Android EventBus 3.1.1最新源码分析

前言

EventBus 是一款在 Android 开发中经常使用的发布/订阅事件总线框架，将事件的接收者和发送者分开；简化了组件之间的通信，使用简单、效率高、体积小！由于经常使用到，所以我们深入理解该框架的原理就很有必要了，下面我们就来分析一下EventBus原理。

EventBus构造方法

大家都知道当我们在使用EventBus时，首先会调用EventBus.getDefault（）方法来获取EventBus的实例。那我们来看一下getDefault（）方法，代码如下：

	/** Convenience singleton for apps using a process-wide EventBus instance. */
    public static EventBus getDefault() {
        if (defaultInstance == null) {
            synchronized (EventBus.class) {
                if (defaultInstance == null) {
                    defaultInstance = new EventBus();
                }
            }
        }
        return defaultInstance;
    }

这是一个双重检查模式单例模式。
接下来是EventBus构造方法，代码如下：

	/**
     * Creates a new EventBus instance; each instance is a separate scope in which events are delivered. To use a
     * central bus, consider {@link #getDefault()}.
     */
    public EventBus() {
        this(DEFAULT_BUILDER);
    }
    
    EventBus(EventBusBuilder builder) {
        logger = builder.getLogger();
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        mainThreadSupport = builder.getMainThreadSupport();
        mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);
        indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
        subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
                builder.strictMethodVerification, builder.ignoreGeneratedIndex);
        logSubscriberExceptions = builder.logSubscriberExceptions;
        logNoSubscriberMessages = builder.logNoSubscriberMessages;
        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
        sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
        throwSubscriberException = builder.throwSubscriberException;
        eventInheritance = builder.eventInheritance;
        executorService = builder.executorService;
    }

这里DEFAULT_BUILDER是默认的EventBusBuilder：

private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();

这里调用了EventBus的另外一个构造方法，采用了建造者模式，通过构造EventBusBuilder来配置EventBus。

订阅者注册

获取到EventBus的实例以后就调用register(this)方法将订阅者注册到EventBus中，register（）方法代码如下：

	public void register(Object subscriber) {
        Class subscriberClass = subscriber.getClass();
        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);//得到一个SubscriberMethod的集合，就是传进来的订阅者的所有订阅方法
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {//遍历订阅者的订阅方法来完成订阅者的注册操作
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

register方法做了两件事：一件事是查找订阅者的订阅方法，另一件事是订阅者的注册。
SubscriberMethod类主要用来保存订阅方法的Method对象、线程模式、事件类型、优先级、是否是黏性事件等属性。

public class SubscriberMethod {
    final Method method;
    final ThreadMode threadMode;
    final Class eventType;
    final int priority;
    final boolean sticky;
    /** Used for efficient comparison */
    String methodString;

    public SubscriberMethod(Method method, Class eventType, ThreadMode threadMode, int priority, boolean sticky) {
        this.method = method;
        this.threadMode = threadMode;
        this.eventType = eventType;
        this.priority = priority;
        this.sticky = sticky;
    }
	....
	....
	其他方法
	....
	....
}

查找订阅者方法

接下来看看findSubscriberMethods(subscriberClass)方法代码：

	List findSubscriberMethods(Class subscriberClass) {
        List subscriberMethods = METHOD_CACHE.get(subscriberClass);//从缓存中查找是否有订阅方法的集合
        if (subscriberMethods != null) {//如果找到就return 
            return subscriberMethods;
        }
		//如果缓存中没有，则根据 ignoreGeneratedIndex属性的值来选择采用何种方法来查找订阅方法的集合
		//ignoreGeneratedIndex 的默认值是 false，可以通过EventBusBuilder来设置它的值
        if (ignoreGeneratedIndex) {
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
            subscriberMethods = findUsingInfo(subscriberClass);
        }
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass
                    + " and its super classes have no public methods with the @Subscribe annotation");
        } else {
            METHOD_CACHE.put(subscriberClass, subscriberMethods);//找到订阅方法的集合后，放入缓存
            return subscriberMethods;
        }
    }

ignoreGeneratedIndex 属性表示是否忽略注解器生成的 MyEventBusIndex，具体可以看官网：http://greenrobot.org/eventbus/documentation/subscriber-index/

项目中经常通过EventBus单例模式来获取默认的EventBus对象，也就是ignoreGeneratedIndex为false的情况，这种情况调用了findUsingInfo(subscriberClass)方法，代码如下：

	private List findUsingInfo(Class subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            findState.subscriberInfo = getSubscriberInfo(findState);//获取订阅者信息
            if (findState.subscriberInfo != null) {
            	//如果我们通过EventBusBuilder配置了MyEventBusIndex，便会获取到subscriberInfo
                SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();//得到订阅方法相关的信息
                for (SubscriberMethod subscriberMethod : array) {
                    if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                        findState.subscriberMethods.add(subscriberMethod);
                    }
                }
            } else {//没有配置MyEventBusIndex
                findUsingReflectionInSingleClass(findState);//将订阅方法保存到findState中
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);//对findState做回收处理并返回订阅方法的List集合
    }

在开始查找订阅方法的时候并没有忽略注解器为我们生成的索引 MyEventBusIndex。

如果我们通过EventBusBuilder配置了MyEventBusIndex，便会获取subscriberInfo（这个参考上面ignoreGeneratedIndex为true的情况所调用的方法findUsingReflection(subscriberClass)）；
没有配置MyEventBusIndex便会执行findUsingReflectionInSingleClass(findState)方法；

最后再通过getMethodsAndRelease(findState)方法对findState做回收处理并返回订阅方法的List集合，这里贴一下getMethodsAndRelease(findState)方法代码：

	private List getMethodsAndRelease(FindState findState) {
        List subscriberMethods = new ArrayList<>(findState.subscriberMethods);
        findState.recycle();
        synchronized (FIND_STATE_POOL) {
            for (int i = 0; i < POOL_SIZE; i++) {
                if (FIND_STATE_POOL[i] == null) {
                    FIND_STATE_POOL[i] = findState;
                    break;
                }
            }
        }
        return subscriberMethods;
    }

默认情况下是没有配置MyEventBusIndex的，所以就会调用findUsingReflectionInSingleClass(findState)方法，代码如下：

	private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // This is faster than getMethods, especially when subscribers are fat classes like Activities
            methods = findState.clazz.getDeclaredMethods();//通过反射来获取订阅者中的方法
        } catch (Throwable th) {
            // Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
            methods = findState.clazz.getMethods();
            findState.skipSuperClasses = true;
        }
        for (Method method : methods) {
            int modifiers = method.getModifiers();
            //根据方法的类型、参数和注解来找到订阅方法。找到订阅方法后将订阅方法的相关信息保存到findState中。
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class[] parameterTypes = method.getParameterTypes();
                if (parameterTypes.length == 1) {
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        Class eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                    subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                        }
                    }
                } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                    String methodName = method.getDeclaringClass().getName() + "." + method.getName();
                    throw new EventBusException("@Subscribe method " + methodName +
                            "must have exactly 1 parameter but has " + parameterTypes.length);
                }
            } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                String methodName = method.getDeclaringClass().getName() + "." + method.getName();
                throw new EventBusException(methodName +
                        " is a illegal @Subscribe method: must be public, non-static, and non-abstract");
            }
        }
    }

注册过程

上面register方法里面遍历SubscriberMethod对所有的订阅方法进行注册，调用了subscribe(subscriber, subscriberMethod)方法，代码如下：

	// Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class eventType = subscriberMethod.eventType;
        //根据subscriber（订阅者）和subscriberMethod（订阅方法）创建一个Subscription（订阅对象）
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        //根据eventType（事件类型）获取Subscriptions（订阅对象集合）
        CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
        	//如果Subscriptions为null则重新创建，并将Subscriptions根据eventType保存在subscriptionsByEventType中
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
        	//判断订阅者是否被注册
            if (subscriptions.contains(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }

        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);//按照订阅方法的优先级插入到订阅对象集合中，完成订阅方法的注册
                break;
            }
        }

        List> subscribedEvents = typesBySubscriber.get(subscriber);//通过subscriber获取subscribedEvents事件类型集合
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();//为空重新创建对象
            typesBySubscriber.put(subscriber, subscribedEvents);//根据subscriber将subscribedEvents存储在typesBySubscriber集合中
        }
        subscribedEvents.add(eventType);//将eventType添加到subscribedEvents中

        if (subscriberMethod.sticky) {
            if (eventInheritance) {//黏性事件处理
                // Existing sticky events of all subclasses of eventType have to be considered.
                // Note: Iterating over all events may be inefficient with lots of sticky events,
                // thus data structure should be changed to allow a more efficient lookup
                // (e.g. an additional map storing sub classes of super classes: Class -> List).
                //从stickyEvents事件保存队列中取出该事件类型的事件发送给当前订阅者
                Set, Object>> entries = stickyEvents.entrySet();
                for (Map.Entry, Object> entry : entries) {
                    Class candidateEventType = entry.getKey();
                    if (eventType.isAssignableFrom(candidateEventType)) {
                        Object stickyEvent = entry.getValue();
                        checkPostStickyEventToSubscription(newSubscription, stickyEvent);
                    }
                }
            } else {
                Object stickyEvent = stickyEvents.get(eventType);
                checkPostStickyEventToSubscription(newSubscription, stickyEvent);
            }
        }
    }

subscribe（）方法主要就是做了两件事：
1、将Subscriptions根据eventType封装到subscriptionsByEventType中，将subscribedEvents根据subscriber封装到typesBySubscriber中；
2、对黏性事件的处理。

发送事件

我们通过post方法来对事件进行发送，所以我们看一下post（）方法源码：

	/** Posts the given event to the event bus. */
    public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List