触摸[4] VelocityTracker

【参考链接】

 

可以使用VelocityTracker类来获得手指滑动时的速度

假设computeCurrentVelocity()中设置的时间间隔是1000ms，在1000ms内手指在水平方向从左向右滑动了100像素，那么计算出来的值就是100。

注意速度的正负值跟屏幕坐标系方向保持一致。

    @Override
    public boolean onTouchEvent(MotionEvent event) {
       super.onTouchEvent(event);

       switch (event.getAction()){
           case MotionEvent.ACTION_DOWN:
               Log.e("shadowfaxghh", "ACTION_DOWN");
               lastX=(int) event.getRawX();

               obtaionVelocityTracker();
               mVelocityTracker.addMovement(event);
               break;

           case MotionEvent.ACTION_MOVE:
               Log.e("shadowfaxghh", "ACTION_MOVE");
               int newX= (int) event.getRawX();
               int deltaX=newX-lastX;

//               if (Math.abs(deltaX) >ViewConfiguration.get(getContext()).getScaledTouchSlop()) {
                   int newScrollX = getScrollX() + (-deltaX);
                   if (newScrollX > maxScrollX)
                       newScrollX = maxScrollX;
                   if (newScrollX < minScrollX)
                       newScrollX = minScrollX;

                   scrollTo(newScrollX, getScrollY());

                   //更新坐标
                   lastX = newX;
//               }

               obtaionVelocityTracker();
               mVelocityTracker.addMovement(event);
               break;

           case MotionEvent.ACTION_UP:
               Log.e("shadowfaxghh", "ACTION_UP");
               lastX=(int) event.getRawX();

               if(mVelocityTracker!=null) {
                   //计算速度
                   mVelocityTracker.computeCurrentVelocity(1000, scaledMaximumFlingVelocity);

                   //获取x方向的速度
                   int xVelocity = (int) mVelocityTracker.getXVelocity();
                   Log.e("shadowfaxghh", "xVelocity=" + xVelocity);

                   recycleVelocityTracker();
               }
               break;

           case MotionEvent.ACTION_CANCEL:
               Log.e("shadowfaxghh", "ACTION_CANCEL");
               lastX=(int) event.getRawX();
               recycleVelocityTracker();
               break;

           default:
               break;
       }
       return true;
    }

    private void obtaionVelocityTracker(){
       if(mVelocityTracker==null)
           mVelocityTracker=VelocityTracker.obtain();
    }

    private void recycleVelocityTracker(){
       if(mVelocityTracker!=null){
           mVelocityTracker.clear();
           mVelocityTracker.recycle();
           mVelocityTracker=null;
       }
    }