Android jetpack viewModel 屏幕旋转数据如何保存。

解决的问题

瞬态数据的丢失。

异步调用的内存泄露

类膨胀提高维护难度和测试难度。

参考：2021年最全面的Jetpack系统学习课程，看他就够了，更新中_哔哩哔哩_bilibili

布局

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".activity.ViewModelActivity">


    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="0"
        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="parent"
        app:layout_constraintVertical_bias="0.396" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="加1"
        android:onClick="addOne"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView"
        app:layout_constraintVertical_bias="0.395" />
</androidx.constraintlayout.widget.ConstraintLayout>

首先我们知道。在旋转屏幕的时候如果不重写他的onSaveInstace的方法。一些状态将无法被保存

也就是会被清空掉

package com.anguomob.jecpack.activity

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import androidx.lifecycle.ViewModelProvider
import com.anguomob.jecpack.R
import com.anguomob.jecpack.databinding.ActivityMainBinding
import com.anguomob.jecpack.databinding.ActivityViewModelBinding
import com.anguomob.jecpack.viewmodel.MyViewModel

class ViewModelActivity : AppCompatActivity() {
    lateinit var binding: ActivityViewModelBinding

    var number = 0;
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityViewModelBinding.inflate(layoutInflater)
        setContentView(binding.root)

 
        binding.textView.text = number.toString()

    }

    fun addOne(view: View) {
        binding.textView.text = (++number).toString()
    }
}

这种数据是在旋转后会变为0的

如果使用viewModel则不会

package com.anguomob.jecpack.activity

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import androidx.lifecycle.ViewModelProvider
import com.anguomob.jecpack.R
import com.anguomob.jecpack.databinding.ActivityMainBinding
import com.anguomob.jecpack.databinding.ActivityViewModelBinding
import com.anguomob.jecpack.viewmodel.MyViewModel

class ViewModelActivity : AppCompatActivity() {
    lateinit var binding: ActivityViewModelBinding
    lateinit var viewModel: MyViewModel
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityViewModelBinding.inflate(layoutInflater)
        setContentView(binding.root)

        viewModel = ViewModelProvider(
            this,
            ViewModelProvider.AndroidViewModelFactory.getInstance(application)
        ).get(
            MyViewModel::class.java
        )
        binding.textView.text = viewModel.number.toString()

    }

    fun addOne(view: View) {
        binding.textView.text = (++viewModel.number).toString()
    }
}

MyViewModel的内容也很简单

package com.anguomob.jecpack.viewmodel

import androidx.lifecycle.ViewModel

class MyViewModel : ViewModel() {
    var number = 0

}

这个原理得亏与viewModel的声明周期比较长

所以不要往viewModel里面传入Context 会造成内存泄露

如果要用Context 请使用 AndroidViewModel中的Application

 什么意思呢

修改下我们刚才的代码

package com.anguomob.jecpack.viewmodel

import android.app.Application
import androidx.lifecycle.AndroidViewModel
import androidx.lifecycle.ViewModel

class MyViewModel(application: Application) : AndroidViewModel(application) {
    var number = 0

}