MainActivity.java

package com.example.androidpowercomsumption;

import android.os.Bundle;
import android.os.Handler;
import android.view.KeyEvent;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.LifecycleObserver;

public class MainActivity extends AppCompatActivity implements LifecycleObserver {

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

    }


    @Override
    protected void onStop() {
        super.onStop();
    }


    private boolean mIsExit;
    /**
     * 双击返回键退出
     */
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {

        if (keyCode == KeyEvent.KEYCODE_BACK) {
            if (mIsExit) {
                this.finish();

            } else {
                Toast.makeText(this, "再按一次退出", Toast.LENGTH_SHORT).show();
                mIsExit = true;
                new Handler().postDelayed(new Runnable() {
                    @Override
                    public void run() {
                        mIsExit = false;
                    }
                }, 2000);
            }
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }

}