android使用JetPack中的ViewModelSavedState保存数据

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解决当进程被系统回收后，如何保存数据。

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使用ViewModel中的SavedStateHandle也可以解决这个问题。
下面代码中存在的一些小知识：
MutableLiveData：个人的理解就是当数据有变化时，可以通知界面更新数据

1. 首先我们创建一个页面SaveStateActivity

package com.java.jetpackdemo.savestate

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.databinding.DataBindingUtil
import androidx.lifecycle.SavedStateViewModelFactory
import androidx.lifecycle.ViewModelProviders
import com.java.jetpackdemo.R
import com.java.jetpackdemo.databinding.ActivitySaveStateBinding

class SaveStateActivity : AppCompatActivity() {
    private lateinit var binding: ActivitySaveStateBinding
    private lateinit var viewModel: SaveStateViewModel
    companion object {
        const val KEY_NUMBER = "KEY_NUMBER"
    }
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        //用databinding来绑定布局
        binding = DataBindingUtil.setContentView(this, R.layout.activity_save_state)
        //创建viewmodel
        /** 不传savedstateviewmodel的话创建viewmodel的方式
         myViewModel = ViewModelProviders.of(this).get(MyViewModel::class.java) **/
        viewModel = ViewModelProviders.of(this, SavedStateViewModelFactory(application, this))
            .get(SaveStateViewModel::class.java)
        //将viewmodel绑定到databinding上
        binding.data = viewModel
        //databinding监听activity的生命周期，实时更新数据
        binding.lifecycleOwner = this
    }
}

2. 创建布局

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">
    <data>
        <variable
            name="data"
            type="com.java.jetpackdemo.savestate.SaveStateViewModel" />
    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".savestate.SaveStateActivity">

        <TextView
            android:id="@+id/tvNumber"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{data.number.toString()}"
            app:layout_constraintBottom_toTopOf="@+id/guideline3"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

        <androidx.constraintlayout.widget.Guideline
            android:id="@+id/guideline3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            app:layout_constraintGuide_percent="0.2" />

        <Button
            android:id="@+id/btnAdd"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="@{()->data.add()}"
            android:text="@string/save_state_btn"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.498"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="@+id/guideline3"
            app:layout_constraintVertical_bias="0.178" />
    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

3. 创建viewmodel

package com.java.jetpackdemo.savestate

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

class SaveStateViewModel(handle: SavedStateHandle) : ViewModel() {
    private var linkNumber: MutableLiveData<Int> = MutableLiveData()

    private var handle = handle

    //初始化数据
    init {
        linkNumber.value = 0
    }

    //将数据加1
    public fun add() {
        getNumber().value = getNumber().value?.plus(1)
    }

    //获取数据
    public fun getNumber(): MutableLiveData<Int> {
        if (!handle.contains(SaveStateActivity.KEY_NUMBER)) {
            handle.set(SaveStateActivity.KEY_NUMBER, 0)
        }
        return handle.getLiveData(SaveStateActivity.KEY_NUMBER)
    }
}

如何测试呢？可以打开开发者模式，然后打开 【不保留活】 的选项，当我们按下home键后，进程就会被系统回收，但是我们重新打开app，数据并没有丢失。