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);
 }
 
 
}