文章目录标题

• 布局
• MainActivity
• 测试

布局

在res/layout 下进行布局

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" xmlns:app="http://schemas.android.com/apk/res-auto" android:layout_width="match_parent" android:layout_height="match_parent" tools:context=".MainActivity">


    <LinearLayout android:orientation="vertical" android:layout_width="match_parent" android:layout_height="match_parent" tools:layout_editor_absoluteY="0dp" tools:layout_editor_absoluteX="0dp">
        <TextView android:text="00:00:00" android:textSize="60sp" android:gravity="center" android:layout_width="match_parent" android:layout_height="wrap_content" android:id="@+id/timeView"/>
        <Button android:text="start" android:onClick="onClickStart" android:layout_width="match_parent" android:layout_height="wrap_content" android:id="@+id/start"/>
        <Button android:text="stop" android:onClick="onClickStop" android:layout_width="match_parent" android:layout_height="wrap_content" android:id="@+id/stop"/>
        <Button android:text="reset" android:onClick="onClickReset" android:layout_width="match_parent" android:layout_height="wrap_content" android:id="@+id/reset"/>
    </LinearLayout>

</android.support.constraint.ConstraintLayout>

MainActivity

package com.test;

import android.os.Handler;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;

import android.widget.TextView;

public class MainActivity extends AppCompatActivity { 


    private int seconds = 0;
    private boolean running = false; //计时状态
    private boolean wasRunning = false; //保存running的状态

    //app进入后台，暂停计时
    @Override
    protected void onStop() { 
        super.onStop();
        wasRunning = running;
        running = false;
    }

    //重新进入app，开始计时
    @Override
    protected void onStart() { 
        super.onStart();
        if(wasRunning) running = true;
    }

    //失去焦点(如分屏)，暂停计时
    @Override
    protected void onPause() { 
        super.onPause();
        wasRunning = running;
        running = false;
    }

    //获得焦点,重新开始计时
    @Override
    protected void onResume() { 
        super.onResume();
        if(wasRunning) running = true;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //获取保存的状态
        if(savedInstanceState!=null){ 
            seconds = savedInstanceState.getInt("seconds");
            running = savedInstanceState.getBoolean("running");
            wasRunning = savedInstanceState.getBoolean("wasRunning");
        }
        runTime();
    }

    
    @Override
    public void onSaveInstanceState(Bundle saveInstanceState) { 

        super.onSaveInstanceState(saveInstanceState);
        saveInstanceState.putInt("seconds",seconds);
        saveInstanceState.putBoolean("running",running);
        saveInstanceState.putBoolean("wasRunning",wasRunning);
    }
    
    public void onClickStart(View button){ 
        running = true;
    }
    public void onClickStop(View button){ 
       running = false;
    }
    public void onClickReset(View button){ 
        running = false;
        seconds = 0;
    }

     
    private void runTime(){ 
        final Handler handler = new Handler();
        handler.post(new Runnable() { 
            @Override
            public void run() { 
                final TextView textView = findViewById(R.id.timeView);
                int hour = seconds /3600%24;
                int minute = seconds%3600/60;
                String time = String.format("%02d:%02d:%02d",hour,minute,seconds%60);
                textView.setText(time);
                if(running) seconds++;
                handler.postDelayed(this,1000);
               }
            }
        );

    }
}

测试

完成~
笔者也是初学者，不足之处，还望指正！