Android Studio 之 数据存活【2】，返回桌面切换回来，内容还保存着

package com.example.viewmodelrestore;

import android.os.Bundle;

import androidx.appcompat.app.AppCompatActivity;
import androidx.databinding.DataBindingUtil;
import androidx.lifecycle.SavedStateVMFactory;
import androidx.lifecycle.ViewModelProviders;

import com.example.viewmodelrestore.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {
    ActivityMainBinding binding;
    MyViewModel myViewModel;
    public final static String KEY_NUMBER  = "my_number"; //常量

    /*
    = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
    利用 ViewModel 进行保存数据，横屏切换后，数据都会保存
    但是如果Activity 杀死后，重新打开，数据就没有了。

        binding = DataBindingUtil.setContentView(this,R.layout.activity_main);
        //ViewModelProviders 需要在 build.gradle 中 添加如下引用：
        //implementation ‘androidx.lifecycle:lifecycle-extensions:2.1.0‘
        myViewModel = ViewModelProviders.of(this).get(MyViewModel.class);
        binding.setData(myViewModel);
        binding.setLifecycleOwner(this);
        这种是之前老的 api 中的方法

        使用 onSaveInstanceState 键值对 ，进入后台，再回到app，数据还会显示之前的
        新的方法是在 build.gradle 中引用 implementation ‘androidx.lifecycle:lifecycle-viewmodel-savedstate:1.0.0-alpha01‘
        在 MyViewModel 类中 添加一个构造函数
        使用：SavedStateHandle handle
        SavedStateHandle.getLiveData
     */

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

        binding = DataBindingUtil.setContentView(this,R.layout.activity_main);

        //ViewModelProviders 需要在 build.gradle 中 添加如下引用：
        //implementation ‘androidx.lifecycle:lifecycle-extensions:2.1.0‘


        //判断是否有数据，如果有直接取出来
        //用 SaveInstanceState 就不需要用下面这个了
        /*
        【1】：老方法
        myViewModel = ViewModelProviders.of(this).get(MyViewModel.class);
        if(savedInstanceState != null){
            myViewModel.getNumber().setValue(savedInstanceState.getInt(KEY_NUMBER));
        }
         */


        //新方法【2】：直接用这句，就不需要判断为不为空了
        //需要在 build.gradle 中引用 implementation ‘androidx.lifecycle:lifecycle-viewmodel-savedstate:1.0.0-alpha01‘
        //注意创建 ViewModel 对象的时候，要带一个参数 ： new SavedStateVMFactory(this)
        myViewModel = ViewModelProviders.of(this,new SavedStateVMFactory(this)).get(MyViewModel.class);

        binding.setData(myViewModel);
        binding.setLifecycleOwner(this);
    }

    //将数据保存起来
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt(KEY_NUMBER,myViewModel.getNumber().getValue());
    }
}

package com.example.viewmodelrestore;

import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.SavedStateHandle;
import androidx.lifecycle.ViewModel;

public class MyViewModel extends ViewModel {
    //【老方法】
    private MutableLiveData<Integer> number;
    
    //【新方法】
    private SavedStateHandle handle;
    public MyViewModel(SavedStateHandle handle) {
        this.handle = handle;
    }

    public MutableLiveData<Integer> getNumber() {
        //【新方法】    
        //判断 handld 键值组里面有没有这个
        if(!handle.contains(MainActivity.KEY_NUMBER)){
            handle.set(MainActivity.KEY_NUMBER,0);
        }
        return handle.getLiveData(MainActivity.KEY_NUMBER);
        

        /* 【老方法】，不用 MyViewModel 构造函数的情况下，不使用 SaveStateHandle
        if(number == null)
        {
            number = new MutableLiveData<>();
            number.setValue(0);
        }
        return number;
         */
    }

    public void add(){
        //【新方法】
        getNumber().setValue(getNumber().getValue() + 1);
        
        //【老方法】
        //number.setValue(number.getValue() + 1);
    }
}