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

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