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一. 背景

这段时间接手了一个比较紧急的项目，它是一个运行在某开发板上的 Android 项目。

该项目采用的架构比较老，例如 RxJava 还在使用 1.x 的版本。起初看到源码，我内心是拒绝的。(这大半年来，我在使用 C++ 开发桌面端、 Java/Kotlin 开发后端，不过没关系。)好在该项目最近开发的部分功能采用 Kotlin 编写，那我开发的功能也打算使用 Kotlin。

二. RxJava 版本的 EventBus

两年前，我在写《RxJava 2.x 实战》的时候，写过一个 RxJava 2 版本的 EventBus，并且在实际的项目中验证过。

它还需要一个第三方库 RxRelay。RxRelay中的各个 Relay 既是 Observable 也是 Consumer 的 RxJava 类型，它们是一个没有 onComplete 和 onError 的Subject。所以不必要担心下游的触发的终止状态（onComplete 或 onError）。

RxRelay的Github地址：https://github.com/JakeWharton/RxRelay

RxBus 的源码：

package com.safframework.study.rxbus4;

import com.jakewharton.rxrelay2.PublishRelay;
import com.jakewharton.rxrelay2.Relay;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import io.reactivex.Observable;
import io.reactivex.ObservableEmitter;
import io.reactivex.ObservableOnSubscribe;
import io.reactivex.Scheduler;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.annotations.NonNull;
import io.reactivex.disposables.Disposable;
import io.reactivex.functions.Action;
import io.reactivex.functions.Consumer;

/**
 * Created by Tony Shen on 2017/6/14.
 */

public class RxBus {

    private Relay<Object> bus = null;
    private static RxBus instance;
    private final Map<Class<?>, Object> mStickyEventMap;

    //禁用构造方法
    private RxBus() {
        bus = PublishRelay.create().toSerialized();
        mStickyEventMap = new ConcurrentHashMap<>();
    }

    public static RxBus get() {
        return Holder.BUS;
    }

    public void post(Object event) {
        bus.accept(event);
    }

    public void postSticky(Object event) {
        synchronized (mStickyEventMap) {
            mStickyEventMap.put(event.getClass(), event);
        }
        bus.accept(event);
    }

    public <T> Observable<T> toObservable(Class<T> eventType) {
        return bus.ofType(eventType);
    }

    /**
     * 根据传递的 eventType 类型返回特定类型(eventType)的 被观察者
     */
    public <T> Observable<T> toObservableSticky(final Class<T> eventType) {
        synchronized (mStickyEventMap) {
            Observable<T> observable = bus.ofType(eventType);
            final Object event = mStickyEventMap.get(eventType);

            if (event != null) {
                return observable.mergeWith(Observable.create(new ObservableOnSubscribe<T>() {
                    @Override
                    public void subscribe(@NonNull ObservableEmitter<T> e) throws Exception {
                        e.onNext(eventType.cast(event));
                    }
                }));
            } else {
                return observable;
            }
        }
    }

    public boolean hasObservers() {
        return bus.hasObservers();
    }

    public <T> Disposable register(Class<T> eventType, Scheduler scheduler, Consumer<T> onNext) {
        return toObservable(eventType).observeOn(scheduler).subscribe(onNext);
    }

    public <T> Disposable register(Class<T> eventType, Scheduler scheduler, Consumer<T> onNext, Consumer onError,
                                   Action onComplete, Consumer onSubscribe) {
        return toObservable(eventType).observeOn(scheduler).subscribe(onNext, onError, onComplete, onSubscribe);
    }

    public <T> Disposable register(Class<T> eventType, Scheduler scheduler, Consumer<T> onNext, Consumer onError,
                                   Action onComplete) {
        return toObservable(eventType).observeOn(scheduler).subscribe(onNext, onError, onComplete);
    }

    public <T> Disposable register(Class<T> eventType, Scheduler scheduler, Consumer<T> onNext, Consumer onError) {
        return toObservable(eventType).observeOn(scheduler).subscribe(onNext, onError);
    }

    public <T> Disposable register(Class<T> eventType, Consumer<T> onNext) {
        return toObservable(eventType).observeOn(AndroidSchedulers.mainThread()).subscribe(onNext);
    }

    public <T> Disposable register(Class<T> eventType, Consumer<T> onNext, Consumer onError,
                                   Action onComplete, Consumer onSubscribe) {
        return toObservable(eventType).observeOn(AndroidSchedulers.mainThread()).subscribe(onNext, onError, onComplete, onSubscribe);
    }

    public <T> Disposable register(Class<T> eventType, Consumer<T> onNext, Consumer onError,
                                   Action onComplete) {
        return toObservable(eventType).observeOn(AndroidSchedulers.mainThread()).subscribe(onNext, onError, onComplete);
    }

    public <T> Disposable register(Class<T> eventType, Consumer<T> onNext, Consumer onError) {
        return toObservable(eventType).observeOn(AndroidSchedulers.mainThread()).subscribe(onNext, onError);
    }

    public <T> Disposable registerSticky(Class<T> eventType, Scheduler scheduler, Consumer<T> onNext) {
        return toObservableSticky(eventType).observeOn(scheduler).subscribe(onNext);
    }

    public <T> Disposable registerSticky(Class<T> eventType, Consumer<T> onNext) {
        return toObservableSticky(eventType).observeOn(AndroidSchedulers.mainThread()).subscribe(onNext);
    }

    public <T> Disposable registerSticky(Class<T> eventType, Consumer<T> onNext, Consumer onError) {
        return toObservableSticky(eventType).observeOn(AndroidSchedulers.mainThread()).subscribe(onNext,onError);
    }

    /**
     * 移除指定eventType的Sticky事件
     */
    public <T> T removeStickyEvent(Class<T> eventType) {
        synchronized (mStickyEventMap) {
            return eventType.cast(mStickyEventMap.remove(eventType));
        }
    }

    /**
     * 移除所有的Sticky事件
     */
    public void removeAllStickyEvents() {
        synchronized (mStickyEventMap) {
            mStickyEventMap.clear();
        }
    }

    public void unregister(Disposable disposable) {
        if (disposable != null && !disposable.isDisposed()) {
            disposable.dispose();
        }
    }

    private static class Holder {
        private static final RxBus BUS = new RxBus();
    }

}

该版本 RxBus 支持异常处理和 Sticky 事件。唯一的缺点是，不支持 Backpressure。

三. Kotlin Coroutine 版本的 EventBus

既然有了之前的 RxBus，为何要重新写一个呢？

首先，我们目前的项目并没有采用 EventBus。但是，我写的某一个 Service 需要跟 Activities 通信。我想偷懒，当然采用 EventBus 会比较简单。但是，我们的 RxJava 版本还在用 1.x！！

幸好，我们用了 Kotlin，部分代码还用了 Coroutine，于是我想到了使用 Coroutine 的 Channel 来实现 EventBus。

Channel 可以实现协程之间的数据通信。Kotlin 的 Channel 与 Java 的 BlockingQueue 类似。BlockingQueue 的 put 和 take 操作，相当于 Channel 的 send 和 receive 操作，但是 BlockingQueue 是阻塞操作而 Channel 都是挂起操作。

EventBus 用于注册普通事件、Sticky 事件，事件的发布等等。

package com.safframework.eventbus

import android.util.Log
import kotlinx.coroutines.*
import java.util.concurrent.ConcurrentHashMap
import kotlin.coroutines.CoroutineContext

/**
 *
 * @FileName:
 *          com.safframework.eventbus.EventBus
 * @author: Tony Shen
 * @date: 2019-08-24 23:28
 * @version: V1.0 <描述当前版本功能>
 */
val UI: CoroutineDispatcher = Dispatchers.Main

object EventBus: CoroutineScope {

    private val TAG = "EventBus"

    private val job = SupervisorJob()

    override val coroutineContext: CoroutineContext = Dispatchers.Default + job

    private val contextMap = ConcurrentHashMap<String, MutableMap<Class<*>, EventData<*>>>()
    private val mStickyEventMap = ConcurrentHashMap<Class<*>, Any>()

    @JvmStatic
    fun <T> register(
        contextName: String,
        eventDispatcher: CoroutineDispatcher = UI,
        eventClass: Class<T>,
        eventCallback: (T) -> Unit
    ) {
        val eventDataMap = if (contextMap.containsKey(contextName)) {
            contextMap[contextName]!!
        } else {
            val eventDataMap = mutableMapOf<Class<*>, EventData<*>>()
            contextMap[contextName] = eventDataMap
            eventDataMap
        }

        eventDataMap[eventClass] = EventData(this, eventDispatcher, eventCallback)
    }

    @JvmStatic
    fun <T> register(
        contextName: String,
        eventDispatcher: CoroutineDispatcher = UI,
        eventClass: Class<T>,
        eventCallback: (T) -> Unit,
        eventFail:(Throwable)->Unit
    ) {
        val eventDataMap = if (contextMap.containsKey(contextName)) {
            contextMap[contextName]!!
        } else {
            val eventDataMap = mutableMapOf<Class<*>, EventData<*>>()
            contextMap[contextName] = eventDataMap
            eventDataMap
        }

        eventDataMap[eventClass] = EventData(this, eventDispatcher, eventCallback, eventFail)
    }

    @JvmStatic
    fun <T> registerSticky(
        contextName: String,
        eventDispatcher: CoroutineDispatcher = UI,
        eventClass: Class<T>,
        eventCallback: (T) -> Unit
    ) {
        val eventDataMap = if (contextMap.containsKey(contextName)) {
            contextMap[contextName]!!
        } else {
            val eventDataMap = mutableMapOf<Class<*>, EventData<*>>()
            contextMap[contextName] = eventDataMap
            eventDataMap
        }

        eventDataMap[eventClass] = EventData(this, eventDispatcher, eventCallback)

        val event = mStickyEventMap[eventClass]
        event?.let {

            postEvent(it)
        }
    }

    @JvmStatic
    fun <T> registerSticky(
        contextName: String,
        eventDispatcher: CoroutineDispatcher = UI,
        eventClass: Class<T>,
        eventCallback: (T) -> Unit,
        eventFail:(Throwable)->Unit
    ) {
        val eventDataMap = if (contextMap.containsKey(contextName)) {
            contextMap[contextName]!!
        } else {
            val eventDataMap = mutableMapOf<Class<*>, EventData<*>>()
            contextMap[contextName] = eventDataMap
            eventDataMap
        }

        eventDataMap[eventClass] = EventData(this, eventDispatcher, eventCallback, eventFail)

        val event = mStickyEventMap[eventClass]
        event?.let {

            postEvent(it)
        }
    }

    @JvmStatic
    fun post(event: Any, delayTime: Long = 0) {

        if (delayTime > 0) {
            launch {
                delay(delayTime)
                postEvent(event)
            }
        } else {
            postEvent(event)
        }
    }

    @JvmStatic
    fun postSticky(event: Any) {

        mStickyEventMap[event.javaClass] = event
    }

    @JvmStatic
    fun unregisterAllEvents() {

        Log.i(TAG,"unregisterAllEvents()")

        coroutineContext.cancelChildren()
        for ((_, eventDataMap) in contextMap) {
            eventDataMap.values.forEach {
                it.cancel()
            }
            eventDataMap.clear()
        }
        contextMap.clear()
    }

    @JvmStatic
    fun unregister(contextName: String) {

        Log.i(TAG,"$contextName")

        val cloneContexMap = ConcurrentHashMap<String, MutableMap<Class<*>, EventData<*>>>()
        cloneContexMap.putAll(contextMap)
        val map = cloneContexMap.filter { it.key == contextName }
        for ((_, eventDataMap) in map) {
            eventDataMap.values.forEach {
                it.cancel()
            }
            eventDataMap.clear()
        }
        contextMap.remove(contextName)
    }

    @JvmStatic
    fun <T> removeStickyEvent(eventType: Class<T>) {
        mStickyEventMap.remove(eventType)
    }

    private fun postEvent(event: Any) {
        
        val cloneContexMap = ConcurrentHashMap<String, MutableMap<Class<*>, EventData<*>>>()
        cloneContexMap.putAll(contextMap)
        for ((_, eventDataMap) in cloneContexMap) {
            eventDataMap.keys
                .firstOrNull { it == event.javaClass || it == event.javaClass.superclass }
                ?.let { key -> eventDataMap[key]?.postEvent(event) }
        }
    }
}

EventData 通过 channel 实现真正的发送、消费事件。

package com.safframework.eventbus

import kotlinx.coroutines.CoroutineDispatcher
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.channels.Channel
import kotlinx.coroutines.channels.consumeEach
import kotlinx.coroutines.launch
import java.lang.Exception

/**
 *
 * @FileName:
 *          com.safframework.eventbus.EventData
 * @author: Tony Shen
 * @date: 2019-08-25 00:20
 * @version: V1.0 <描述当前版本功能>
 */

data class EventData<T>(
    val coroutineScope: CoroutineScope,
    val eventDispatcher: CoroutineDispatcher,
    val onEvent: (T) -> Unit,
    val exception: ((Throwable)->Unit)? = null
) {

    private val channel = Channel<T>()

    init {
        coroutineScope.launch {
            channel.consumeEach { // 消费者循环地消费消息
                launch(eventDispatcher) {

                    if (exception!=null) {

                        try{
                            onEvent(it)
                        } catch (e:Exception) {

                            exception.invoke(e)
                        }
                    } else {

                        onEvent(it)
                    }
                }
            }
        }
    }

    fun postEvent(event: Any) {
        if (!channel.isClosedForSend) {

            coroutineScope.launch {
                channel.send(event as T)
            }
        } else {
            println("Channel is closed for send")
        }
    }

    fun cancel() {
        channel.cancel()
    }
}

EventBus github 地址：https://github.com/fengzhizi715/EventBus

该版本的 EventBus 跟 RxBus 的功能基本一致。上述 github 地址中，包含有 demo 介绍了 EventBus 的具体使用，其实也跟 RxBus 的使用一致。

题外话，最近还抽空优化了另外一个跟 Coroutines 相关的项目：https://github.com/fengzhizi715/Lifecycle-Coroutines-Extension

四. 总结

该版本的 EventBus 是给不使用 RxBus 或者其他版本 EventBus 提供了另一种选择。当然，该项目未来还有优化的空间。