Android仿美团拖拽效果实例代码

 @Override
 public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
 slideUp = dy > 0;//正为收缩，负为展开
 onPanelDragged(top);
 }

 @Override
 public void onViewReleased(View releasedChild, float xvel, float yvel) {
 int target;
 if (!slideUp) {
  if (mSlideOffset >= mAnchorPoint / 6) {
  target = computePanelToPosition(mAnchorPoint);
  } else {
  target = computePanelToPosition(0f);
  }
 }else {
  if (mSlideOffset >= mAnchorPoint / 3) {
  target = computePanelToPosition(0f);
  } else {
  target = computePanelToPosition(mAnchorPoint);
  }
 }

 if (mDragHelper != null) {
  mDragHelper.settleCapturedViewAt(releasedChild.getLeft(), target);
 }
 }

 /**
 * 拖拽状态更新以及位置的更新
 * */
 private void onPanelDragged(int newTop) {
 setPanelStateInternal(PanelState.DRAGGING);
 //重新计算距离顶部偏移
 mSlideOffset = computeSlideOffset(newTop);
 //更新视觉差效果和分发事件
 applyParallaxForCurrentSlideOffset();
 //如果偏移是向上，覆盖则无效，需要增加main的高度
 LayoutParams lp = mMainView.getLayoutParams();
 int defaultHeight = getHeight() - getPaddingBottom() - getPaddingTop() - mPanelHeight;
 if (mSlideOffset <= 0 && !mOverlayFlag) {
  lp.height = (newTop - getPaddingBottom());
  if (lp.height == defaultHeight) {
  lp.height = LayoutParams.MATCH_PARENT;
  }
 } else if (lp.height != LayoutParams.MATCH_PARENT && !mOverlayFlag) {
  lp.height = LayoutParams.MATCH_PARENT;
 }
 mMainView.requestLayout();
 }

case MotionEvent.ACTION_MOVE:{
 float dx = x - mPrevMotionX;
 float dy = y - mPrevMotionY;
 mPrevMotionX = x;
 mPrevMotionY = y;

 //横向滑动就不分发了
 if (Math.abs(dx) > Math.abs(dy)) {
  return true;
 }

 //滑动向上、向下
 if (dy > 0) { //收缩
  if (mScrollableViewHelper.getScrollableViewScrollPosition(mScrollView, true) > 0) {
  isMyHandleTouch = true;
  return super.dispatchTouchEvent(ev);
  }

 //之前子view处理了事件
 //我们就需要重新组合一下让面板得到一个合理的点击事件
 if (isMyHandleTouch) {
  MotionEvent up = MotionEvent.obtain(ev);
  up.setAction(MotionEvent.ACTION_CANCEL);
  super.dispatchTouchEvent(up);
  up.recycle();
  ev.setAction(MotionEvent.ACTION_DOWN);
  }

  isMyHandleTouch = false;
  return this.onTouchEvent(ev);
 } else { //展开

  //scrollY=0表示没滑动过，canScroll(1)表示可scroll up
  //逻辑或的意义：拖拽到顶后，要不要禁用外部拖拽
  if (isOnTopFlag == 1) {
  int offset = mDragView.getScrollY();
  boolean scroll = mScrollableViewHelper.getScrollableViewScrollPosition(mScrollView, true) > 0;
  setEnabled(offset == 0 || scroll);
  mDragHelper.abort();
  return super.dispatchTouchEvent(ev);
  }

  //面板是否全部展开
  if (mSlideOffset < mAnchorPoint) {
  isMyHandleTouch = false;
  return this.onTouchEvent(ev);
  }

  if (!isMyHandleTouch && mDragHelper.isDragging()) {
  mDragHelper.cancel();
  ev.setAction(MotionEvent.ACTION_DOWN);
  }

  isMyHandleTouch = true;
  return super.dispatchTouchEvent(ev);
 }
}

@Override
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
 ...(省略一些代码)
 //没有合适的回调方法，只能另辟蹊径了
 //在这里判断dragView有没有到顶，然后把事件给内嵌view
 final int targetY = computePanelToPosition(mAnchorPoint);
 final int originalY = computePanelToPosition(0f);
 if (mDragView.getTop() == targetY) {
 //避免多次回调
 if (isOnTopFlag != 1 && stateCallback != null) {
  stateCallback.onExpandedState();
 }
 isOnTopFlag = 1;
 }else if (mDragView.getTop() == originalY){
 if (isOnTopFlag == -1 && stateCallback != null) {
  stateCallback.onCollapsedState();
 }
 isOnTopFlag = 0;
 }else {
 isOnTopFlag = -1;
 }
...(省略一些代码)
}