Android EventBus基础入门及源码分析

—— 迷茫是什么？迷茫是大事干不了，小事不想干。能力配不上欲望，才华配不上梦想。

目录

前言

一、简介

​二、基本使用

三、源码分析

四、内容推荐

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前言

时隔多年，那些曾经学过且用过的知识早已记忆模糊。如果不反复研究学习，使用起来也会很生涩，如新知识一样。本编为巩固EventBus所写。一个人为什么要努力，因为喜欢的东西很贵想去的地方都很远，想爱的人很完美。

一、简介

官方文档：https://greenrobot.org/eventbus/documentation/

Github：https://github.com/greenrobot/EventBus

（1）是什么：是一个事件发布/订阅的轻量级框架。基于观察者模式，实现组件间的通讯。代码简洁且解耦。

（2）有什么用：可以替代传统的Intent,Handler,Broadcast或接口函数。

​二、基本使用

（1）添加依赖  （不是最新） 基于以前学过的版本

implementation 'org.greenrobot:eventbus:3.0.0'

（2）定义消息事件（可以配置传递参数）

public static class MessageEvent { /* Additional fields if needed */ }

（3）定义接收事件的线程方法（发送的事件，将在该方法中收到）

@Subscribe(threadMode = ThreadMode.MAIN)  
public void onMessageEvent(MessageEvent event) {/* Do something */};

（4）EventBus初始化 （与广播相似，需要订阅与取消）

 @Override
 public void onStart() {
     super.onStart();
     EventBus.getDefault().register(this);
 }

 @Override
 public void onStop() {
     super.onStop();
     EventBus.getDefault().unregister(this);
 }

（5）发送事件

EventBus.getDefault().post(new MessageEvent());

（6）添加混淆

-keepattributes *Annotation*
-keepclassmembers class * {
    @org.greenrobot.eventbus.Subscribe <methods>;
}
-keep enum org.greenrobot.eventbus.ThreadMode { *; }
 
# Only required if you use AsyncExecutor
-keepclassmembers class * extends org.greenrobot.eventbus.util.ThrowableFailureEvent {
    <init>(java.lang.Throwable);
}

简单的整理了一下 。正确姿势参考官方文档。

三、源码分析

（1）EventBus.getDefault()

* 单例模式 双重效验锁  线程安全 懒加载
public static EventBus getDefault() {
   if (defaultInstance == null) {
       synchronized (EventBus.class) {
          if (defaultInstance == null) {
              defaultInstance = new EventBus();
          }
       }
    }
    return defaultInstance;
}

* 使用构建者配置EventBus 属性
    private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
    public EventBus() {
        this(DEFAULT_BUILDER);
    }

    * 属性简介
   EventBus(EventBusBuilder builder) {
        * 保存Event集合
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        * 线程调度
        mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);
        * 索引
        indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
        * EventBus订阅方法
        subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
                builder.strictMethodVerification, builder.ignoreGeneratedIndex);
        * EventBus日志
        logSubscriberExceptions = builder.logSubscriberExceptions;
        logNoSubscriberMessages = builder.logNoSubscriberMessages;
        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
        * 无消息发送
        sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
        * 异常事件
        throwSubscriberException = builder.throwSubscriberException;
        * EventBus继承关系
        eventInheritance = builder.eventInheritance;
        * 线程池
        executorService = builder.executorService;
    }

（2）EventBus.getDefault().register(this)

* 注册给定的订阅方以接收事件
public void register(Object subscriber) {
	* 利用反射获取订阅的类
	Class subscriberClass = subscriber.getClass();
	* 根据订阅的类找到 该类下的订阅方法   -> 1
	List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
	synchronized (this) {
		* 便利所有的订阅方法
		for (SubscriberMethod subscriberMethod : subscriberMethods) {
                        * --> 2
			subscribe(subscriber, subscriberMethod);
		}
	}
}

1.subscriberMethodFinder.findSubscriberMethods(subscriberClass)

* 获取所有的订阅方法
List findSubscriberMethods(Class subscriberClass) {
	* 判断是否已缓存
	List subscriberMethods = METHOD_CACHE.get(subscriberClass);
	if (subscriberMethods != null) {
		return subscriberMethods;
	}
	* ignoreGeneratedIndex 忽略生成的索引 默认为false
	if (ignoreGeneratedIndex) {
		subscriberMethods = findUsingReflection(subscriberClass);
	} else {
		* 通过反射获取到订阅方法列表 -> 1.1
		subscriberMethods = findUsingInfo(subscriberClass);
	}
	* 订阅方法列表为空时 抛出异常
	if (subscriberMethods.isEmpty()) {
		throw new EventBusException("Subscriber" + subscriberClass
				+ " and its super classes have no public methods with the @Subscribe annotation");
	} else {
		* 缓存该订阅类的所有订阅方法
		METHOD_CACHE.put(subscriberClass, subscriberMethods);
		return subscriberMethods;
	}
}

1.1.findUsingInfo(Class subscriberClass)

* 通过反射获取到订阅方法列表
private List findUsingInfo(Class subscriberClass) {
	* 创建FindState 并初始化
	FindState findState = prepareFindState();
	findState.initForSubscriber(subscriberClass);
	while (findState.clazz != null) {
		* 判断findState是否已经有缓存订阅信息
		findState.subscriberInfo = getSubscriberInfo(findState);
		if (findState.subscriberInfo != null) {
			SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
			for (SubscriberMethod subscriberMethod : array) {
				if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
					findState.subscriberMethods.add(subscriberMethod);
				}
			}
		} else {
			* 利用反射机制 将订阅方法信息 存储在findState 中
			findUsingReflectionInSingleClass(findState);
		}
		* 移除订阅类
		findState.moveToSuperclass();
	}
	* 回收FindState对象，获取订阅方法列表
	return getMethodsAndRelease(findState);
}

2.subscribe(Object subscriber, SubscriberMethod subscriberMethod)

* 判断是否已经注册/订阅过该事件
* 按照优先级缓存订阅事件
* 判断是否已经缓存在typesBySubscriber中 
* 判断是否是粘性事件  并分发粘性事件

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
	* 订阅方法类型
	Class< eventType = subscriberMethod.eventType;
	* 创建 订阅事件
	Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
	* 判断是否缓存过 订阅事件 列表
	CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
	if (subscriptions == null) {
		subscriptions = new CopyOnWriteArrayList<>();
		subscriptionsByEventType.put(eventType, subscriptions);
	} else {
		* 判断是否已经订阅过  订阅过则抛出异常
		if (subscriptions.contains(newSubscription)) {
			throw new EventBusException("Subscriber" + subscriber.getClass() + " already registered to event "+ eventType);
		}
	}
	* 按照优先级缓存订阅事件  subscriptionsByEventType
	int size = subscriptions.size();
	for (int i = 0; i <= size; i++) {
		if (i == size || subscriberMethod.priority >subscriptions.get(i).subscriberMethod.priority) {
			subscriptions.add(i, newSubscription);
			break;
		}
	}
	* 判断是否已经缓存在typesBySubscriber中  
	List> subscribedEvents = typesBySubscriber.get(subscriber);
	if (subscribedEvents == null) {
		subscribedEvents = new ArrayList<>();
		typesBySubscriber.put(subscriber, subscribedEvents);
	}
	subscribedEvents.add(eventType);
	* 判断是否是粘性事件  并分发粘性事件
	if (subscriberMethod.sticky) {
		if (eventInheritance) {
			Set, Object>> entries = stickyEvents.entrySet();
			for (Map.Entry, Object> entry : entries) {
				Class candidateEventType = entry.getKey();
				if (eventType.isAssignableFrom(candidateEventType)) {
					Object stickyEvent = entry.getValue();
					* 分发事件
					checkPostStickyEventToSubscription(newSubscription, stickyEvent);
				}
			}
		} else {
			Object stickyEvent = stickyEvents.get(eventType);
			* 分发事件  -->2.1
			checkPostStickyEventToSubscription(newSubscription, stickyEvent);
		}
	}
}

2.1checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent)

 * 判断粘性事件是否为空  
private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
	if (stickyEvent != null) {
		// If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
		// --> Strange corner case, which we don't take care of here.
		postToSubscription(newSubscription, stickyEvent, Looper.getMainLooper() == Looper.myLooper());
	}
}
* 根据线程模式进行事件分发
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
	switch (subscription.subscriberMethod.threadMode) {
		case POSTING:
			invokeSubscriber(subscription, event);
			break;
		case MAIN:
			if (isMainThread) {
				invokeSubscriber(subscription, event);
			} else {
				mainThreadPoster.enqueue(subscription, event);
			}
			break;
		case BACKGROUND:
			if (isMainThread) {
				backgroundPoster.enqueue(subscription, event);
			} else {
				invokeSubscriber(subscription, event);
			}
			break;
		case ASYNC:
			asyncPoster.enqueue(subscription, event);
			break;
		default:
			throw new IllegalStateException("Unknown thread mode:" + subscription.subscriberMethod.threadMode);
	}
}


* 利用反射 执行订阅方法
void invokeSubscriber(Subscription subscription, Object event) {
	try {
		subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
	} catch (InvocationTargetException e) {
		handleSubscriberException(subscription, event, e.getCause());
	} catch (IllegalAccessException e) {
		throw new IllegalStateException("Unexpected exception", e);
	}
}

*  将事件添加到 PendingPostQueue 队列中  执行handler 从队列冲取出消息进行处理 并利用反射 执行订阅方法 
void enqueue(Subscription subscription, Object event) {
	PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
	synchronized (this) {
		queue.enqueue(pendingPost);
		if (!handlerActive) {
			handlerActive = true;
			if (!sendMessage(obtainMessage())) {
				throw new EventBusException("Could not send handler message");
			}
		}
	}
}

总结:

1.利用反射获取到订阅类的所有订阅方法

2.判断是否已经注册/订阅过该事件

3.按照优先级缓存订阅事件

4.判断是否是粘性事件 并分发粘性事件  (1)同一个线程 利用反射 执行订阅方法 (2)不同线程 将事件添加到 PendingPostQueue 队列中 执行handler 从队列冲取出消息进行处理 并利用反射 执行订阅方法 

（3）EventBus.getDefault().post(new MessageEvent());

* 发送事件
public void post(Object event) {
	* 获取当前线程的信息
	PostingThreadState postingState = currentPostingThreadState.get();
	* 将事件添加到当前线程的队列中
	List