EventBus源码解析

如果有朋友对EventBus的使用还不熟悉,建议看看
http://www.jianshu.com/p/887421af4cc1

(1)从构造方法入手

EventBus.getDefault()

单例模式
 public static EventBus getDefault() {
        if (defaultInstance == null) {
            synchronized (EventBus.class) {
                if (defaultInstance == null) {
                    defaultInstance = new EventBus();
                }
            }
        }
        return defaultInstance;
}

然后看看new EventBus()

    public EventBus() {
        this(DEFAULT_BUILDER);
    }
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();

然后看看this()方法做了什么

一系列的初始化操作
   EventBus(EventBusBuilder builder) {
        //记住这个map,它的key是订阅方法的类型(也就是订阅方法的参数类型),value是一个list,后面会用到
        subscriptionsByEventType = new HashMap<>();
        //记住这个map,它的key是订阅者,参数是该订阅者的所有订阅放方法的参数类型的集合,后面会用到
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);
        indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
//关注下这里,创建SubscriberMethodFinder的第三个参数传入的是默认值false,后面会用到
        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;
    }

(2)接着看register()方法

  public void register(Object subscriber) {
        Class subscriberClass = subscriber.getClass();
//寻找该类中所有的订阅方法,就是有@Subscribe的那些方法
        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
//把这些方法注册到EventBus
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

我们先看SubscriberMethod是什么

//这里包含了我们在写订阅方法所要考虑的那些
public class SubscriberMethod {
    final Method method;//方法
    final ThreadMode threadMode;//在哪个线程执行
    final Class eventType;//参数类型
    final int priority;//优先级
    final boolean sticky;//是否是粘性方法
    /** Used for efficient comparison */
    String methodString;
    ...
}

再看

 List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);

  List findSubscriberMethods(Class subscriberClass) {
      //如果内存中有就直接在内存中取
        List subscriberMethods = METHOD_CACHE.get(subscriberClass);
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
        ////是否忽略注解器生成的MyEventBusIndex类
        //ignoreGeneratedIndex这个参数我们在第一步就说明了,默认为false
        if (ignoreGeneratedIndex) {
            //利用反射来获取订阅类中的订阅方法信息
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
        //所以我们要看这个方法
        //从注解器生成的MyEventBusIndex类中获得订阅类的订阅方法信息
            subscriberMethods = findUsingInfo(subscriberClass);
        }
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass
                    + " and its super classes have no public methods with the @Subscribe annotation");
        } else {
            //写进缓存,下次获取更快
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }

  private List findUsingInfo(Class subscriberClass) {
        //FindState其实就是一个里面保存了订阅者和订阅方法信息的一个实体类，包括订阅类中所有订阅的事件类型和所有的订阅方法等
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            findState.subscriberInfo = getSubscriberInfo(findState);
            //findState.subscriberInfo 这个变量在FindState初始化的时候被设置为null,所以这里先不用看
            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 {
              //着重看这个方法
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // This is faster than getMethods, especially when subscribers are fat classes like Activities
          //通过反射获取到订阅类中的所有方法
            methods = findState.clazz.getDeclaredMethods();
        } 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) {
        //获取方法的声明,比如public,abstract,static等
            int modifiers = method.getModifiers();
        //该方法必须是public,而且不能是MODIFIERS_IGNORE
    //private static final int MODIFIERS_IGNORE = Modifier.ABSTRACT | Modifier.STATIC | BRIDGE | SYNTHETIC
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                //获取方法的参数
                Class[] parameterTypes = method.getParameterTypes();
              //参数只能有1个
                if (parameterTypes.length == 1) {
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                //该方法必须有Subscribe注解
                    if (subscribeAnnotation != null) {
                        Class eventType = parameterTypes[0];
                        //校验是否添加该方法,具体怎么校验的大家可以自己看看源码
                        if (findState.checkAdd(method, eventType)) {
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            //把该方法添加到list集合
                            //这个list集合的声明是final List subscriberMethods = new ArrayList<>();
                            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");
            }
        }
    }

执行完这个方法我们就获取到了该类中所有的订阅方法,并把这些订阅方法放入到FindState的List subscriberMethods = new ArrayList<>()里面
我们再返回去看看findUsingInfo()这个方法,这个方法最终的返回
getMethodsAndRelease(findState)

   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;
    }

到这里就把我们FindState中的subscriberMethods集合返回了,经过以上步骤我们得到了类中的所有订阅的方法
下面我们看看怎么注册这些方法,就是register()方法的后半部分

//这个方法的第一个参数是订阅者,第二个参数是该订阅者的所有订阅方法
  private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        //获取订阅方法的订阅类型
        Class eventType = subscriberMethod.eventType;
      //这个Subscription就两个成员变量,一个是订阅者subscriber,一个是订阅方法subscriberMethod
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {
                //当重复执行register()方法就会报此异常
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }
        //根据优先级将订阅者和订阅方法组成的newSubscription添加到list里面,
        //这个list存放的是订阅方法类型相同的Subscription.
        //优先级高的放到list的前面位置
        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;
            }
        }
        //typesBySubscriber它的key是订阅者,参数是该订阅者的所有订阅放方法的参数类型的集合
        List> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);
        //如果是粘性事件,则立即分发,具体事件分发,我们稍后再说
        if (subscriberMethod.sticky) {
            if (eventInheritance) {
                // 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).
                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);
            }
        }
    }

至此,register()方法结束,虽然代码很多,但是总结起来就是维护了两个主要的map
subscriptionsByEventType:它的key是订阅方法的类型(也就是订阅方法的参数类型),value是一个list
typesBySubscriber:它的key是订阅者,参数是该订阅者的所有订阅放方法的参数类型的集合
我们可以猜测,unregister()方法就是在这两个map中移除相应的订阅者和订阅方法

(3)下面看下事件分发post()

   public void post(Object event) {
        //获取当前线程的postingState
        //currentPostingThreadState = new ThreadLocal() 
        PostingThreadState postingState = currentPostingThreadState.get();
        //取得当前线程的事件队列
        List