How ScrollView works ?

ScrollView 继承自 FrameLayout，是什么让 FrameLayout 变成了可以滚动，可 fling 的呢？
是因为 onInterceptTouchEvent 和 onTouchEvent 的重写。

ScrollView 只能包含一个子视图，常常是一个线性布局。ScrollView 所关心的是，子视图是否消费了 Down 事件。不关心的是，子视图内部又经历了多少次事件分发。

---

当子视图不消费 DOWN 事件

onInterceptTouchEvent 函数会被调用。
（先无视掉无关紧要的代码：诸如简单判断，确保滚动结束，确保父视图不拦截事件，边缘效果，多点触控，内部滚动，越界滚动，回收工作等等。）

case MotionEvent.ACTION_DOWN: {
final int y = (int) ev.getY();
mLastMotionY = y;
mActivePointerId = ev.getPointerId(0);
}
// DOWN 事件产生时，一般 mIsBeingDragged 已经被重置为 false 了
return mIsBeingDragged;

DOWN 事件不会被拦截，在子视图中寻找能够接收到事件的是视图，让它处理。
根据假设，子视图不消费 DOWN 事件，最后由 ScrollView 的 onTouchEvent 处理。

case MotionEvent.ACTION_DOWN: {
mLastMotionY = (int) ev.getY();
mActivePointerId = ev.getPointerId(0);
break;
}
return true;

ScrollView 会消费掉 Down 事件（否则将收不到后续事件），并记录位置。

MOVE 事件

由于子视图没有消费 DOWN 事件，即 touchTarget 为 null，后续事件将被拦截并由 ScrollView 的 onTouchEvent 处理。

case MotionEvent.ACTION_MOVE: {
final int y = (int) ev.getY(activePointerIndex);
// 相对坐标，绝对距离
int deltaY = mLastMotionY - y;
// 如果移动距离大于最小距离，则标识为正在拖拽，并消耗掉这段距离
if (!mIsBeingDragged && Math.abs(deltaY) > mTouchSlop) {  
mIsBeingDragged = true;
if (deltaY > 0) {
deltaY -= mTouchSlop;
} else {
deltaY += mTouchSlop;
}}
// 如果正在拖拽
if (mIsBeingDragged) {
final int range = getScrollRange();
if (overScrollBy(0, deltaY, 0, mScrollY, 0, range, 0, mOverscrollDistance, true)) {}
break;
// ！！！ ---- MOVE 和 UP 的返回值有何意义？ ---- ！！！
return true;

protected boolean overScrollBy(int deltaX, int deltaY, int scrollX, int scrollY, int scrollRangeX, int scrollRangeY, int maxOverScrollX, int maxOverScrollY, boolean isTouchEvent) {     
int newScrollY = scrollY + deltaY;
onOverScrolled(newScrollX, newScrollY, clampedX, clampedY);
}

protected void onOverScrolled(int scrollX, int scrollY, boolean clampedX, boolean clampedY) {
super.scrollTo(scrollX, scrollY);
}

如果移动距离小于 TouchSlop，则不做任何响应。
如果移动距离大于 TouchSlop，则确认正在拖拽，并消耗这段距离。
如果正在拖拽，则更新 mScrollY 并重画。

UP 事件

case MotionEvent.ACTION_UP:
if (mIsBeingDragged) {
// 计算滑动速度，并 fling
// 下拉向上滚，所以速度是取负值
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int initialVelocity = (int) velocityTracker.getYVelocity(mActivePointerId);
if ((Math.abs(initialVelocity) > mMinimumVelocity)) {
flingWithNestedDispatch(-initialVelocity); }}
// 重置
mActivePointerId = INVALID_POINTER;
endDrag();
}

private void flingWithNestedDispatch(int velocityY) {
if (canFling) {  fling(velocityY); }
}

public void fling(int velocityY) {
int height = getHeight() - mPaddingBottom - mPaddingTop;
int bottom = getChildAt(0).getHeight();
mScroller.fling(mScrollX, mScrollY, 0, velocityY, 0, 0, 0,Math.max(0, bottom - height), 0, height/2);
}

滑动日志

4-16 13:10:15.497 5038-5038/ivolianer.viewgroup E/result: ScrollView dispatch event 0
04-16 13:10:15.497 5038-5038/ivolianer.viewgroup E/result: LinearLayout dispatch event 0
04-16 13:10:15.497 5038-5038/ivolianer.viewgroup E/result: ScrollView onTouch dispatch event 0
04-16 13:10:15.532 5038-5038/ivolianer.viewgroup E/result: ScrollView dispatch event 2
04-16 13:10:15.532 5038-5038/ivolianer.viewgroup E/result: ScrollView onTouch dispatch event 2
04-16 13:10:15.549 5038-5038/ivolianer.viewgroup E/result: ScrollView dispatch event 2
04-16 13:10:15.549 5038-5038/ivolianer.viewgroup E/result: ScrollView onTouch dispatch event 2
04-16 13:10:15.560 5038-5038/ivolianer.viewgroup E/result: ScrollView dispatch event 2
04-16 13:10:15.561 5038-5038/ivolianer.viewgroup E/result: ScrollView onTouch dispatch event 2
04-16 13:10:15.561 5038-5038/ivolianer.viewgroup E/result: ScrollView dispatch event 1
04-16 13:10:15.561 5038-5038/ivolianer.viewgroup E/result: ScrollView onTouch dispatch event 1

---

当子视图消费 DOWN 事件（比上面的情况复杂）

interceptTouchEvent 被调用

case MotionEvent.ACTION_DOWN: {   
final int y = (int) ev.getY(); 
mLastMotionY = y;
mActivePointerId = ev.getPointerId(0);
}

不拦截 DOWN 事件，寻找能接收到事件的子视图，让它处理。
根据假设，子视图消费 DOWN 事件，ScrollView 本身的 onTouchEvent 不会被调用。

MOVE 事件

因为 target 不为 null，intereceptTouchEvent 被调用。

case MotionEvent.ACTION_MOVE: {
  final int y = (int) ev.getY(pointerIndex);
  final int yDiff = Math.abs(y - mLastMotionY);
  if (yDiff > mTouchSlop && (getNestedScrollAxes() & SCROLL_AXIS_VERTICAL) == 0){
    mIsBeingDragged = true;
    mLastMotionY = y;
  }
  break;
}
return mIsBeingDragged;

如果滑动距离始终小于 TouchSlop

事件不会被拦截。
所有 MOVE 事件都会 TouchTarget 处理。
最后的 UP 事件也会传递给 TouchTarget。

日志

04-16 13:12:03.104 5038-5038/ivolianer.viewgroup E/result: ScrollView dispatch event 0
04-16 13:12:03.104 5038-5038/ivolianer.viewgroup E/result: LinearLayout dispatch event 0
04-16 13:12:03.126 5038-5038/ivolianer.viewgroup E/result: ScrollView dispatch event 2
04-16 13:12:03.126 5038-5038/ivolianer.viewgroup E/result: LinearLayout dispatch event 2
04-16 13:12:03.176 5038-5038/ivolianer.viewgroup E/result: ScrollView dispatch event 2
04-16 13:12:03.176 5038-5038/ivolianer.viewgroup E/result: LinearLayout dispatch event 2
04-16 13:12:04.101 5038-5038/ivolianer.viewgroup E/result: ScrollView dispatch event 1
04-16 13:12:04.101 5038-5038/ivolianer.viewgroup E/result: LinearLayout dispatch event 1

如果滑动距离大于 TouchSlop

mIsBeingDragged = true，并拦截事件。

if ((action == MotionEvent.ACTION_MOVE) && (mIsBeingDragged)) {
  return true;
}

后续的 MOVE 事件也会被拦截。

拦截事件的实质

final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted;
if (dispatchTransformedTouchEvent(ev, cancelChild,  target.child,target.pointerIdBits)) {
handled = true;
}

把事件传递给 TouchTarget 时。
如果不拦截，正常分发事件。
如果拦截了，事件会被替换成 CANCEL 事件。

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,  View child, int desiredPointerIdBits) {
  final boolean handled; 
  final int oldAction = event.getAction();
  if (cancel || oldAction == MotionEvent.ACTION_CANCEL){
    event.setAction(MotionEvent.ACTION_CANCEL);
    if (child == null) {
      handled = super.dispatchTouchEvent(event);
    } else {
      handled = child.dispatchTouchEvent(event);
    }
  return handled;    
}

并且会重置 TouchTarget ，这会导致后续 UP 事件也无法收到。

if (canceled || actionMasked == MotionEvent.ACTION_UP || actionMasked ==MotionEvent.ACTION_HOVER_MOVE) {
  resetTouchState();
} 

日志

04-16 13:01:47.191 17678-17678/ivolianer.viewgroup E/result: ScrollView receive event 0
04-16 13:01:47.195 17678-17678/ivolianer.viewgroup E/result: LinearLayout receive event 0
04-16 13:01:47.231 17678-17678/ivolianer.viewgroup E/result: ScrollView receive event 2
04-16 13:01:47.231 17678-17678/ivolianer.viewgroup E/result: LinearLayout receive event 2
04-16 13:01:47.247 17678-17678/ivolianer.viewgroup E/result: ScrollView receive event 2
04-16 13:01:47.247 17678-17678/ivolianer.viewgroup E/result: LinearLayout receive event 3
04-16 13:01:47.264 17678-17678/ivolianer.viewgroup E/result: ScrollView receive event 2
04-16 13:01:47.264 17678-17678/ivolianer.viewgroup E/result: ScrollView onTouch receive event 2
04-16 13:01:47.328 17678-17678/ivolianer.viewgroup E/result: ScrollView receive event 2
04-16 13:01:47.328 17678-17678/ivolianer.viewgroup E/result: ScrollView onTouch receive event 2
04-16 13:01:47.328 17678-17678/ivolianer.viewgroup E/result: ScrollView receive event 1
04-16 13:01:47.328 17678-17678/ivolianer.viewgroup E/result: ScrollView onTouch receive event 1

---

BB了这多，总结下：

ScrollView 给了你传递 DOWN 和 UP 事件的机会，为了让子视图有机会实现点击或长按。但当你企图传递 MOVE 事件时（滑动距离大于 TouchSlop），他会无情拦截下来，并重置 TouchTarget。