Android判断Touch为滑动事件还是操作控件
因为在项目中要判断WebView是否处于滚动状态，但它不像ListView有onScrollStateChanged方法来监听，要实现就得手动监听它的Touch事件。
谈起Touch事件不得说提到 onInterceptTouchEvent，它是ViewGroup的一个扩展函数，要实现高级效果 必不可少的就需要重写 onInterceptTouchEvent、对所有的 MotionEvent 起到一个过滤的作用，最终的目的是防止一些事件传递到它包含的子控件中。
以ScrollView为例介绍滑动滚动条的判定。
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
/*
* This method JUST determines whether we want to intercept the motion.
* If we return true, onMotionEvent will be called and we do the actual
* scrolling there.
*/
/*
* Shortcut the most recurring case: the user is in the dragging
* state and he is moving his finger. We want to intercept this
* motion.
*/
final int action = ev.getAction();
if ((action == MotionEvent.ACTION_MOVE) && (mIsBeingDragged)) {
return true;
}
switch (action & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_MOVE: {
/*
* mIsBeingDragged == false, otherwise the shortcut would have caught it. Check
* whether the user has moved far enough from his original down touch.
*/
/*
* Locally do absolute value. mLastMotionY is set to the y value
* of the down event.
*/
final int activePointerId = mActivePointerId;
if (activePointerId == INVALID_POINTER) {
// If we don't have a valid id, the touch down wasn't on content.
break;
}
//通过activePointerId 的值来获得 当前 触摸点的索引
final int pointerIndex = ev.findPointerIndex(activePointerId);
//通过索引的值获得当前触摸的y的值
final float y = ev.getY(pointerIndex);
final int yDiff = (int) Math.abs(y - mLastMotionY);
//根据绝对值和零界点来判断是单击还是滑动滚动条
if (yDiff > mTouchSlop) {
mIsBeingDragged = true;
mLastMotionY = y;
initVelocityTrackerIfNotExists();
mVelocityTracker.addMovement(ev);
if (mScrollStrictSpan == null) {
mScrollStrictSpan = StrictMode.enterCriticalSpan("ScrollView-scroll");
}
}
break;
}
case MotionEvent.ACTION_DOWN: {
final float y = 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.
*/
//记录 mLastMotionY 和 mActivePointerId
//在 MotionEvent.ACTION_MOVE 状态时会用到
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.
*/
mIsBeingDragged = !mScroller.isFinished();
if (mIsBeingDragged && mScrollStrictSpan == null) {
mScrollStrictSpan = StrictMode.enterCriticalSpan("ScrollView-scroll");
}
break;
}
case MotionEvent.ACTION_CANCEL:
case MotionEvent.ACTION_UP:
/* Release the drag */
mIsBeingDragged = false;
mActivePointerId = INVALID_POINTER;
recycleVelocityTracker();
if (mScroller.springBack(mScrollX, mScrollY, 0, 0, 0, getScrollRange())) {
invalidate();
}
break;
case MotionEvent.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
}
/*
* The only time we want to intercept motion events is if we are in the
* drag mode.
*/
return mIsBeingDragged;
}