怎么在Android中利用activity与view对滑动进行判断

//手指按下的点为(x1, y1)手指离开屏幕的点为(x2, y2)  float x1 = 0;  float x2 = 0;  float y1 = 0;  float y2 = 0;   @Override  public boolean onTouchEvent(MotionEvent event) {  //继承了Activity的onTouchEvent方法，直接监听点击事件  if(event.getAction() == MotionEvent.ACTION_DOWN) {  //当手指按下的时候  x1 = event.getX();  y1 = event.getY();  }  if(event.getAction() == MotionEvent.ACTION_UP) {  //当手指离开的时候  x2 = event.getX();  y2 = event.getY();  if(y1 - y2 > 50) {  Toast.makeText(MainActivity.this, "向上滑", Toast.LENGTH_SHORT).show();  } else if(y2 - y1 > 50) {  Toast.makeText(MainActivity.this, "向下滑", Toast.LENGTH_SHORT).show();  } else if(x1 - x2 > 50) {  Toast.makeText(MainActivity.this, "向左滑", Toast.LENGTH_SHORT).show();  } else if(x2 - x1 > 50) {  Toast.makeText(MainActivity.this, "向右滑", Toast.LENGTH_SHORT).show();  }  }  return super.onTouchEvent(event);  }