android中使用Activity实现监听手指上下左右滑动

package com.example.testtt;import android.app.Activity;import android.os.Bundle;import android.view.MotionEvent;import android.widget.Toast;public class MainActivity extends Activity { //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 protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); }  @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); }  }