Android Animation.setAnimationListener()失效问题

Android执行动画，使用Animation情景如下：

Animation animation = new Animation();

如果需要监听动画执行

animation.setAnimationListener()

需要在

view.startAnimation(animation)

前设置listener，否则会小概率出现listener回调无法执行的情况，原因是因为view.startAnimation时会调用invalidate方法更新UI

   /**
     * Start the specified animation now.
     *
     * @param animation the animation to start now
     */
    public void startAnimation(Animation animation) {
        animation.setStartTime(Animation.START_ON_FIRST_FRAME);
        setAnimation(animation);
        invalidateParentCaches();
        invalidate(true);
    }

并在调用到draw()方法时进行animationListener的回调

 /**
     * 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();
        if (a != null) {
            //在这里对animation进行处理
            more = applyLegacyAnimation(parent, drawingTime, a, scalingRequired);
            concatMatrix = a.willChangeTransformationMatrix();
       ....//省略部分
        return more;
    }

跟进applyLegacyAnimation方法中

  /**
     * 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);
            //在这里调用aniamtion的setListenerHandler方法
            if (mAttachInfo != null) a.setListenerHandler(mAttachInfo.mHandler);
            onAnimationStart();
        }
}

最后则是Animation自身会检测当前listener是否为空进行回调了

    /**
     * Sets the handler used to invoke listeners.
     *
     * @hide
     */
    public void setListenerHandler(Handler handler) {
        if (mListenerHandler == null) {
            mOnStart = new Runnable() {
                public void run() {
                    if (mListener != null) {
                        mListener.onAnimationStart(Animation.this);
                    }
                }
            };
            mOnRepeat = new Runnable() {
                public void run() {
                    if (mListener != null) {
                        mListener.onAnimationRepeat(Animation.this);
                    }
                }
            };
            mOnEnd = new Runnable() {
                public void run() {
                    if (mListener != null) {
                        mListener.onAnimationEnd(Animation.this);
                    }
                }
            };
        }
        mListenerHandler = handler;
    }