详解Android如何设计一个全局可调用的ViewModel对象
作者:TimeFine
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
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