对于NestedScrollView ，支持NestedScrollingChild 和 NestedScrollingParent方法，需要先理解这个内嵌的模式。
- 处理流程
- 先调用父ViewGroup dispatchNestedPreScroll () 处理需要消耗的距离
- 再自身调用overScrollByCompat()处理需要消耗的距离
- 再自身调用dispatchNestedScroll()处理需要消耗的距离
- dispatchNestedScroll()方法里面的参数int dxConsumed, int dyConsumed, int dxUnconsumed,
int dyUnconsumed 为什么不是int数组，参考mScrollOffset的使用
重点记录这两个方法，是NestedScrollView的精髓
- onInterceptTouchEvent
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();
//如果当前是移动事件，并且 mIsBeingDragged = true,表示正在拖拽，这个时候，直接拦截事件
//Android 里面有一些都是这么处理，比如disaptchTouchEvent中的mFisrtTarget也是有这方面的处理
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;
//这里是还没有获取到出点，只有在 MotionEvent.ACTION_DOWN事件中才能获取到activePointerId
if (activePointerId ==INVALID_POINTER) {
// If we don't have a valid id, the touch down wasn't on content.
break;
}
//需要注意这里的获取当前触点的序号，如果不支持多触点，activePointerId 为MotionEvent对象方法getPointerId里面的第0个节点
final int pointerIndex = ev.findPointerIndex(activePointerId);
if (pointerIndex == -1) {
Log.e(TAG, "Invalid pointerId=" + activePointerId
+" in onInterceptTouchEvent");
break;
}
final int y = (int) ev.getY(pointerIndex);
final int yDiff = Math.abs(y -mLastMotionY);
if (yDiff >mTouchSlop
//这里判断是没有与父ViewGroup相关联或者父View没有Y抽的滑动，参考NestedScrollingChildHelper类的startNestedScroll()方法
//getNestedScrollAxes()返回的是滚动方向
&& (getNestedScrollAxes() & ViewCompat.SCROLL_AXIS_VERTICAL) ==0) {
//这里会刷新mLastMotionY值，这里很重要，主要注意
mIsBeingDragged =true;
mLastMotionY = y;
initVelocityTrackerIfNotExists();
mVelocityTracker.addMovement(ev);
mNestedYOffset =0;
final ViewParent parent = getParent();
if (parent !=null) {
//父类不拦截
parent.requestDisallowInterceptTouchEvent(true);
}
}
break;
}
case MotionEvent.ACTION_DOWN: {
final int y = (int) ev.getY();
if (!inChild((int) ev.getX(), 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();
//开始Y抽内嵌滑动，
startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL, ViewCompat.TYPE_TOUCH);
break;
}
case MotionEvent.ACTION_CANCEL:
case MotionEvent.ACTION_UP:
/* Release the drag */
mIsBeingDragged =false;
mActivePointerId =INVALID_POINTER;
recycleVelocityTracker();
if (mScroller.springBack(getScrollX(), getScrollY(), 0, 0, 0, getScrollRange())) {
ViewCompat.postInvalidateOnAnimation(this);
}
//停止内嵌滑动设置
stopNestedScroll(ViewCompat.TYPE_TOUCH);
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;
}
-
onTouchEvent 这里就是NestedScroll核心处理
public boolean onTouchEvent(MotionEvent ev) { initVelocityTrackerIfNotExists(); MotionEvent vtev = MotionEvent.obtain(ev); final int actionMasked = ev.getActionMasked(); if (actionMasked == MotionEvent.ACTION_DOWN) { //如果是down事件，NestedYOffset重置为0，表示是新一轮的内嵌滑动处理 mNestedYOffset = 0; } //因为父ViewGroup能滑动，所以事件的位置也需要偏移滑动的距离 vtev.offsetLocation(0, mNestedYOffset); switch (actionMasked) { case MotionEvent.ACTION_DOWN: { //没有View，后面的逻辑就不走了 if (getChildCount() == 0) { return false; } if ((mIsBeingDragged = !mScroller.isFinished())) { final ViewParent parent = getParent(); if (parent != null) { parent.requestDisallowInterceptTouchEvent(true); } } /* * If being flinged and user touches, stop the fling. isFinished * will be false if being flinged. */ if (!mScroller.isFinished()) { mScroller.abortAnimation(); } // Remember where the motion event started //down事件的时候，会获取点击点的位置，还有一次可能是从onInterceptTouchEvent里面获取，可以看相关源码 mLastMotionY = (int) ev.getY(); mActivePointerId = ev.getPointerId(0); //开始内嵌滑动 startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL, ViewCompat.TYPE_TOUCH); break; } case MotionEvent.ACTION_MOVE: final int activePointerIndex = ev.findPointerIndex(mActivePointerId); if (activePointerIndex == -1) { Log.e(TAG, "Invalid pointerId=" + mActivePointerId + " in onTouchEvent"); break; } final int y = (int) ev.getY(activePointerIndex); int deltaY = mLastMotionY - y; //先父ViewGroup处理dispatchNestedPreScroll //注意，这里需要思考mScrollConsumed 是一个int[]数据 if (dispatchNestedPreScroll(0, deltaY, mScrollConsumed, mScrollOffset, ViewCompat.TYPE_TOUCH)) { //这里需要减去消耗的距离 deltaY -= mScrollConsumed[1]; //事件偏移设置 vtev.offsetLocation(0, mScrollOffset[1]); mNestedYOffset += mScrollOffset[1]; } if (!mIsBeingDragged && Math.abs(deltaY) > mTouchSlop) { final ViewParent parent = getParent(); if (parent != null) { parent.requestDisallowInterceptTouchEvent(true); } mIsBeingDragged = true; if (deltaY > 0) { deltaY -= mTouchSlop; } else { deltaY += mTouchSlop; } } if (mIsBeingDragged) { // Scroll to follow the motion event mLastMotionY = y - mScrollOffset[1]; final int oldY = getScrollY(); final int range = getScrollRange(); final int overscrollMode = getOverScrollMode(); boolean canOverscroll = overscrollMode == View.OVER_SCROLL_ALWAYS || (overscrollMode == View.OVER_SCROLL_IF_CONTENT_SCROLLS && range > 0); // Calling overScrollByCompat will call onOverScrolled, which // calls onScrollChanged if applicable. //自身滑动实现 if (overScrollByCompat(0, deltaY, 0, getScrollY(), 0, range, 0, 0, true) && !hasNestedScrollingParent(ViewCompat.TYPE_TOUCH)) { // Break our velocity if we hit a scroll barrier. mVelocityTracker.clear(); } final int scrolledDeltaY = getScrollY() - oldY; final int unconsumedY = deltaY - scrolledDeltaY; //父ViewGroup处理dispatchNestedScroll，如果需要有滑动距离消耗的话 //注意dispatchNestedScroll的参数为啥不是int[]，而是int 值 if (dispatchNestedScroll(0, scrolledDeltaY, 0, unconsumedY, mScrollOffset, ViewCompat.TYPE_TOUCH)) { mLastMotionY -= mScrollOffset[1]; vtev.offsetLocation(0, mScrollOffset[1]); mNestedYOffset += mScrollOffset[1]; } else if (canOverscroll) { ensureGlows(); final int pulledToY = oldY + deltaY; if (pulledToY < 0) { EdgeEffectCompat.onPull(mEdgeGlowTop, (float) deltaY / getHeight(), ev.getX(activePointerIndex) / getWidth()); if (!mEdgeGlowBottom.isFinished()) { mEdgeGlowBottom.onRelease(); } } else if (pulledToY > range) { EdgeEffectCompat.onPull(mEdgeGlowBottom, (float) deltaY / getHeight(), 1.f - ev.getX(activePointerIndex) / getWidth()); if (!mEdgeGlowTop.isFinished()) { mEdgeGlowTop.onRelease(); } } if (mEdgeGlowTop != null && (!mEdgeGlowTop.isFinished() || !mEdgeGlowBottom.isFinished())) { ViewCompat.postInvalidateOnAnimation(this); } } } break; case MotionEvent.ACTION_UP: final VelocityTracker velocityTracker = mVelocityTracker; velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity); int initialVelocity = (int) velocityTracker.getYVelocity(mActivePointerId); if ((Math.abs(initialVelocity) > mMinimumVelocity)) { flingWithNestedDispatch(-initialVelocity); } else if (mScroller.springBack(getScrollX(), getScrollY(), 0, 0, 0, getScrollRange())) { ViewCompat.postInvalidateOnAnimation(this); } mActivePointerId = INVALID_POINTER; endDrag(); break; case MotionEvent.ACTION_CANCEL: if (mIsBeingDragged && getChildCount() > 0) { if (mScroller.springBack(getScrollX(), getScrollY(), 0, 0, 0, getScrollRange())) { ViewCompat.postInvalidateOnAnimation(this); } } mActivePointerId = INVALID_POINTER; endDrag(); break; case MotionEvent.ACTION_POINTER_DOWN: { final int index = ev.getActionIndex(); mLastMotionY = (int) ev.getY(index); mActivePointerId = ev.getPointerId(index); break; } case MotionEvent.ACTION_POINTER_UP: onSecondaryPointerUp(ev); mLastMotionY = (int) ev.getY(ev.findPointerIndex(mActivePointerId)); break; } if (mVelocityTracker != null) { mVelocityTracker.addMovement(vtev); } vtev.recycle(); return true; }