ViewModelSavedState（即使进程在后台被系统杀死数据也能保存下来）

引入

正常情况下，如果一个进程被移到后台，且当前系统内存不足时，会有一定概率会将该进程杀死来释放内存资源，而被系统杀死时，ViewModel也会被删除，UI的数据就会遭到丢失。

如下图所示，一旦切出后台进程被系统杀死数据就会丢失。

不过好在系统杀死进程时，会调用 onSaveInstanceState() 方法，顾名思义就是保存实例状态的，我们重写这个方法来保存我们的数据。

代码

1. onSaveInstanceState

ViewModel部分

package com.example.viewmodelsavedstate;

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

public class MyViewModel extends ViewModel {
    private MutableLiveData<Integer> number;

    public MutableLiveData<Integer> getNumber() {
        if(number == null){
            number = new MutableLiveData<>();
            number.setValue(0);
        }
        return number;
    }

    public void add(int x){
        number.setValue(number.getValue() + x);
    }
}

MainActivity，21 - 23行是载入 savedInstanceState，28 - 32是写入 savedInstanceState。

package com.example.viewmodelsavedstate;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.databinding.DataBindingUtil;
import androidx.lifecycle.ViewModelProvider;
import android.os.Bundle;
import com.example.viewmodelsavedstate.databinding.ActivityMainBinding;


public class MainActivity extends AppCompatActivity {
    MyViewModel myViewModel;
    ActivityMainBinding binding;
    public final static String SAVED_KEY_NAME = "number";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        myViewModel = new ViewModelProvider(this).get(MyViewModel.class);
        if(savedInstanceState != null){
            myViewModel.getNumber().setValue(savedInstanceState.getInt(SAVED_KEY_NAME));
        }
        binding.setData(myViewModel);
        binding.setLifecycleOwner(this);
    }

    @Override
    protected void onSaveInstanceState(@NonNull Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt(SAVED_KEY_NAME, myViewModel.getNumber().getValue());
    }
}

2. SavedStateHandle

ViewModel部分，使用 SavedStateHandle 来代替 MutableLiveData 存储数据，同时在创建 ViewModel 时将SavedStateHandle 传入供后续使用。

package com.example.viewmodelsavedstate;

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

public class MyViewModel extends ViewModel {
    private SavedStateHandle handle;
    public MyViewModel(SavedStateHandle handle){
        this.handle = handle;
    }

    public MutableLiveData<Integer> getNumber() {
        if(!handle.contains(MainActivity.SAVED_KEY_NAME)){
            handle.set(MainActivity.SAVED_KEY_NAME, 0);
        }
        return handle.getLiveData(MainActivity.SAVED_KEY_NAME);
    }

    public void add(int x){
        getNumber().setValue(getNumber().getValue() + x);
    }
}

MainActivity部分

package com.example.viewmodelsavedstate;

import androidx.appcompat.app.AppCompatActivity;
import androidx.databinding.DataBindingUtil;
import androidx.lifecycle.ViewModelProvider;

import android.os.Bundle;

import com.example.viewmodelsavedstate.databinding.ActivityMainBinding;


public class MainActivity extends AppCompatActivity {
    MyViewModel myViewModel;
    ActivityMainBinding binding;
    public final static String SAVED_KEY_NAME = "number";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        myViewModel = new ViewModelProvider(this).get(MyViewModel.class);
        binding.setData(myViewModel);
        binding.setLifecycleOwner(this);
    }

}