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詳解Android如何設(shè)計(jì)一個(gè)全局可調(diào)用的ViewModel對象

 更新時(shí)間：2023年05月17日 11:22:28   作者：TimeFine  
很多時(shí)候我們需要維護(hù)一個(gè)全局可用的ViewModel，因?yàn)檫@樣可以維護(hù)全局同一份數(shù)據(jù)源，且方便使用協(xié)程綁定App的生命周期，那如何設(shè)計(jì)全局可用的ViewModel對象，文中介紹的非常詳細(xì),需要的朋友可以參考下

一、思路

viewModel對象是存儲在ViewModelStore中的，那么如果我們創(chuàng)建一個(gè)全局使用的ViewModelStore并且在獲取viewModel對象的時(shí)候從它里面獲取就可以了。

viewModel是通過ViewModelProvider的get方法獲取的，一般是ViewModelProvider(owner: ViewModelStoreOwner, factory: Factory).get(ViewModel::class.java)。

如何將ViewModelProvider與ViewModelStore關(guān)聯(lián)起來? 紐帶就是ViewModelStoreOwner, ViewModelStoreOwner是一個(gè)接口，需要實(shí)現(xiàn)getViewModelStore()方法，而該方法返回的就是ViewModelStore:

public interface ViewModelStoreOwner {
    /**
     * Returns owned {@link ViewModelStore}
     *
     * @return a {@code ViewModelStore}
     */
    @NonNull
    ViewModelStore getViewModelStore();   //返回一個(gè)ViewModelStore
}

讓某個(gè)類實(shí)現(xiàn)這個(gè)接口，重寫方法返回我們定義的ViewModelStore就可以了。

至于上面ViewModelProvider構(gòu)造方法的第二個(gè)參數(shù)Factory是什么呢?

源碼中提供了二種Factory，一種是NewInstanceFactory，一種是AndroidViewModelFactory，它們的主要區(qū)別是：

  • NewInstanceFactory創(chuàng)建ViewModel時(shí)，會為每個(gè)Activity或Fragment創(chuàng)建一個(gè)新的ViewModel實(shí)例，這會導(dǎo)致ViewModel無法在應(yīng)用程序的不同部分共享數(shù)據(jù)。（ComponentActivity源碼getDefaultViewModelProviderFactory方法）

  • AndroidViewModelFactory可以訪問應(yīng)用程序的全局狀態(tài)，并且ViewModel實(shí)例可以在整個(gè)應(yīng)用程序中是共享的。

根據(jù)我們的需求，需要用的是AndroidViewModelFactory。

二、具體實(shí)現(xiàn)

1、方式一：可以全局添加和獲取任意ViewModel

定義Application，Ktx.kt文件

import android.app.Application

lateinit var appContext: Application

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

定義全局可用的ViewModelOwner實(shí)現(xiàn)類

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)
    }
}

定義一個(gè)ViewModel通過StateFlow定義發(fā)送和訂閱事件的方法

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)
            }
        }
    }
}

定義一個(gè)調(diào)用的類

object FlowEvent {

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

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

測試代碼如下:

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        //打印協(xié)程名稱
        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
內(nèi)容:MainActivity observeEvent before :0 線程:main @coroutine#1
內(nèi)容:MainActivity observeEvent before :1 線程:main @coroutine#1
內(nèi)容:MainActivity observeEvent after :1 線程:main @coroutine#2

2、方式二：更方便在Activity和Fragment中調(diào)用

定義Application，讓BaseApplication實(shí)現(xiàn)ViewModelStoreOwner

//BaseApplication實(shí)現(xiàn)ViewModelStoreOwner接口
class BaseApplication : Application(), ViewModelStoreOwner {

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

    override fun onCreate() {
        super.onCreate()
        //設(shè)置全局的上下文
        setApplicationContext(this)
        //創(chuàng)建ViewModelStore
        mAppViewModelStore = ViewModelStore()

    }

    override fun getViewModelStore(): ViewModelStore = mAppViewModelStore

    /**
     * 獲取一個(gè)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文件也有變化，如下

lateinit var appContext: Application

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

//定義擴(kuò)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)
        }
    }
}

//定義擴(kuò)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中調(diào)用上述擴(kuò)展方法

abstract class BaseActivity: AppCompatActivity() {

    //創(chuàng)建ViewModel對象
    val eventViewModel: EventViewModel by lazy { getAppViewModel() }

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

    }
}

abstract class BaseFragment: Fragment() {

    //創(chuàng)建ViewModel對象
    val eventViewModel: EventViewModel by lazy { getAppViewModel() }

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

    }
}

測試代碼

class MainActivity : BaseActivity() {

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

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

        findViewById<AppCompatButton>(R.id.bt).setOnClickListener {
            //點(diǎn)擊按鈕修改值
            eventViewModel.postEvent(1)
            //跳轉(zhuǎn)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

內(nèi)容:MainActivity observeEvent :0 線程:main @coroutine#1
內(nèi)容:MainActivity observeEvent :1 線程:main @coroutine#1
內(nèi)容:TwoActivity observeEvent :1 線程:main @coroutine#2

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