1. 简介
（1）ViewModel 类旨在以注重生命周期的方式存储和管理界面相关数据。ViewModel 类让数据可在发生屏幕旋转等配置更改后继续留存。
（2）对于简单的数据，activity 可以使用 onSaveInstanceState() 方法从 onCreate() 中的捆绑包恢复其数据，但此方法仅适合可以序列化再反序列化的少量数据，而不适合数量可能较大的数据，如用户列表或位图。这种情况就可以使用ViewModel来解决！
（3）根据之前在公司实习的经验，数据相关的都放在ViewModel中，一般就没大问题！
2. 使用
Step1: 引入依赖
plugins {
id 'com.android.application'
id 'org.jetbrains.kotlin.android'
id 'kotlin-kapt'
}
android {
compileSdk 32
defaultConfig {
applicationId "com.exam.viewmodel"
minSdk 21
targetSdk 32
versionCode 1
versionName "1.0"
testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
}
buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
}
}
compileOptions {
sourceCompatibility JavaVersion.VERSION_1_8
targetCompatibility JavaVersion.VERSION_1_8
}
kotlinOptions {
jvmTarget = '1.8'
}
buildFeatures{
viewBinding = true
dataBinding = true
}
}
dependencies {
implementation 'androidx.core:core-ktx:1.7.0'
implementation 'androidx.appcompat:appcompat:1.3.0'
implementation 'com.google.android.material:material:1.4.0'
implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
testImplementation 'junit:junit:4.13.2'
androidTestImplementation 'androidx.test.ext:junit:1.1.3'
androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
// 监听整个应用程序的生命周期
implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.3.1'
implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.3.1'
implementation 'androidx.lifecycle:lifecycle-process:2.5.1'
// viewModel的获取
implementation 'androidx.fragment:fragment-ktx:1.5.2'
// room
implementation "androidx.room:room-runtime:2.4.3"
kapt "androidx.room:room-compiler:2.4.3"
// navigation
implementation "androidx.navigation:navigation-fragment-ktx:2.5.2"
implementation "androidx.navigation:navigation-ui-ktx:2.5.2"
// paging
implementation "androidx.paging:paging-runtime:3.1.1"
// retrofit
implementation "com.squareup.retrofit2:retrofit:2.9.0"
implementation "com.squareup.retrofit2:converter-gson:2.9.0"
// hilt注入
implementation "com.google.dagger:hilt-android:2.28-alpha"
kapt "com.google.dagger:hilt-android-compiler:2.28-alpha"
// 使viewModel在Activity中可以向之前一样实例化
implementation 'androidx.hilt:hilt-lifecycle-viewmodel:1.0.0-alpha02'
kapt 'androidx.hilt:hilt-compiler:1.0.0-alpha02'
// 协程
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.1.1"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.1.1"
}
2.1 在Activity中使用
Step1： 建立ViewModel类
class MainActivityViewModel: ViewModel() {
private var i = 0
private val testNum = MutableLiveData<Int>()
fun getTestNum(): MutableLiveData<Int>{
return testNum
}
fun addOne(){
i++
testNum.value = i
}
}
Step2: 在activity中实例化它，并使用
class MainActivity : AppCompatActivity() {
private val viewModel: MainActivityViewModel by viewModels()
private lateinit var binding: ActivityMainBinding
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
// 使用viewmodel
viewModel.getTestNum().observe(this){
binding.tvText.text = it.toString()
}
click()
}
private fun click(){
binding.btnAdd.setOnClickListener {
viewModel.addOne()
}
binding.btnGoToSecondActivity.setOnClickListener {
val intent: Intent = Intent(this, SecondActivity::class.java)
startActivity(intent)
}
}
}
这样在数据ViewModel中的数据使用LiveData，在Activity中不对数据进行处理，这样即使屏幕翻转，数据也不会消失。
2.2 fragment共享数据
有了ViewModel后， fragment之间的数据共享变得简单了起来。思路非常简单：
Step1: 新建公共的ViewModel类
class FragmentViewModel: ViewModel() {
private val testLiveData = MutableLiveData<Int>()
private var i = 0
fun getLiveData(): MutableLiveData<Int> {
return testLiveData
}
fun addOne(){
i++
testLiveData.value = i
}
}
Step2： 在两个fragment中分别实例化同一个ViewModel即可
LeftFragment:
class LeftFragment : Fragment() {
private val viewModel: FragmentViewModel by activityViewModels()
private lateinit var binding: FragmentLeftBinding
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
binding = FragmentLeftBinding.inflate(layoutInflater, container, false)
return binding.root
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
activity?.let {
viewModel.getLiveData().observe(it){
binding.tvLeft.text = it.toString()
}
}
}
}
RightFragment:
class RightFragment : Fragment() {
private val viewModel: FragmentViewModel by activityViewModels()
private lateinit var binding: FragmentRightBinding
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
binding = FragmentRightBinding.inflate(layoutInflater, container, false)
return binding.root
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
activity?.let {
viewModel.getLiveData().observe(it){
binding.tvRight.text = it.toString()
}
}
}
}
这样就能非常高效的使得数据共享。
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