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ViewModelProvider的get方法获取的，一般是ViewModelProvider(owner: ViewModelStoreOwner, factory: Factory).get(ViewModel::class.java)。
如何将ViewModelProvider与ViewModelStore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)
}
}
}
在BaseActivity和BaseFragment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
三、应用场景
很多同学思维还局限在单例，下面是最近写的一篇文章，刚好用到了，抛砖引玉大家看看：
Android-换个思路实现H5唤醒App并跳转到指定页面
