Android活动中onCreate参数Bundle savedInstanceState的作用

当我们使用Android Stdio的活动自动向导自动生成活动时，在活动中Android Stdio会自动为我们生成这样一段代码。

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

这段方法会在活动被系统创建后被调用。而这个活动的参数Bundle使用来恢复状态的。活动在一些状况可能会被系统撤销而后重新创建，例如屏幕方向发生旋转。这时onCreate方法可以通过Bundle参数中保存的数值来恢复活动被系统撤销前的一些参数。Android允许开发者通过onSaveInstanceState(Bundle)方法来保存当前活动的信息参数，在活动被撤销前Android会调用此方法来保存信息。
使用方式如下:

@Override//声明覆盖
    public void onSaveInstanceState(Bundle saveInstanceState) {

        super.onSaveInstanceState(saveInstanceState);
        /*↑必须先调用父类中的改方法以保证父类中该方法的操作全部被实现*/
        saveInstanceState.putInt("seconds", seconds);
        /*通过.put*（“信息名”），信息值；的方式对数据进行保存*/
        saveInstanceState.putBoolean("running",running);
        saveInstanceState.putBoolean("wasRunning",wasRunning);

    }

这样如果活动被重新构建，onCreate会自动被传入anSaveInstanceState方法中使用的Bundle，恢复数据的方式如下：

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if(savedInstanceState != null){
        //如果活动是第一次被创建，则传入的Bundle的值为null
            seconds = savedInstanceState.getInt("seconds");
            running = savedInstanceState.getBoolean("running");
            wasRunning = savedInstanceState.getBoolean("wasRunning");
        }
    }

下面是一个实现了数据自动恢复的秒针计时Android活动代码，可以参考一下

package com.hdad.stopwatch;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.os.Handler;
import android.view.View;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    private int seconds = 0;
    private boolean running = false;
    private boolean wasRunning ;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if(savedInstanceState != null){
            seconds = savedInstanceState.getInt("seconds");
            running = savedInstanceState.getBoolean("running");
            wasRunning = savedInstanceState.getBoolean("wasRunning");
        }
        runningTime();
    }

    @Override
    public void onSaveInstanceState(Bundle saveInstanceState) {

        super.onSaveInstanceState(saveInstanceState);
        saveInstanceState.putInt("seconds", seconds);
        saveInstanceState.putBoolean("running",running);
        saveInstanceState.putBoolean("wasRunning",wasRunning);

    }

    @Override
    public void onStop() {

        super.onStop();
        wasRunning = running;
        running = false;
    }

    @Override
    public void onStart() {

        super.onStart();
        running = wasRunning;
    }

    public void onClickStart(View view){
        running = true;
    }

    public void onClickStop(View view){
        running = false;
    }

    public void onClickReset(View view){
        running = false;
        seconds = 0;
    }

    public void runningTime(){
        final TextView textView = (TextView)findViewById(R.id.time_view);
        final Handler handler = new Handler();
        handler.post(//本线程自活动创建伊始便开始运行
                new Runnable() {
                    @Override
                    public void run() {
                        int hours = seconds/3600;
                        int minutes = seconds/60;
                        int secs = seconds%60;
                        String time = String.format("%d:%02d:%02d",hours,minutes,secs);
                        textView.setText(time);
                        if(running){
                            seconds++;
                        }
                        handler.postDelayed(this,1000);//1000毫秒后再次请求运行本类
                    }
                }
        );

    }
}