    boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
        final boolean hardwareAcceleratedCanvas = canvas.isHardwareAccelerated();
        /* If an attached view draws to a HW canvas, it may use its RenderNode + DisplayList.
         *
         * If a view is dettached, its DisplayList shouldn't exist. If the canvas isn't
         * HW accelerated, it can't handle drawing RenderNodes.
         */
        boolean drawingWithRenderNode = mAttachInfo != null
                && mAttachInfo.mHardwareAccelerated
                && hardwareAcceleratedCanvas;

        boolean more = false;
        final boolean childHasIdentityMatrix = hasIdentityMatrix();
        final int parentFlags = parent.mGroupFlags;

        if ((parentFlags & ViewGroup.FLAG_CLEAR_TRANSFORMATION) != 0) {
            parent.getChildTransformation().clear();
            parent.mGroupFlags &= ~ViewGroup.FLAG_CLEAR_TRANSFORMATION;
        }

        Transformation transformToApply = null;
        boolean concatMatrix = false;
        final boolean scalingRequired = mAttachInfo != null && mAttachInfo.mScalingRequired;
        final Animation a = getAnimation();//得到动画
        if (a != null) {
            more = applyLegacyAnimation(parent, drawingTime, a, scalingRequired);
            concatMatrix = a.willChangeTransformationMatrix();
            if (concatMatrix) {
                mPrivateFlags3 |= PFLAG3_VIEW_IS_ANIMATING_TRANSFORM;
            }
            transformToApply = parent.getChildTransformation();
        } else {
            if ((mPrivateFlags3 & PFLAG3_VIEW_IS_ANIMATING_TRANSFORM) != 0) {
                // No longer animating: clear out old animation matrix
                mRenderNode.setAnimationMatrix(null);
                mPrivateFlags3 &= ~PFLAG3_VIEW_IS_ANIMATING_TRANSFORM;
            }
            if (!drawingWithRenderNode
                    && (parentFlags & ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS) != 0) {
                final Transformation t = parent.getChildTransformation();
                final boolean hasTransform = parent.getChildStaticTransformation(this, t);
                if (hasTransform) {
                    final int transformType = t.getTransformationType();
                    transformToApply = transformType != Transformation.TYPE_IDENTITY ? t : null;
                    concatMatrix = (transformType & Transformation.TYPE_MATRIX) != 0;
                }
            }
        }

        concatMatrix |= !childHasIdentityMatrix;

        // Sets the flag as early as possible to allow draw() implementations
        // to call invalidate() successfully when doing animations
        mPrivateFlags |= PFLAG_DRAWN;

        if (!concatMatrix &&
                (parentFlags & (ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS |
                        ViewGroup.FLAG_CLIP_CHILDREN)) == ViewGroup.FLAG_CLIP_CHILDREN &&
                canvas.quickReject(mLeft, mTop, mRight, mBottom, Canvas.EdgeType.BW) &&
                (mPrivateFlags & PFLAG_DRAW_ANIMATION) == 0) {
            mPrivateFlags2 |= PFLAG2_VIEW_QUICK_REJECTED;
            return more;
        }
        mPrivateFlags2 &= ~PFLAG2_VIEW_QUICK_REJECTED;

        if (hardwareAcceleratedCanvas) {
            // Clear INVALIDATED flag to allow invalidation to occur during rendering, but
            // retain the flag's value temporarily in the mRecreateDisplayList flag
            mRecreateDisplayList = (mPrivateFlags & PFLAG_INVALIDATED) != 0;
            mPrivateFlags &= ~PFLAG_INVALIDATED;
        }

        RenderNode renderNode = null;
        Bitmap cache = null;
        int layerType = getLayerType(); // TODO: signify cache state with just 'cache' local
        if (layerType == LAYER_TYPE_SOFTWARE
                || (!drawingWithRenderNode && layerType != LAYER_TYPE_NONE)) {
            // If not drawing with RenderNode, treat HW layers as SW
            layerType = LAYER_TYPE_SOFTWARE;
            buildDrawingCache(true);
            cache = getDrawingCache(true);
        }

        if (drawingWithRenderNode) {
            // Delay getting the display list until animation-driven alpha values are
            // set up and possibly passed on to the view
            renderNode = updateDisplayListIfDirty();
            if (!renderNode.isValid()) {
                // Uncommon, but possible. If a view is removed from the hierarchy during the call
                // to getDisplayList(), the display list will be marked invalid and we should not
                // try to use it again.
                renderNode = null;
                drawingWithRenderNode = false;
            }
        }

        int sx = 0;
        int sy = 0;
        if (!drawingWithRenderNode) {
            computeScroll();
            sx = mScrollX;
            sy = mScrollY;
        }

        final boolean drawingWithDrawingCache = cache != null && !drawingWithRenderNode;
        final boolean offsetForScroll = cache == null && !drawingWithRenderNode;

        int restoreTo = -1;
        if (!drawingWithRenderNode || transformToApply != null) {
            restoreTo = canvas.save();
        }
        if (offsetForScroll) {
            canvas.translate(mLeft - sx, mTop - sy);
        } else {
            if (!drawingWithRenderNode) {
                canvas.translate(mLeft, mTop);
            }
            if (scalingRequired) {
                if (drawingWithRenderNode) {
                    // TODO: Might not need this if we put everything inside the DL
                    restoreTo = canvas.save();
                }
                // mAttachInfo cannot be null, otherwise scalingRequired == false
                final float scale = 1.0f / mAttachInfo.mApplicationScale;
                canvas.scale(scale, scale);
            }
        }

        float alpha = drawingWithRenderNode ? 1 : (getAlpha() * getTransitionAlpha());
        if (transformToApply != null
                || alpha < 1
                || !hasIdentityMatrix()
                || (mPrivateFlags3 & PFLAG3_VIEW_IS_ANIMATING_ALPHA) != 0) {
            if (transformToApply != null || !childHasIdentityMatrix) {
                int transX = 0;
                int transY = 0;

                if (offsetForScroll) {
                    transX = -sx;
                    transY = -sy;
                }
                //Animation真正实现效果的地方
                if (transformToApply != null) {
                    if (concatMatrix) {
                        if (drawingWithRenderNode) {
                            renderNode.setAnimationMatrix(transformToApply.getMatrix());
                        } else {
                            // Undo the scroll translation, apply the transformation matrix,
                            // then redo the scroll translate to get the correct result.
                            canvas.translate(-transX, -transY);
                            canvas.concat(transformToApply.getMatrix());//矩阵变换
                            canvas.translate(transX, transY);
                        }
                        parent.mGroupFlags |= ViewGroup.FLAG_CLEAR_TRANSFORMATION;
                    }

                    float transformAlpha = transformToApply.getAlpha();
                    if (transformAlpha < 1) {
                        alpha *= transformAlpha;
                        parent.mGroupFlags |= ViewGroup.FLAG_CLEAR_TRANSFORMATION;
                    }
                }

                if (!childHasIdentityMatrix && !drawingWithRenderNode) {
                    canvas.translate(-transX, -transY);
                    canvas.concat(getMatrix());
                    canvas.translate(transX, transY);
                }
            }

            // Deal with alpha if it is or used to be <1
            if (alpha < 1 || (mPrivateFlags3 & PFLAG3_VIEW_IS_ANIMATING_ALPHA) != 0) {
                if (alpha < 1) {
                    mPrivateFlags3 |= PFLAG3_VIEW_IS_ANIMATING_ALPHA;
                } else {
                    mPrivateFlags3 &= ~PFLAG3_VIEW_IS_ANIMATING_ALPHA;
                }
                parent.mGroupFlags |= ViewGroup.FLAG_CLEAR_TRANSFORMATION;
                if (!drawingWithDrawingCache) {
                    final int multipliedAlpha = (int) (255 * alpha);
                    if (!onSetAlpha(multipliedAlpha)) {
                        if (drawingWithRenderNode) {
                            renderNode.setAlpha(alpha * getAlpha() * getTransitionAlpha());
                        } else if (layerType == LAYER_TYPE_NONE) {
                            canvas.saveLayerAlpha(sx, sy, sx + getWidth(), sy + getHeight(),
                                    multipliedAlpha);
                        }
                    } else {
                        // Alpha is handled by the child directly, clobber the layer's alpha
                        mPrivateFlags |= PFLAG_ALPHA_SET;
                    }
                }
            }
        } else if ((mPrivateFlags & PFLAG_ALPHA_SET) == PFLAG_ALPHA_SET) {
            onSetAlpha(255);
            mPrivateFlags &= ~PFLAG_ALPHA_SET;
        }

        if (!drawingWithRenderNode) {
            // apply clips directly, since RenderNode won't do it for this draw
            if ((parentFlags & ViewGroup.FLAG_CLIP_CHILDREN) != 0 && cache == null) {
                if (offsetForScroll) {
                    canvas.clipRect(sx, sy, sx + getWidth(), sy + getHeight());
                } else {
                    if (!scalingRequired || cache == null) {
                        canvas.clipRect(0, 0, getWidth(), getHeight());
                    } else {
                        canvas.clipRect(0, 0, cache.getWidth(), cache.getHeight());
                    }
                }
            }

            if (mClipBounds != null) {
                // clip bounds ignore scroll
                canvas.clipRect(mClipBounds);
            }
        }

        if (!drawingWithDrawingCache) {
            if (drawingWithRenderNode) {
                mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                ((DisplayListCanvas) canvas).drawRenderNode(renderNode);
            } else {
                // Fast path for layouts with no backgrounds
                if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
                    mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                    dispatchDraw(canvas);
                } else {
                    draw(canvas);
                }
            }
        } else if (cache != null) {
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
            if (layerType == LAYER_TYPE_NONE) {
                // no layer paint, use temporary paint to draw bitmap
                Paint cachePaint = parent.mCachePaint;
                if (cachePaint == null) {
                    cachePaint = new Paint();
                    cachePaint.setDither(false);
                    parent.mCachePaint = cachePaint;
                }
                cachePaint.setAlpha((int) (alpha * 255));
                canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint);
            } else {
                // use layer paint to draw the bitmap, merging the two alphas, but also restore
                int layerPaintAlpha = mLayerPaint.getAlpha();
                mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
                canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
                mLayerPaint.setAlpha(layerPaintAlpha);
            }
        }

        if (restoreTo >= 0) {
            canvas.restoreToCount(restoreTo);
        }

        if (a != null && !more) {
            if (!hardwareAcceleratedCanvas && !a.getFillAfter()) {
                onSetAlpha(255);
            }
            parent.finishAnimatingView(this, a);
        }

        if (more && hardwareAcceleratedCanvas) {
            if (a.hasAlpha() && (mPrivateFlags & PFLAG_ALPHA_SET) == PFLAG_ALPHA_SET) {
                // alpha animations should cause the child to recreate its display list
                invalidate(true);
            }
        }

        mRecreateDisplayList = false;

        return more;
    }
  
    /**
     * Utility function, called by draw(canvas, parent, drawingTime) to handle the less common
     * case of an active Animation being run on the view.
     */
    private boolean applyLegacyAnimation(ViewGroup parent, long drawingTime,
            Animation a, boolean scalingRequired) {
        Transformation invalidationTransform;
        final int flags = parent.mGroupFlags;
        final boolean initialized = a.isInitialized();
        if (!initialized) {
            a.initialize(mRight - mLeft, mBottom - mTop, parent.getWidth(), parent.getHeight());
            a.initializeInvalidateRegion(0, 0, mRight - mLeft, mBottom - mTop);
            if (mAttachInfo != null) a.setListenerHandler(mAttachInfo.mHandler);
            onAnimationStart();
        }

        final Transformation t = parent.getChildTransformation();
        boolean more = a.getTransformation(drawingTime, t, 1f);
        if (scalingRequired && mAttachInfo.mApplicationScale != 1f) {
            if (parent.mInvalidationTransformation == null) {
                parent.mInvalidationTransformation = new Transformation();
            }
            invalidationTransform = parent.mInvalidationTransformation;
            a.getTransformation(drawingTime, invalidationTransform, 1f);//回调Animation的applyTransformation方法
        } else {
            invalidationTransform = t;
        }

        if (more) {
            if (!a.willChangeBounds()) {
                if ((flags & (ViewGroup.FLAG_OPTIMIZE_INVALIDATE | ViewGroup.FLAG_ANIMATION_DONE)) ==
                        ViewGroup.FLAG_OPTIMIZE_INVALIDATE) {
                    parent.mGroupFlags |= ViewGroup.FLAG_INVALIDATE_REQUIRED;
                } else if ((flags & ViewGroup.FLAG_INVALIDATE_REQUIRED) == 0) {
                    // The child need to draw an animation, potentially offscreen, so
                    // make sure we do not cancel invalidate requests
                    parent.mPrivateFlags |= PFLAG_DRAW_ANIMATION;
                    parent.invalidate(mLeft, mTop, mRight, mBottom);
                }
            } else {
                if (parent.mInvalidateRegion == null) {
                    parent.mInvalidateRegion = new RectF();
                }
                final RectF region = parent.mInvalidateRegion;
                a.getInvalidateRegion(0, 0, mRight - mLeft, mBottom - mTop, region,
                        invalidationTransform);

                // The child need to draw an animation, potentially offscreen, so
                // make sure we do not cancel invalidate requests
                parent.mPrivateFlags |= PFLAG_DRAW_ANIMATION;

                final int left = mLeft + (int) region.left;
                final int top = mTop + (int) region.top;
                parent.invalidate(left, top, left + (int) (region.width() + .5f),
                        top + (int) (region.height() + .5f));
            }
        }
        return more;
    }
}

流程总结

  1. View#startAnimation 调用 View#invalidate()
  2. View#invalidate() 调用 ViewGroup#invalidateChild()
  3. ViewGroup#invalidateChild() 调用 ViewRootImpl#invalidateChildInParent()
  4. ViewRootImpl#invalidateChildInParent() 调用 ViewRootImpl#scheduleTraversals()
  5. ViewRootImpl#scheduleTraversals() 调用 Choreographer#postCallback(Runnable)
  6. Choreographer 等待 SYNC 信号，调用 Runnable 的 run 方法，执行 ViewRootImpl#performTraversals()
  7. 经过一系列传递，来到 View#draw(cavas, viewGroup, drawingTime)
  8. View#draw(cavas, viewGroup, drawingTime) 调用 View#applyLegacyAniamtion()
  9. View#applyLegacyAniamtion() 调用 Animation#getTransFormation()，回调 Animation#applyTransFormation()
  10. View#draw(cavas, viewGroup, drawingTime) 通过 Matrix 实现了动画
  11. View#applyLegacyAniamtion() 返回 true, 继续执行 View#invalidate()，回到了第二步

Animator

从 ObjectAnimator 开始分析：

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public final class ObjectAnimator extends ValueAnimator {
    public static ObjectAnimator ofInt(Object target, String propertyName, int... values) {
        ObjectAnimator anim = new ObjectAnimator(target, propertyName);
        anim.setIntValues(values);
        return anim;
    }
  
    private ObjectAnimator(Object target, String propertyName) {
        setTarget(target);
        setPropertyName(propertyName);
    }
  
    private WeakReference<Object> mTarget;

    @Override
    public void setTarget(@Nullable Object target) {
        final Object oldTarget = getTarget();
        if (oldTarget != target) {
            if (isStarted()) {
                cancel();
            }
            mTarget = target == null ? null : new WeakReference<Object>(target);
            // New target should cause re-initialization prior to starting
            mInitialized = false;
        }
    }
  
    PropertyValuesHolder[] mValues;
    HashMap<String, PropertyValuesHolder> mValuesMap;
    private String mPropertyName;
    boolean mInitialized = false;
  
    public void setPropertyName(@NonNull String propertyName) {
        // mValues could be null if this is being constructed piecemeal. Just record the
        // propertyName to be used later when setValues() is called if so.
        if (mValues != null) {
            PropertyValuesHolder valuesHolder = mValues[0];
            String oldName = valuesHolder.getPropertyName();
            valuesHolder.setPropertyName(propertyName);
            mValuesMap.remove(oldName);
            mValuesMap.put(propertyName, valuesHolder);
        }
        mPropertyName = propertyName;
        // New property/values/target should cause re-initialization prior to starting
        mInitialized = false;
    }
  
    @Override
    public void setIntValues(int... values) {
        if (mValues == null || mValues.length == 0) {
            // No values yet - this animator is being constructed piecemeal. Init the values with
            // whatever the current propertyName is
            if (mProperty != null) {
                setValues(PropertyValuesHolder.ofInt(mProperty, values));
            } else {
                setValues(PropertyValuesHolder.ofInt(mPropertyName, values));
            }
        } else {
            super.setIntValues(values);
        }
    }
}

将 target 保存，是要操作的对象，通过 PropertyValuesHolder 和 mPropertyName 存储 propertyName ，是要操作的属性，调用 start() 方法开始动画：

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public final class ObjectAnimator extends ValueAnimator {
    protected static ThreadLocal<AnimationHandler> sAnimationHandler =
            new ThreadLocal<AnimationHandler>();
    @Override
    public void start() {
        // See if any of the current active/pending animators need to be canceled
        AnimationHandler handler = sAnimationHandler.get();
        if (handler != null) {
            int numAnims = handler.mAnimations.size();
            for (int i = numAnims - 1; i >= 0; i--) {
                if (handler.mAnimations.get(i) instanceof ObjectAnimator) {
                    ObjectAnimator anim = (ObjectAnimator) handler.mAnimations.get(i);
                    if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
                        anim.cancel();
                    }
                }
            }
            numAnims = handler.mPendingAnimations.size();
            for (int i = numAnims - 1; i >= 0; i--) {
                if (handler.mPendingAnimations.get(i) instanceof ObjectAnimator) {
                    ObjectAnimator anim = (ObjectAnimator) handler.mPendingAnimations.get(i);
                    if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
                        anim.cancel();
                    }
                }
            }
            numAnims = handler.mDelayedAnims.size();
            for (int i = numAnims - 1; i >= 0; i--) {
                if (handler.mDelayedAnims.get(i) instanceof ObjectAnimator) {
                    ObjectAnimator anim = (ObjectAnimator) handler.mDelayedAnims.get(i);
                    if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
                        anim.cancel();
                    }
                }
            }
        }
        if (DBG) {
            Log.d(LOG_TAG, "Anim target, duration: " + getTarget() + ", " + getDuration());
            for (int i = 0; i < mValues.length; ++i) {
                PropertyValuesHolder pvh = mValues[i];
                Log.d(LOG_TAG, "   Values[" + i + "]: " +
                    pvh.getPropertyName() + ", " + pvh.mKeyframes.getValue(0) + ", " +
                    pvh.mKeyframes.getValue(1));
            }
        }
        super.start();
    }
}

public class ValueAnimator extends Animator {
  
    @Override
    public void start() {
        start(false);
    }
  
    private void start(boolean playBackwards) {
        if (Looper.myLooper() == null) {
            throw new AndroidRuntimeException("Animators may only be run on Looper threads");
        }
        mReversing = playBackwards;
        mPlayingBackwards = playBackwards;
        if (playBackwards && mSeekFraction != -1) {
            if (mSeekFraction == 0 && mCurrentIteration == 0) {
                // special case: reversing from seek-to-0 should act as if not seeked at all
                mSeekFraction = 0;
            } else if (mRepeatCount == INFINITE) {
                mSeekFraction = 1 - (mSeekFraction % 1);
            } else {
                mSeekFraction = 1 + mRepeatCount - (mCurrentIteration + mSeekFraction);
            }
            mCurrentIteration = (int) mSeekFraction;
            mSeekFraction = mSeekFraction % 1;
        }
        if (mCurrentIteration > 0 && mRepeatMode == REVERSE &&
                (mCurrentIteration < (mRepeatCount + 1) || mRepeatCount == INFINITE)) {
            // if we were seeked to some other iteration in a reversing animator,
            // figure out the correct direction to start playing based on the iteration
            if (playBackwards) {
                mPlayingBackwards = (mCurrentIteration % 2) == 0;
            } else {
                mPlayingBackwards = (mCurrentIteration % 2) != 0;
            }
        }
        int prevPlayingState = mPlayingState;
        mPlayingState = STOPPED;
        mStarted = true;
        mStartedDelay = false;
        mPaused = false;
        updateScaledDuration(); // in case the scale factor has changed since creation time
        AnimationHandler animationHandler = getOrCreateAnimationHandler();
        animationHandler.mPendingAnimations.add(this);
        if (mStartDelay == 0) {
            // This sets the initial value of the animation, prior to actually starting it running
            if (prevPlayingState != SEEKED) {
                setCurrentPlayTime(0);
            }
            mPlayingState = STOPPED;
            mRunning = true;
            notifyStartListeners();//动画开始的回调
        }
        animationHandler.start();//最终调用的是AnimationHandler的start()方法
    }
    protected static class AnimationHandler {
        private AnimationHandler() {
            mChoreographer = Choreographer.getInstance();//还是Choreographer
        }
      
        public void start() {
            scheduleAnimation();
        }
      
        private void scheduleAnimation() {
            if (!mAnimationScheduled) {
                mChoreographer.postCallback(Choreographer.CALLBACK_ANIMATION, mAnimate, null);
                mAnimationScheduled = true;
            }
        }
      
        // Called by the Choreographer.
        final Runnable mAnimate = new Runnable() {
            @Override
            public void run() {
                mAnimationScheduled = false;
                doAnimationFrame(mChoreographer.getFrameTime());
            }
        };
      
        void doAnimationFrame(long frameTime) {
            mLastFrameTime = frameTime;

            // mPendingAnimations holds any animations that have requested to be started
            // We're going to clear mPendingAnimations, but starting animation may
            // cause more to be added to the pending list (for example, if one animation
            // starting triggers another starting). So we loop until mPendingAnimations
            // is empty.
            while (mPendingAnimations.size() > 0) {
                ArrayList<ValueAnimator> pendingCopy =
                        (ArrayList<ValueAnimator>) mPendingAnimations.clone();
                mPendingAnimations.clear();
                int count = pendingCopy.size();
                for (int i = 0; i < count; ++i) {
                    ValueAnimator anim = pendingCopy.get(i);
                    // If the animation has a startDelay, place it on the delayed list
                    if (anim.mStartDelay == 0) {
                        anim.startAnimation(this);
                    } else {
                        mDelayedAnims.add(anim);
                    }
                }
            }

            // Next, process animations currently sitting on the delayed queue, adding
            // them to the active animations if they are ready
            int numDelayedAnims = mDelayedAnims.size();
            for (int i = 0; i < numDelayedAnims; ++i) {
                ValueAnimator anim = mDelayedAnims.get(i);
                if (anim.delayedAnimationFrame(frameTime)) {
                    mReadyAnims.add(anim);
                }
            }
            int numReadyAnims = mReadyAnims.size();
            if (numReadyAnims > 0) {
                for (int i = 0; i < numReadyAnims; ++i) {
                    ValueAnimator anim = mReadyAnims.get(i);
                    anim.startAnimation(this);
                    anim.mRunning = true;
                    mDelayedAnims.remove(anim);
                }
                mReadyAnims.clear();
            }

            // Now process all active animations. The return value from animationFrame()
            // tells the handler whether it should now be ended
            int numAnims = mAnimations.size();
            for (int i = 0; i < numAnims; ++i) {
                mTmpAnimations.add(mAnimations.get(i));
            }
            for (int i = 0; i < numAnims; ++i) {
                ValueAnimator anim = mTmpAnimations.get(i);
                if (mAnimations.contains(anim) && anim.doAnimationFrame(frameTime)) {
                    mEndingAnims.add(anim);
                }
            }
            mTmpAnimations.clear();
            if (mEndingAnims.size() > 0) {
                for (int i = 0; i < mEndingAnims.size(); ++i) {
                    mEndingAnims.get(i).endAnimation(this);
                }
                mEndingAnims.clear();
            }

            // Schedule final commit for the frame.
            mChoreographer.postCallback(Choreographer.CALLBACK_COMMIT, mCommit, null);

            // If there are still active or delayed animations, schedule a future call to
            // onAnimate to process the next frame of the animations.
            if (!mAnimations.isEmpty() || !mDelayedAnims.isEmpty()) {//动画没有完的话
                scheduleAnimation();
            }
        }
    }
}

通过调用 start() 方法，最终执行的是 AnimationHandler#start() 方法，该方法也是通过 Choreographer.postCallback 去执行，回调 mAnimate 这个 Runnable，执行 doAnimationFrame() 方法，调用ValueAnimator#doAnimationFrame(),如果动画没有结束的话，继续调用scheduleAnimation()` 。

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public class ValueAnimator extends Animator {
    /**
     * Processes a frame of the animation, adjusting the start time if needed.
     *
     * @param frameTime The frame time.
     * @return true if the animation has ended.
     */
    final boolean doAnimationFrame(long frameTime) {
        if (mPlayingState == STOPPED) {
            mPlayingState = RUNNING;
            if (mSeekFraction < 0) {
                mStartTime = frameTime;
            } else {
                long seekTime = (long) (mDuration * mSeekFraction);
                mStartTime = frameTime - seekTime;
                mSeekFraction = -1;
            }
            mStartTimeCommitted = false; // allow start time to be compensated for jank
        }
        if (mPaused) {
            if (mPauseTime < 0) {
                mPauseTime = frameTime;
            }
            return false;
        } else if (mResumed) {
            mResumed = false;
            if (mPauseTime > 0) {
                // Offset by the duration that the animation was paused
                mStartTime += (frameTime - mPauseTime);
                mStartTimeCommitted = false; // allow start time to be compensated for jank
            }
        }
        // The frame time might be before the start time during the first frame of
        // an animation.  The "current time" must always be on or after the start
        // time to avoid animating frames at negative time intervals.  In practice, this
        // is very rare and only happens when seeking backwards.
        final long currentTime = Math.max(frameTime, mStartTime);
        return animationFrame(currentTime);
    }
  
    boolean animationFrame(long currentTime) {
        boolean done = false;
        switch (mPlayingState) {
        case RUNNING:
        case SEEKED:
            float fraction = mDuration > 0 ? (float)(currentTime - mStartTime) / mDuration : 1f;
            if (mDuration == 0 && mRepeatCount != INFINITE) {
                // Skip to the end
                mCurrentIteration = mRepeatCount;
                if (!mReversing) {
                    mPlayingBackwards = false;
                }
            }
            if (fraction >= 1f) {
                if (mCurrentIteration < mRepeatCount || mRepeatCount == INFINITE) {
                    // Time to repeat
                    if (mListeners != null) {
                        int numListeners = mListeners.size();
                        for (int i = 0; i < numListeners; ++i) {
                            mListeners.get(i).onAnimationRepeat(this);
                        }
                    }
                    if (mRepeatMode == REVERSE) {
                        mPlayingBackwards = !mPlayingBackwards;
                    }
                    mCurrentIteration += (int) fraction;
                    fraction = fraction % 1f;
                    mStartTime += mDuration;
                    // Note: We do not need to update the value of mStartTimeCommitted here
                    // since we just added a duration offset.
                } else {
                    done = true;
                    fraction = Math.min(fraction, 1.0f);
                }
            }
            if (mPlayingBackwards) {
                fraction = 1f - fraction;
            }
            animateValue(fraction);
            break;
        }

        return done;
    }
  
    void animateValue(float fraction) {
        fraction = mInterpolator.getInterpolation(fraction);//通过插值器拿到数
        mCurrentFraction = fraction;
        int numValues = mValues.length;
        for (int i = 0; i < numValues; ++i) {
            mValues[i].calculateValue(fraction);
        }
        if (mUpdateListeners != null) {
            int numListeners = mUpdateListeners.size();
            for (int i = 0; i < numListeners; ++i) {
                mUpdateListeners.get(i).onAnimationUpdate(this);//回调动画的listener
            }
        }
    }
}

public final class ObjectAnimator extends ValueAnimator {
    @Override
    void animateValue(float fraction) {
        final Object target = getTarget();
        if (mTarget != null && target == null) {
            // We lost the target reference, cancel and clean up.
            cancel();
            return;
        }

        super.animateValue(fraction);
        int numValues = mValues.length;
        for (int i = 0; i < numValues; ++i) {
            mValues[i].setAnimatedValue(target);
        }
    }
}

这里主要就是 setAnimatedValue(target) 来执行动画效果：

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public class PropertyValuesHolder implements Cloneable {
    static class IntPropertyValuesHolder extends PropertyValuesHolder {
        @Override
        void setAnimatedValue(Object target) {
            if (mIntProperty != null) {
                mIntProperty.setValue(target, mIntAnimatedValue);
                return;
            }
            if (mProperty != null) {
                mProperty.set(target, mIntAnimatedValue);
                return;
            }
            if (mJniSetter != 0) {
                nCallIntMethod(target, mJniSetter, mIntAnimatedValue);
                return;
            }
            if (mSetter != null) {
                try {
                    mTmpValueArray[0] = mIntAnimatedValue;
                    mSetter.invoke(target, mTmpValueArray);//反射改变属性
                } catch (InvocationTargetException e) {
                    Log.e("PropertyValuesHolder", e.toString());
                } catch (IllegalAccessException e) {
                    Log.e("PropertyValuesHolder", e.toString());
                }
            }
        }
    }
}

流程总结

  1. ObjectAnimator#ofInt() 设置 target，propName 和 value(PropertyValuesHolder)
  2. 调用 start() 方法，最终执行的是 AnimationHandler#start()
  3. AnimationHandler#start() 会调用 AnimationHandler#scheduleAnimation()
  4. AnimationHandler#scheduleAnimation() 调用 Choreographer ，执行 Runnable 的 run 方法，调用 AnimationHandler#doAnimationFrame()
  5. AnimationHandler#doAnimationFrame() 调用 ObjectAnimator#doAnimationFrame()
  6. 如果动画没有结束，AnimationHandler#doAnimationFrame() 继续调用 AnimationHandler#scheduleAnimation() ，回到第四步
  7. ObjectAnimator#doAnimationFrame() 调用 ObjectAnimator#animationFrame()
  8. ObjectAnimator#animationFrame() 调用 PropertyValuesHolder#setAnimatedValue()
  9. PropertyValuesHolder#setAnimatedValue() 通过反射改变对应 propName 的值