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Android -- EventBus解析

EventBus

EventBus 是一个 Android 事件发布/订阅框架,通过解耦发布者和订阅者简化 Android 事件传递。传统的事件传递方式包括:Handler、BroadCastReceiver、Interface 回调,相比之下 EventBus 的优点是代码简洁,使用简单,并将事件发布和订阅充分解耦。

事件(Event):又可称为消息。其实就是一个对象,可以是网络请求返回的字符串,也可以是某个开关状态等等。 事件类型(EventType) 指事件所属的 Class。事件分为一般事件和 Sticky 事件,相对于一般事件,Sticky 事件不同之处在于,当事件发布后,再有订阅者开始订阅该类型事件,依然能收到该类型事件最近一个 Sticky 事件。

订阅者(Subscriber):订阅某种事件类型的对象。当有发布者发布这类事件后,EventBus 会执行订阅者的 onEvent 函数,这个函数叫 事件响应函数 。订阅者通过 register 接口订阅某个事件类型,unregister 接口退订。订阅者存在优先级,优先级高的订阅者可以取消事件继续向优先级低的订阅者分发,默认所有订阅者优先级都为 0。

发布者(Publisher):发布某事件的对象,通过 post 接口发布事件。

类关系

Android -- EventBus解析

流程

Android -- EventBus解析

EventBus 负责存储订阅者、事件相关信息,订阅者和发布者都只和 EventBus 关联。

Android -- EventBus解析

订阅者首先调用 EventBus 的 register 接口订阅某种类型的事件,当发布者通过 post 接口发布该类型的事件时,EventBus 执行调用者的事件响应函数。

解析

EventBus 类负责所有对外暴露的 API,其中的 register()、post()、unregister() 函数配合上自定义的 EventType 及事件响应函数即可完成核心功能。

EventBus 默认可通过静态函数 getDefault() 获取单例,当然有需要也可以通过 EventBusBuilder 或 构造函数新建一个 EventBus,每个新建的 EventBus 发布和订阅事件都是相互隔离的,即一个 EventBus 对象中的发布者发布事件,另一个 EventBus 对象中的订阅者不会收到该订阅。

EventBus.getDefault().register(this);

register()方法解析:

 public void register(Object subscriber) {  Class<?> subscriberClass = subscriber.getClass();  // @Subscribe in anonymous classes is invisible to annotation processing, always fall back to reflection  boolean forceReflection = subscriberClass.isAnonymousClass();  List<SubscriberMethod> subscriberMethods =    subscriberMethodFinder.findSubscriberMethods(subscriberClass, forceReflection);//主要查找又什么方法是在这个函数里面  for (SubscriberMethod subscriberMethod : subscriberMethods) {   subscribe(subscriber, subscriberMethod);  } } 

通过一个findSubscriberMethods方法找到了一个订阅者中的所有订阅方法,返回一个 List

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass, boolean forceReflection) {   //类名    String key = subscriberClass.getName();    List<SubscriberMethod> subscriberMethods;    synchronized (METHOD_CACHE) {    //判断是否有缓存,有缓存直接返回缓存       subscriberMethods = METHOD_CACHE.get(key);    }   //第一次进来subscriberMethods肯定是Null    if (subscriberMethods != null) {     return subscriberMethods;    }   //INDEX是GeneratedSubscriberIndex    if (INDEX != null && !forceReflection) {     subscriberMethods = findSubscriberMethodsWithIndex(subscriberClass);//后面再来看这个函数     if (subscriberMethods.isEmpty()) {      subscriberMethods = findSubscriberMethodsWithReflection(subscriberClass);//通过反射来找到方法     }    } else {     subscriberMethods = findSubscriberMethodsWithReflection(subscriberClass);//通过反射来找到方法    }    if (subscriberMethods.isEmpty()) {     throw new EventBusException("Subscriber " + subscriberClass       + " and its super classes have no public methods with the @Subscribe annotation");    } else {     synchronized (METHOD_CACHE) {      METHOD_CACHE.put(key, subscriberMethods);//放入缓存     }     return subscriberMethods;    }   } 

findSubscriberMethodsWithReflection:

private List<SubscriberMethod> findSubscriberMethodsWithReflection(Class<?> subscriberClass) {   List<SubscriberMethod> subscriberMethods = new ArrayList<SubscriberMethod>();   Class<?> clazz = subscriberClass;   HashSet<String> eventTypesFound = new HashSet<String>();   StringBuilder methodKeyBuilder = new StringBuilder();   while (clazz != null) {    String name = clazz.getName();      //过滤掉系统类      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)      //通过反射,获取到订阅者的所有方法    Method[] methods = clazz.getDeclaredMethods();    for (Method method : methods) {     int modifiers = method.getModifiers();       //判断是否是public,是否有修饰符     if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {        //获得订阅函数的参数       Class<?>[] parameterTypes = method.getParameterTypes();        //参数只能是1个      if (parameterTypes.length == 1) {         //通过Annotation去拿一些数据       Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);       if (subscribeAnnotation != null) {        String methodName = method.getName();//方法名字        Class<?> eventType = parameterTypes[0];//类型          //获取参数类型,其实就是接收事件的类型          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         ThreadMode threadMode = subscribeAnnotation.threadMode();           //封装一个订阅方法对象,这个对象包含Method对象,threadMode对象,eventType对象,优先级prority,sticky         subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,           subscribeAnnotation.priority(), subscribeAnnotation.sticky()));        }       }      } else if (strictMethodVerification) {       if (method.isAnnotationPresent(Subscribe.class)) {        String methodName = name + "." + method.getName();        throw new EventBusException("@Subscribe method " + methodName +          "must have exactly 1 parameter but has " + parameterTypes.length);       }      }     } else if (strictMethodVerification) {      if (method.isAnnotationPresent(Subscribe.class)) {       String methodName = name + "." + method.getName();       throw new EventBusException(methodName +         " is a illegal @Subscribe method: must be public, non-static, and non-abstract");      }     }    }    //再去查找父类    clazz = clazz.getSuperclass();   }   return subscriberMethods;  } 

方法的讲解都在注释里面。

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

每个订阅方法都调用subscribe方法:

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {     //从订阅方法中拿到订阅事件的类型     Class<?> eventType = subscriberMethod.eventType;     //通过订阅事件类型,找到所有的订阅(Subscription),订阅中包含了订阅者,订阅方法   CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);     //创建一个新的订阅    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);     //将新建的订阅加入到这个事件类型对应的所有订阅列表     if (subscriptions == null) {      //如果该事件目前没有订阅列表,那么创建并加入该订阅      subscriptions = new CopyOnWriteArrayList<Subscription>();    subscriptionsByEventType.put(eventType, subscriptions);   } else {      //如果有订阅列表,检查是否已经加入过      if (subscriptions.contains(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);   // Got to synchronize to avoid shifted positions when adding/removing concurrently     //根据优先级插入订阅   synchronized (subscriptions) {    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;     }    }   }   //将这个订阅事件加入到订阅者的订阅事件列表中   List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);   if (subscribedEvents == null) {    subscribedEvents = new ArrayList<Class<?>>();    typesBySubscriber.put(subscriber, subscribedEvents);   }   subscribedEvents.add(eventType);   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<Class>).     Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();     for (Map.Entry<Class<?>, 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);    }   }  } 

第一步:通过 subscriptionsByEventType 得到该事件类型所有订阅者信息队列,根据优先级将当前订阅者信息插入到订阅者队列 subscriptionsByEventType 中;

第二步:在 typesBySubscriber 中得到当前订阅者订阅的所有事件队列,将此事件保存到队列 typesBySubscriber 中,用于后续取消订阅;

第三步:检查这个事件是否是 Sticky 事件,如果是则从 stickyEvents 事件保存队列中取出该事件类型最后一个事件发送给当前订阅者

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());         }     }
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);  } } 

过程:

  1. 找到被注册者中所有的订阅方法。
  2. 依次遍历订阅方法,找到EventBus中eventType对应的订阅列表,然后根据当前订阅者和订阅方法创建一个新的订阅加入到订阅列表。
  3. 找到EvnetBus中subscriber订阅的事件列表,将eventType加入到这个事件列表。
public void post(Object event) {   //拿到PostingThreadState    PostingThreadState postingState = currentPostingThreadState.get();    List<Object> eventQueue = postingState.eventQueue;   //将事件放入队列     eventQueue.add(event);    if (!postingState.isPosting) {     postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();     postingState.isPosting = true;//设置为正在post     if (postingState.canceled) {      throw new EventBusException("Internal error. Abort state was not reset");     }     try {      while (!eventQueue.isEmpty()) {      //分发事件       postSingleEvent(eventQueue.remove(0), postingState);      }     } finally {      postingState.isPosting = false;      postingState.isMainThread = false;     }    }   } 
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {  Class<?> eventClass = event.getClass();  boolean subscriptionFound = false;  if (eventInheritance) {     //找到eventClass对应的事件,包含父类对应的事件和接口对应的事件    List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);   int countTypes = eventTypes.size();   for (int h = 0; h < countTypes; h++) {    Class<?> clazz = eventTypes.get(h);      //postSingleEventForEventType去查找,其中里面的数据都是通过subscribe()缓存进去的    subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);   }  } else {   subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);  }  if (!subscriptionFound) {   if (logNoSubscriberMessages) {    Log.d(TAG, "No subscribers registered for event " + eventClass);   }   if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&     eventClass != SubscriberExceptionEvent.class) {      //如果没有订阅发现,那么会Post一个NoSubscriberEvent事件    post(new NoSubscriberEvent(this, event));   }  } } 
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {  CopyOnWriteArrayList<Subscription> subscriptions;  synchronized (this) {     //subscriptionsByEventType是从subscribe()缓存进去的   subscriptions = subscriptionsByEventType.get(eventClass);  }  if (subscriptions != null && !subscriptions.isEmpty()) {   for (Subscription subscription : subscriptions) {    postingState.event = event;    postingState.subscription = subscription;    boolean aborted = false;    try {       //对每个订阅调用该方法     postToSubscription(subscription, event, postingState.isMainThread);     aborted = postingState.canceled;    } finally {     postingState.event = null;     postingState.subscription = null;     postingState.canceled = false;    }    if (aborted) {     break;    }   }   return true;  }  return false; } 

post 函数会首先得到当前线程的 post 信息 PostingThreadState ,其中包含事件队列,将当前事件添加到其事件队列中,然后循环调用 postSingleEvent 函数发布队列中的每个事件。

postSingleEvent 函数会先去 eventTypesCache 得到该事件对应类型的的父类及接口类型,没有缓存则查找并插入缓存。循环得到的每个类型和接口,调用 postSingleEventForEventType 函数发布每个事件到每个订阅者。

postSingleEventForEventType 函数在 subscriptionsByEventType 查找该事件订阅者订阅者队列,调用 postToSubscription 函数向每个订阅者发布事件。

postToSubscription 函数中会判断订阅者的 ThreadMode,从而决定在什么 Mode 下执行事件响应函数。

ThreadMode

  1. PostThread :默认的 ThreadMode,表示在执行 Post 操作的线程直接调用订阅者的事件响应方法,不论该线程是否为主线程(UI 线程)。当该线程为主线程时,响应方法中不能有耗时操作,否则有卡主线程的风险。适用场景: 对于是否在主线程执行无要求,但若 Post 线程为主线程,不能耗时的操作
  2. MainThread :在主线程中执行响应方法。如果发布线程就是主线程,则直接调用订阅者的事件响应方法,否则通过主线程的 Handler 发送消息在主线程中处理——调用订阅者的事件响应函数。显然, MainThread 类的方法也不能有耗时操作,以避免卡主线程。适用场景: 必须在主线程执行的操作
  3. BackgroundThread :在后台线程中执行响应方法。如果发布线程 不是 主线程,则直接调用订阅者的事件响应函数,否则启动 唯一的 后台线程去处理。由于后台线程是唯一的,当事件超过一个的时候,它们会被放在队列中依次执行,因此该类响应方法虽然没有 PostThread 类和 MainThread 类方法对性能敏感,但最好不要有重度耗时的操作或太频繁的轻度耗时操作,以造成其他操作等待。适用场景: 操作轻微耗时且不会过于频繁 ,即一般的耗时操作都可以放在这里;
  4. Async :不论发布线程是否为主线程,都使用一个空闲线程来处理。和 BackgroundThread 不同的是, Async 类的所有线程是相互独立的,因此不会出现卡线程的问题。适用场景: 长耗时操作,例如网络访问

我是天王盖地虎的分割线

dependencies {     compile fileTree(include: ['*.jar'], dir: 'libs')     compile 'de.greenrobot:eventbus:3.0.0-beta1' }

Android -- EventBus使用: http://www.cnblogs.com/yydcdut/p/4290846.html

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