背景

跨页面通信是一个比较常见的场景，通常我们会选择使用EventBus，但EventBus无法感知声明周期，收到消息就会回调，所以有了LiveData之后很快就有了LiveEventBus。不过它也有缺点，比如不能切换线程。现在SharedFlow稳定了，那是不是也能搞一波？

于是有了FlowEventBus

常用消息总线对比

消息总线	延迟发送	有序接收消息	Sticky	生命周期感知	跨进程/APP	线程分发
EventBus	❌	✅	✅	❌	❌	✅
RxBus	❌	✅	✅	❌	❌	✅
LiveEventBus	✅	✅	✅	✅	✅	❌
FlowEventBus	✅	✅	✅	✅	❌	✅

设计构思

通过学习从 LiveData 迁移到 Kotlin 数据流得到思路：

SharedFlow作为事件载体：
优点：
依托协程轻松切换线程
可以通过replay实现粘性效果
可以被多个观察者订阅
无观察者自动清除事件不会造成积压

结合 Lifecycle 感知生命周期，做到响应时机可控

依赖库版本

关键在于 kotlinx-coroutines > 1.4.x 和 lifecycle-runtime-ktx > 2.3.x

API

一般注册与发送

简单的事件接收注册

observeEvent<String>(eventName="SimpleEvent") { value ->
   ...
}

简单的事件发送

postEvent(eventName="SimpleEvent",eventValue="Let's do it")

自定义事件发送

postEvent(CustomEvent(value = "Hello Word"))

自定义事件接收注册

observeEvent<CustomEvent> { event ->
  ...
}

延迟发送

postDelayEvent(CustomEvent(name = "Hello Word"),1000)

发送粘性事件

postStickyEvent(eventName = STICKY,value = "☝ 粘性事件️")

指定最小感知生命周期(默认 Lifecycle.State.Started)

observeEvent<String>("SimpleEvent",Lifecycle.State.DESTROYED) { value ->
   ...
}

切换接收线程

observeEvent<String>("SimpleEvent",Dispatchers.IO) { value ->
    ...
}

原理

以上功能依托于Kotlin协程的SharedFlow和Lifecycle 因此实现起来非常简单。

粘性事件

MutableSharedFlow<Any>(
    replay = if (isSticky) 1 else 0,
    extraBufferCapacity = Int.MAX_VALUE //避免挂起导致数据发送失败
)

生命周期感知

fun <T> LifecycleOwner.launchWhenStateAtLeast(
    minState: Lifecycle.State,
    block: suspend CoroutineScope.() -> T
) {
    lifecycleScope.launch {
        lifecycle.whenStateAtLeast(minState, block)
    }
}

切换线程 whenStateAtLeast 由于执行的block默认是在主线程，因此需要手动切换线程：

lifecycleOwner.launchWhenStateAtLeast(minState) {
    flow.collect { value ->
        lifecycleOwner.lifecycleScope.launch(dispatcher) {
                onReceived.invoke(value as T)
        }
    }
}

延迟事件

viewModelScope.launch {
    delay(time)
    flow.emit(value)
}

有序分发
SharedFlow本质类似阻塞队列,是有序的。
全局单例
使用全局ViewModel，主要是因为有ViewModelScope，可以避免使用GlobalScope，如果想要单页面内部组件通信，那就使用ActivityScope的ViewModel就行啦：

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(EventBusInitializer.application)
        )
    }

    fun <T : ViewModel> getApplicationScopeViewModel(modelClass: Class<T>): T {
        return mApplicationProvider[modelClass]
    }
}

ViewModel内部就一个map和注册，发送方法，非常简单：

internal class EventBusViewModel : ViewModel() {

    private val eventFlows: HashMap<String, MutableSharedFlow<Any>> = HashMap()
   
    ...

}

总结

站在巨人的肩膀上的同时也可以简单了解下原理。不过挺复杂的，需要下点功夫😄

kotlinx.coroutines.flow

使用

Add it in your root build.gradle at the end of repositories:

allprojects {
		repositories {
			...
			maven { url 'https://jitpack.io' }
		}
	}

Step 2. Add the dependency

dependencies {
	        implementation 'com.github.biubiuqiu0:flow-event-bus:0.0.1'
	}

Step 3. Init

class MyApplication: Application() {
    override fun onCreate() {
        super.onCreate()
        EventBusInitializer.init(this)
    }
}

License

MIT License

Copyright (c) 2021 Compose-Museum

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

xiangyao0906/flow-event-bus

背景

常用消息总线对比

设计构思

依赖库版本

API

原理

总结

使用

License