Android实现点击两次返回键退出

package com.gaolei.exitdemo;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.KeyEvent;
import android.widget.Toast;

public class MainActivity extends Activity {

  // 定义一个变量，来标识是否退出
  private static boolean isExit = false;

  Handler mHandler = new Handler() {

    @Override
    public void handleMessage(Message msg) {
      super.handleMessage(msg);
      isExit = false;
    }
  };

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
  }

  @Override
  public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
      exit();
      return false;
    }
    return super.onKeyDown(keyCode, event);
  }

  private void exit() {
    if (!isExit) {
      isExit = true;
      Toast.makeText(getApplicationContext(), "再按一次退出程序",
          Toast.LENGTH_SHORT).show();
      // 利用handler延迟发送更改状态信息
      mHandler.sendEmptyMessageDelayed(0, 2000);
    } else {
      finish();
      System.exit(0);
    }