ScrollView源码分析

最新推荐文章于 2021-05-26 02:14:37 发布

总是很累的泡面头

最新推荐文章于 2021-05-26 02:14:37 发布

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分类专栏： android 文章标签： android ScrollView 源码

本文链接：https://blog.youkuaiyun.com/qq_29207593/article/details/88688302

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本文详细分析了Android ScrollView的源码，重点探讨了onInterceptTouchEvent()和onTouchEvent(MotionEvent ev)两个方法。在onInterceptTouchEvent()中，ScrollView只在拖动时拦截事件，而onTouchEvent()包含了滚动逻辑的关键实现，包括对down、move和up事件的处理，以及如何与OverScroller、EdgeEffect和VelocityTracker等组件协同工作。通过这个分析，理解ScrollView的滚动细节至关重要。

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onInterceptTouchEvent()

先看返回值

onInterceptTouchEvent(){
       /*
        * The only time we want to intercept motion events is if we are in the
        * drag mode.
        */
        return mIsBeingDragged;
}
/**
 * True if the user is currently dragging this ScrollView around. This is
  * not the same as 'is being flinged', which can be checked by
  * mScroller.isFinished() (flinging begins when the user lifts his finger).
 */
    private boolean mIsBeingDragged = false;

可以看到ScrollView只在被拖动的时候拦截掉，来自己处理事件。
那么什么时候被拖动呢，一定是在上面的手势识别中处理的，显而易见move的时候一定是mIsBeingDragged = true的，up的时候一定是false的，可以在源码得到验证。有趣的是Down的时候。

case MotionEvent.ACTION_DOWN: {
                final int y = (int) ev.getY();
                if (!inChild((int) ev.getX(), (int) y)) {
                    mIsBeingDragged = false;
                    recycleVelocityTracker();
                    break;
                }

                /*
                 * Remember location of down touch.
                 * ACTION_DOWN always refers to pointer index 0.
                 */
                mLastMotionY = y;
                mActivePointerId = ev.getPointerId(0);

                initOrResetVelocityTracker();
                mVelocityTracker.addMovement(ev);
                /*
                 * If being flinged and user touches the screen, initiate drag;
                 * otherwise don't. mScroller.isFinished should be false when
                 * being flinged. We need to call computeScrollOffset() first so that
                 * isFinished() is correct.
                */
                mScroller.computeScrollOffset();
                mIsBeingDragged = !mScroller.isFinished();
                if (mIsBeingDra

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