Android中ViewPager帶來的滑動卡頓問題解決要點解析

問題說明:
當SwipeRefreshLayout中放置了ViewPager控件,兩者的滑動會相互沖突.具體表現為ViewPager的左右滑動不順暢,容易被SwipeRefreshLayout攔截(即出現刷新的View).

問題原因:
ViewPager本身是處理了滾動事件的沖突,它在橫向滑動時會調用requestDisallowInterceptTouchEvent()方法使父控件不攔截當前的Touch事件序列.但是SwipeRefreshLayout的requestDisallowInterceptTouchEvent()方法什麼也沒有做,所以仍然會攔截當前的Touch事件序列.

問題分析:
為什麼SwipeRefreshLayout的requestDisallowInterceptTouchEvent()方法什麼都不做?

首先SwipeRefreshLayout繼承自ViewGroup.

在requestDisallowInterceptTouchEvent()方法什麼都不做的情況下,用戶可以從底部下拉刷新一次拉出LoadingView.
如果方法調用ViewGroup的requestDisallowInterceptTouchEvent()方法, 可以解決ViewPager的兼容問題,但是用戶在界面底部下拉至頭部後,無法繼續下拉,需要手指放開一次才能拉出LoadingView.
目標分析:
那麼為了更加順滑地滾動,想要的效果當然是一次性拉出LoadingView.既然ViewPager在左右滑動時才會調用requestDisallowInterceptTouchEvent()方法,那麼SwipeRefreshLayout只應該在上下滑動時才攔截Touch事件.

具體邏輯如下:

記錄是否調用了requestDisallowInterceptTouchEvent()方法,並且設置為true.
在SwipeRefreshLayout中判斷是否是上下滑動.
如果同時滿足1,2,則調用super.requestDisallowInterceptTouchEvent(true).
否則調用super.requestDisallowInterceptTouchEvent(false).
注意:因為ViewGroup的requestDisallowInterceptTouchEvent方法設置true後,Touch事件在dispatchTouchEvent()方法中就會被攔截,所以需要在dispatchTouchEvent()方法中判斷是否為上下滑動.

實現代碼(部分):

//非法按鍵
private static final int INVALID_POINTER = -1;

//dispatch方法記錄第一次按下的x
private float mInitialDisPatchDownX;

//dispatch方法記錄第一次按下的y
private float mInitialDisPatchDownY;

//dispatch方法記錄的手指
private int mActiveDispatchPointerId = INVALID_POINTER;

//是否請求攔截
private boolean hasRequestDisallowIntercept = false;

@Override
public void requestDisallowInterceptTouchEvent(boolean b) {
  hasRequestDisallowIntercept = b;
  // Nope.
}

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
  switch (ev.getAction()) {
    case MotionEvent.ACTION_DOWN:
      mActiveDispatchPointerId = MotionEventCompat.getPointerId(ev, 0);
      final float initialDownX = getMotionEventX(ev, mActiveDispatchPointerId);
      if (initialDownX != INVALID_POINTER) {
        mInitialDisPatchDownX = initialDownX;
      }
      final float initialDownY = getMotionEventY(ev, mActiveDispatchPointerId);
      if (mInitialDisPatchDownY != INVALID_POINTER) {
        mInitialDisPatchDownY = initialDownY;
      }
      break;
    case MotionEvent.ACTION_MOVE:
      if (hasRequestDisallowIntercept) {
        //解決viewPager滑動沖突問題
        final float x = getMotionEventX(ev, mActiveDispatchPointerId);
        final float y = getMotionEventY(ev, mActiveDispatchPointerId);
        if (mInitialDisPatchDownX != INVALID_POINTER && x != INVALID_POINTER &&
            mInitialDisPatchDownY != INVALID_POINTER && y != INVALID_POINTER) {
          final float xDiff = Math.abs(x - mInitialDisPatchDownX);
          final float yDiff = Math.abs(y - mInitialDisPatchDownY);
          if (xDiff > mTouchSlop && xDiff * 0.7f > yDiff) {
            //橫向滾動不需要攔截
            super.requestDisallowInterceptTouchEvent(true);
          } else {
            super.requestDisallowInterceptTouchEvent(false);
          }
        } else {
          super.requestDisallowInterceptTouchEvent(false);
        }
      }
      break;
    case MotionEvent.ACTION_UP:
    case MotionEvent.ACTION_CANCEL:
      if (ev.getAction() == MotionEvent.ACTION_UP || ev.getAction() == MotionEvent.ACTION_CANCEL) {
        hasRequestDisallowIntercept = false;
      }
      break;
  }

  return super.dispatchTouchEvent(ev);
}

private float getMotionEventY(MotionEvent ev, int activePointerId) {
  final int index = MotionEventCompat.findPointerIndex(ev, activePointerId);
  if (index < 0) {
    return -1;
  }
  return MotionEventCompat.getY(ev, index);
}

private float getMotionEventX(MotionEvent ev, int activePointerId) {
  final int index = MotionEventCompat.findPointerIndex(ev, activePointerId);
  if (index < 0) {
    return -1;
  }
  return MotionEventCompat.getX(ev, index);
}