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

作者:互联网

使用ViewMode SavedState来使得ViewModel在系统杀死后台进程的时候得以存活。

本集使用到的库,需要在gradle中添加: implementation 'androidx.lifecycle:lifecycle-viewmodel-savedstate:1.0.0-alpha01'

 

方法1:只用 savedInstanceState

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());
    }
}

  

ViewModel 类

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); } }

  

 

标签:handle,桌面,androidx,number,Studio,import,Android,MyViewModel,lifecycle
来源: https://www.cnblogs.com/gfwei/p/11780236.html