EvenBus源码分析

概述

一般使用EventBus的组件类,类似下面这种方式:

public class SampleComponent extends Fragment
{

    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        EventBus.getDefault().register(this);
    }

    public void onEventMainThread(param)
    {
    }
    public void onEventPostThread(param)
    {
    }
    public void onEventBackgroundThread(param)
    {
    }
    public void onEventAsync(param)
    {
    }
    @Override
    public void onDestroy()
    {
        super.onDestroy();
        EventBus.getDefault().unregister(this);
    }
}

 

大多情况下,都会在onCreate中进行register,在onDestory中进行unregister ;

代码中以onEvent开头的方法的作用?

register(this)就是去当前类,遍历所有的方法,找到onEvent开头的然后进行存储。

onEvent后面可以写四种,也就是上面出现的四个方法,决定了当前的方法最终在什么线程运行,怎么运行

register后,调用:

EventBus.getDefault().post(param);

调用也可以叫发布,只要把这个param发布出去,EventBus会在它内部存储的方法中,进行扫描,找到参数匹配的,就使用反射进行调用。

其实EventBus就是在内部存储了一堆onEvent开头的方法,然后post的时候,根据post传入的参数,去找到匹配的方法,反射调用之。

它内部使用了Map进行存储,键就是参数的Class类型。知道是这个类型,根据post传入的参数就可以进行查找.


register

EventBus.getDefault().register(this);

首先:

EventBus.getDefault()是个单例模式,和传统的getInstance一样:

public static EventBus getDefault() {
    if(defaultInstance == null) {
        Class var0 = EventBus.class;
        synchronized(EventBus.class) {
            if(defaultInstance == null) {
                defaultInstance = new EventBus();
            }
        }
    }

    return defaultInstance;
}

 使用了双重判断的方式,防止并发的问题,还能极大的提高效率。

register是一个普通的方法:

register使用的一般有4个:

public void register(Object subscriber) {
   register(subscriber, DEFAULT_METHOD_NAME, false, 0);
}
public void register(Object subscriber, int priority) {
   register(subscriber, DEFAULT_METHOD_NAME, false, priority);
}
public void registerSticky(Object subscriber) {
   register(subscriber, DEFAULT_METHOD_NAME, true, 0);
}
public void registerSticky(Object subscriber, int priority) {
   register(subscriber, DEFAULT_METHOD_NAME, true, priority);
}

 本质上就调用了同一个:

private synchronized void register(Object subscriber, String methodName, boolean sticky, int priority) {
   List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriber.getClass(),
         methodName);
   for (SubscriberMethod subscriberMethod : subscriberMethods) {
      subscribe(subscriber, subscriberMethod, sticky, priority);
   }
}

 四个参数

subscriber 是我们扫描类的对象,也就是我们代码中常见的this;

methodName 这个是写死的:“onEvent”,用于确定扫描什么开头的方法,可见我们的类中都是以这个开头。

sticky 这个参数,解释源码的时候解释,暂时不用管

priority 优先级,优先级越高,在调用的时候会越先调用。

下面开始看代码:

List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriber.getClass(), methodName);

 调用内部类SubscriberMethodFinder的findSubscriberMethods方法,传入了subscriber 的class,以及methodName,

返回一个List<SubscriberMethod>。

然后去遍历该类内部所有方法,然后根据methodName去匹配,匹配成功的封装成SubscriberMethod,最后返回一个List。下面看代码

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass, String eventMethodName) {
   String key = subscriberClass.getName() + '.' + eventMethodName;
   List<SubscriberMethod> subscriberMethods;
   synchronized (methodCache) {
      subscriberMethods = methodCache.get(key);
   }
   if (subscriberMethods != null) {
      return subscriberMethods;
   }
   subscriberMethods = new ArrayList<SubscriberMethod>();
   Class<?> clazz = subscriberClass;
   HashSet<String> eventTypesFound = new HashSet<String>();
   StringBuilder methodKeyBuilder = new StringBuilder();
   while (clazz != null) {
      String name = clazz.getName();
      if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {
         // Skip system classes, this just degrades performance  
         break;
      }

      // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)  
      Method[] methods = clazz.getMethods();
      for (Method method : methods) {
         String methodName = method.getName();
         if (methodName.startsWith(eventMethodName)) {
            int modifiers = method.getModifiers();
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
               Class<?>[] parameterTypes = method.getParameterTypes();
               if (parameterTypes.length == 1) {
                  String modifierString = methodName.substring(eventMethodName.length());
                  ThreadMode threadMode;
                  if (modifierString.length() == 0) {
                     threadMode = ThreadMode.PostThread;
                  } else if (modifierString.equals("MainThread")) {
                     threadMode = ThreadMode.MainThread;
                  } else if (modifierString.equals("BackgroundThread")) {
                     threadMode = ThreadMode.BackgroundThread;
                  } else if (modifierString.equals("Async")) {
                     threadMode = ThreadMode.Async;
                  } else {
                     if (skipMethodVerificationForClasses.containsKey(clazz)) {
                        continue;
                     } else {
                        throw new EventBusException("Illegal onEvent method, check for typos: " + method);
                     }
                  }
                  Class<?> eventType = parameterTypes[0];
                  methodKeyBuilder.setLength(0);
                  methodKeyBuilder.append(methodName);
                  methodKeyBuilder.append('>').append(eventType.getName());
                  String methodKey = methodKeyBuilder.toString();
                  if (eventTypesFound.add(methodKey)) {
                     // Only add if not already found in a sub class  
                     subscriberMethods.add(new SubscriberMethod(method, threadMode, eventType));
                  }
               }
            } else if (!skipMethodVerificationForClasses.containsKey(clazz)) {
               Log.d(EventBus.TAG, "Skipping method (not public, static or abstract): " + clazz + "."
                     + methodName);
            }
         }
      }
      clazz = clazz.getSuperclass();
   }
   if (subscriberMethods.isEmpty()) {
      throw new EventBusException("Subscriber " + subscriberClass + " has no public methods called "
            + eventMethodName);
   } else {
      synchronized (methodCache) {
         methodCache.put(key, subscriberMethods);
      }
      return subscriberMethods;
   }
}  

 核心代码说明:

代码中clazz.getMethods();去得到所有的方法:

就开始遍历每一个方法了,去匹配封装了。

分别判断了是否以onEvent开头,是否是public且非static和abstract方法,是否是一个参数。如果都复合,才进入封装的部分。

根据方法的后缀,来确定threadMode,threadMode是个枚举类型:就四种情况。

最后将method, threadMode, eventType传入构造了:new SubscriberMethod(method, threadMode, eventType)。添加到List,最终放回。

代码clazz = clazz.getSuperclass();可以看到,会扫描所有的父类,不仅仅是当前类。

继续回到register:

for (SubscriberMethod subscriberMethod : subscriberMethods) {
        subscribe(subscriber, subscriberMethod, sticky, priority);
}

 for循环扫描到的方法,然后去调用suscribe方法。

// Must be called in synchronized block  
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod, boolean sticky, int priority) {
    subscribed = true;
    Class<?> eventType = subscriberMethod.eventType;
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod, priority);
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<Subscription>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        for (Subscription subscription : subscriptions) {
            if (subscription.equals(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }
    }

    // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)  
    // subscriberMethod.method.setAccessible(true);  

    int size = subscriptions.size();
    for (int i = 0; i <= size; i++) {
        if (i == size || newSubscription.priority > subscriptions.get(i).priority) {
            subscriptions.add(i, newSubscription);
            break;
        }
    }

    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<Class<?>>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);

    if (sticky) {
        Object stickyEvent;
        synchronized (stickyEvents) {
            stickyEvent = stickyEvents.get(eventType);
        }
        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());
        }
    }
}  

 

subscriberMethod中保存了method, threadMode, eventType

根据subscriberMethod.eventType,去subscriptionsByEventType去查找一个CopyOnWriteArrayList<Subscription> ,如果没有则创建。

然后传入的参数封装成了一个:Subscription(subscriber, subscriberMethod, priority);

其实subscriptionsByEventType是个Map,key:eventType ; value:CopyOnWriteArrayList<Subscription> ;

这个Map其实就是EventBus存储方法的地方,一定要记住!

代码中就是添加newSubscription;并且是按照优先级添加的。可以看到,优先级越高,会插到在当前List的前面。

代码中根据subscriber存储它所有的eventType ; 依然是map;key:subscriber ,value:List<eventType> ;知道就行,非核心代码,主要用于isRegister的判断。

代码中判断sticky;如果为true,从stickyEvents中根据eventType去查找有没有stickyEvent,如果有则立即发布去执行。stickyEvent其实就是我们post时的参数。

postToSubscription这个方法,我们在post的时候会介绍。

到此,我们register就介绍完了。

你只要记得一件事:扫描了所有的方法,把匹配的方法最终保存在subscriptionsByEventType(Map,key:eventType ; value:CopyOnWriteArrayList<Subscription> )中;

eventType是我们方法参数的Class,Subscription中则保存着subscriber, subscriberMethod(method, threadMode, eventType), priority;包含了执行改方法所需的一切。


post

post它又是如何调用我们的方法的。

register时,把方法存在subscriptionsByEventType;那么post肯定会去subscriptionsByEventType去取方法,然后调用。

看源码

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

    if (postingState.isPosting) {
        return;
    } else {
        postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
        postingState.isPosting = true;
        if (postingState.canceled) {
            throw new EventBusException("Internal error. Abort state was not reset");
        }
        try {
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}  

 currentPostingThreadState是一个ThreadLocal类型的,里面存储了PostingThreadState;PostingThreadState包含了一个eventQueue和一些标志位。

private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
    @Override
    protected PostingThreadState initialValue() {
        return new PostingThreadState();
    }
}

把我们传入的event,保存到了当前线程中的一个变量PostingThreadState的eventQueue中。

判断当前是否是UI线程。

遍历队列中的所有的event,调用postSingleEvent(eventQueue.remove(0), postingState)方法。

每次post都会去调用整个队列么,那么不会造成方法多次调用么?

可以看到代码中,有个判断,就是防止该问题的,isPosting=true了,就不会往下走了。

下面看postSingleEvent

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<? extends Object> eventClass = event.getClass();
    List<Class<?>> eventTypes = findEventTypes(eventClass);
    boolean subscriptionFound = false;
    int countTypes = eventTypes.size();
    for (int h = 0; h < countTypes; h++) {
        Class<?> clazz = eventTypes.get(h);
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            subscriptions = subscriptionsByEventType.get(clazz);
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
            for (Subscription subscription : subscriptions) {
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted = false;
                try {
                    postToSubscription(subscription, event, postingState.isMainThread);
                    aborted = postingState.canceled;
                } finally {
                    postingState.event = null;
                    postingState.subscription = null;
                    postingState.canceled = false;
                }
                if (aborted) {
                    break;
                }
            }
            subscriptionFound = true;
        }
    }
    if (!subscriptionFound) {
        Log.d(TAG, "No subscribers registered for event " + eventClass);
        if (eventClass != NoSubscriberEvent.class && eventClass != SubscriberExceptionEvent.class) {
            post(new NoSubscriberEvent(this, event));
        }
    }
}

 将我们的event,即post传入的实参;以及postingState传入到postSingleEvent中。

根据event的Class,去得到一个List<Class<?>>;其实就是得到event当前对象的Class,以及父类和接口的Class类型;主要用于匹配,比如你传入Dog extends Dog,他会把Animal也装到该List中。

遍历所有的Class,到subscriptionsByEventType去查找subscriptions;register里面把方法存的那个Map;

遍历每个subscription,依次去调用postToSubscription(subscription, event, postingState.isMainThread);
这个方法就是去反射执行方法了,大家还记得在register,if(sticky)时,也会去执行这个方法。

下面看它如何反射执行:

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    switch (subscription.subscriberMethod.threadMode) {
        case PostThread:
            invokeSubscriber(subscription, event);
            break;
        case MainThread:
            if (isMainThread) {
                invokeSubscriber(subscription, event);
            } else {
                mainThreadPoster.enqueue(subscription, event);
            }
            break;
        case BackgroundThread:
            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);
    }
}  

subscription包含了所有执行需要的东西,大致有:subscriber, subscriberMethod(method, threadMode, eventType), priority;

那么这个方法:第一步根据threadMode去判断应该在哪个线程去执行该方法;
case PostThread:

void invokeSubscriber(Subscription subscription, Object event) throws Error {
    subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
    }

直接反射调用;也就是说在当前的线程直接调用该方法;

case MainThread:

首先去判断当前如果是UI线程,则直接调用;否则: mainThreadPoster.enqueue(subscription, event);把当前的方法加入到队列,然后直接通过handler去发送一个消息,在handler的handleMessage中,去执行我们的方法。就是通过Handler去发送消息,然后执行的。

case BackgroundThread:

如果当前非UI线程,则直接调用;如果是UI线程,则将任务加入到后台的一个队列,最终由Eventbus中的一个线程池去调用

executorService = Executors.newCachedThreadPool();。

case Async:将任务加入到后台的一个队列,最终由Eventbus中的一个线程池去调用;线程池与BackgroundThread用的是同一个。

BackgroundThread和Async有什么区别呢?

BackgroundThread中的任务,一个接着一个去调用,中间使用了一个布尔型变量handlerActive进行的控制。

Async则会动态控制并发。

总结:register会把当前类中匹配的方法,存入一个map,而post会根据实参去map查找进行反射调用。

可以说,就是在一个单例内部维持着一个map对象存储了一堆的方法;post无非就是根据参数去查找方法,进行反射调用。


其余方法

介绍了register和post;大家获取还能想到一个词sticky,在register中,如何sticky为true,会去stickyEvents去查找事件,然后立即去post;

那么这个stickyEvents何时进行保存事件呢?

其实evevntbus中,除了post发布事件,还有一个方法也可以:

public void postSticky(Object event) {
    synchronized (stickyEvents) {
        stickyEvents.put(event.getClass(), event);
    }
    // Should be posted after it is putted, in case the subscriber wants to remove immediately  
    post(event);
}

 和post功能类似,但是会把方法存储到stickyEvents中去;

大家再去看看EventBus中所有的public方法,无非都是一些状态判断,获取事件,移除事件的方法;

 

posted on 2017-02-27 09:51  LoaderMan  阅读(177)  评论(0编辑  收藏  举报

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