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Android如何设计一个全局可调用的ViewModel对象?

很多时候我们需要维护一个全局可用的ViewModel,因为这样可以维护全局同一份数据源,且方便使用协程绑定App的生命周期。那如何设计全局可用的ViewModel对象?


一、思路


viewModel对象是存储在ViewModelStore中的,那么如果我们创建一个全局使用的ViewModelStore并且在获取viewModel对象的时候从它里面获取就可以了。


viewModel是通过ViewModelProviderget方法获取的,一般是ViewModelProvider(owner: ViewModelStoreOwner, factory: Factory).get(ViewModel::class.java)


如何将ViewModelProviderViewModelStore关联起来? 纽带就是ViewModelStoreOwner, ViewModelStoreOwner是一个接口,需要实现getViewModelStore()方法,而该方法返回的就是ViewModelStore:

public interface ViewModelStoreOwner {
/**
* Returns owned {@link ViewModelStore}
*
* @return a {@code ViewModelStore}
*/
@NonNull
ViewModelStore getViewModelStore(); //返回一个ViewModelStore
}

让某个类实现这个接口,重写方法返回我们定义的ViewModelStore就可以了。


至于上面ViewModelProvider构造方法的第二个参数Factory是什么呢?


源码中提供了二种Factory,一种是NewInstanceFactory,一种是AndroidViewModelFactory,它们的主要区别是:




  • NewInstanceFactory创建ViewModel时,会为每个Activity或Fragment创建一个新的ViewModel实例,这会导致ViewModel无法在应用程序的不同部分共享数据。(ComponentActivity源码getDefaultViewModelProviderFactory方法)




  • AndroidViewModelFactory可以访问应用程序的全局状态,并且ViewModel实例可以在整个应用程序中是共享的。




根据我们的需求,需要用的是AndroidViewModelFactory。


二、具体实现


1、方式一:可以全局添加和获取任意ViewModel


定义Application,Ktx.kt文件

import android.app.Application

lateinit var appContext: Application

fun setApplicationContext(context: Application) {
appContext = context
}

定义全局可用的ViewModelOwner实现类

object ApplicationScopeViewModelProvider : ViewModelStoreOwner {

private val eventViewModelStore: ViewModelStore = ViewModelStore()

override fun getViewModelStore(): ViewModelStore {
return eventViewModelStore
}

private val mApplicationProvider: ViewModelProvider by lazy {
ViewModelProvider(
ApplicationScopeViewModelProvider,
ViewModelProvider.AndroidViewModelFactory.getInstance(appContext)
)
}

fun <T : ViewModel> getApplicationScopeViewModel(modelClass: Class<T>): T {
return mApplicationProvider.get(modelClass)
}
}

定义一个ViewModel通过StateFlow定义发送和订阅事件的方法

class EventViewModel : ViewModel() {

private val mutableStateFlow = MutableStateFlow(0)

fun postEvent(state: Int) {
mutableStateFlow.value = state
}

fun observeEvent(scope: CoroutineScope? = null, method: (Int) -> Unit = { _ -> }) {
val eventScope = scope ?: viewModelScope
eventScope.launch {
mutableStateFlow.collect {
method.invoke(it)
}
}
}
}

定义一个调用的类



object FlowEvent {

//发送事件
fun postEvent(state: Int) {
ApplicationScopeViewModelProvider.getApplicationScopeViewModel(EventViewModel::class.java)
.postEvent(state)
}

//订阅事件
fun observeEvent(scope: CoroutineScope? = null, method: (Int) -> Unit = { _ -> }) {
ApplicationScopeViewModelProvider.getApplicationScopeViewModel(EventViewModel::class.java)
.observeEvent(scope, method)
}
}

测试代码如下:

class MainActivity : AppCompatActivity() {

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
//打印协程名称
System.setProperty("kotlinx.coroutines.debug", "on")

FlowEvent.observeEvent {
printMsg("MainActivity observeEvent before :$it")
}
//修改值
FlowEvent.postEvent(1)


FlowEvent.observeEvent {
printMsg("MainActivity observeEvent after :$it")
}

}

}

//日志
内容:MainActivity observeEvent before :0 线程:main @coroutine#1
内容:MainActivity observeEvent before :1 线程:main @coroutine#1
内容:MainActivity observeEvent after :1 线程:main @coroutine#2

2、方式二:更方便在Activity和Fragment中调用


定义Application,让BaseApplication实现ViewModelStoreOwner

//BaseApplication实现ViewModelStoreOwner接口
class BaseApplication : Application(), ViewModelStoreOwner {

private lateinit var mAppViewModelStore: ViewModelStore
private var mFactory: ViewModelProvider.Factory? = null

override fun onCreate() {
super.onCreate()
//设置全局的上下文
setApplicationContext(this)
//创建ViewModelStore
mAppViewModelStore = ViewModelStore()

}

override fun getViewModelStore(): ViewModelStore = mAppViewModelStore

/**
* 获取一个全局的ViewModel
*/
fun getAppViewModelProvider(): ViewModelProvider {
return ViewModelProvider(this, this.getAppFactory())
}

private fun getAppFactory(): ViewModelProvider.Factory {
if (mFactory == null) {
mFactory = ViewModelProvider.AndroidViewModelFactory.getInstance(this)
}
return mFactory as ViewModelProvider.Factory
}
}

Ktx.kt文件也有变化,如下

lateinit var appContext: Application

fun setApplicationContext(context: Application) {
appContext = context
}

//定义扩展方法
inline fun <reified VM : ViewModel> Fragment.getAppViewModel(): VM {
(this.requireActivity().application as? BaseApplication).let {
if (it == null) {
throw NullPointerException("Application does not inherit from BaseApplication")
} else {
return it.getAppViewModelProvider().get(VM::class.java)
}
}
}

//定义扩展方法
inline fun <reified VM : ViewModel> AppCompatActivity.getAppViewModel(): VM {
(this.application as? BaseApplication).let {
if (it == null) {
throw NullPointerException("Application does not inherit from BaseApplication")
} else {
return it.getAppViewModelProvider().get(VM::class.java)
}
}
}

BaseActivityBaseFragment中调用上述扩展方法

abstract class BaseActivity: AppCompatActivity() {

//创建ViewModel对象
val eventViewModel: EventViewModel by lazy { getAppViewModel() }

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)

}
}
abstract class BaseFragment: Fragment() {

//创建ViewModel对象
val eventViewModel: EventViewModel by lazy { getAppViewModel() }

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)

}
}

测试代码

class MainActivity : BaseActivity() {

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
//打印协程名称
System.setProperty("kotlinx.coroutines.debug", "on")

eventViewModel.observeEvent {
printMsg("MainActivity observeEvent :$it")
}

findViewById<AppCompatButton>(R.id.bt).setOnClickListener {
//点击按钮修改值
eventViewModel.postEvent(1)
//跳转到其他Activity
Intent(this, TwoActivity::class.java).also { startActivity(it) }
}
}

}
class TwoActivity : BaseActivity() {

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_two)

eventViewModel.observeEvent {
printMsg("TwoActivity observeEvent :$it")
}
}
}

日志

内容:MainActivity observeEvent :0 线程:main @coroutine#1
内容:MainActivity observeEvent :1 线程:main @coroutine#1
内容:TwoActivity observeEvent :1 线程:main @coroutine#2

作者:TimeFine
链接:https://juejin.cn/post/7233686528328286245
来源:稀土掘金
著作权归作者所有。商业转载请联系作者获得授权,非商业转载请注明出处。

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