BroadcastReceiver

​ 本质上是一个监听器，分为接收者和发送者。用于监听（接收）应用发出的广播消息，并做出响应。可用于进程间通信、线程间通信和监听系统的特定时间（如：电话呼入，网络状态变更等）。

• 使用观察者模式：基于消息的发布/订阅事件模型。

• 常用做与android 系统事件的监听和交互。

• 消耗时间、空间较多，在onRecieve()方法内可以获得Context和Intent参数，通过这两个方法，可以调用AndroidSDK中的很多方法，属于重量级组件。

• 可跨进程通信。

Eventbus

​ 针对android跨进程、线程通信的优化方案，一定程度上可以代替广播、handler、Intent。

优点

• 调度灵活，不依赖于context。
• 可继承。
• 有事件优先级。
• 粘性事件机制。
• 量级较轻。

1. 实现原理

• getDefault()：通过懒汉单例模式获取到对象

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

  在new对象时，使用builder模式：

  EventBus(EventBusBuilder builder) {
          subscriptionsByEventType = new HashMap<>();
          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;
          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;
      }
  

• register()实现如下：

  public void register(Object subscriber) {
          Class<?> subscriberClass = subscriber.getClass();
          List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);//通过反射的方式获取到所有通过注解注册的回调方法
          synchronized (this) {
              for (SubscriberMethod subscriberMethod : subscriberMethods) {
                  subscribe(subscriber, subscriberMethod);//这里是具体的注册方法
              }
          }
      }
  

  Subscribe()：

  private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
          Class<?> eventType = subscriberMethod.eventType;//获取触发本次消费的事件类型
          Subscription newSubscription = new Subscription(subscriber, subscriberMethod);//创建订阅对象（把注册者和事件消费方法绑定）
          CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
          if (subscriptions == null) {
              subscriptions = new CopyOnWriteArrayList<>();
              subscriptionsByEventType.put(eventType, subscriptions);
          } else {
              if (subscriptions.contains(newSubscription)) {//若该事件（注册者和消费方法）已注册，则抛异常
                  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) {//轮询的目的是为了按照优先级插入到指定的位置(事件消费队列)
                  subscriptions.add(i, newSubscription);
                  break;
              }
          }
  
          List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);//获取该订阅者下的所有订阅事件
          if (subscribedEvents == null) {
              subscribedEvents = new ArrayList<>();
              typesBySubscriber.put(subscriber, subscribedEvents);
          }
          subscribedEvents.add(eventType);//将本事件添加进触发事件列表中
  
          if (subscriberMethod.sticky) {//粘性事件处理
              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);
              }
          }
      }
  

  以上就是EventBus注册时的逻辑，接下来我们看看发布事件的时候的逻辑：

• post()：

  public void post(Object event) {
          PostingThreadState postingState = currentPostingThreadState.get();
          List<Object> eventQueue = postingState.eventQueue;//获取当前线程的消息队列
          eventQueue.add(event);//将本事件添加进消息队列中
  
          if (!postingState.isPosting) {//若当前
              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()) {//循环遍历事件队列（在这儿可以类比handler机制不过不是死循环，也不存在阻塞，每post一次就进行判断，是否处于posting）
                      postSingleEvent(eventQueue.remove(0), postingState);//主要的消费逻辑
                  }
              } finally {
                  postingState.isPosting = false;
                  postingState.isMainThread = false;
              }
          }
      }
  

  这个currentPostingThreadState是做什么的呢，我们可以看一下它的声明和实现：

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

  这里使用了ThreadLocal，说明在每个线程里都会有这个对象的副本。我们接着往下看PostingThreadState是维护什么的类：

  final static class PostingThreadState {
          final List<Object> eventQueue = new ArrayList<Object>();//事件队列，用一个list维护
          boolean isPosting;//是否在发布事件（就是之后的循环遍历是否在执行）
          boolean isMainThread;//是否为主线程，通过判断当前looper是否为MainLooper
          Subscription subscription;//当前的订阅类，上面有说明
          Object event;//当前正在处理的事件
          boolean canceled;//当前事件是否被取消
      }
  

  我们继续看postSingleEvent()方法，这里是事件消费的主要逻辑：

  private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
          Class<?> eventClass = event.getClass();
          boolean subscriptionFound = false;
          if (eventInheritance) {
              List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
              int countTypes = eventTypes.size();
              for (int h = 0; h < countTypes; h++) {
                  Class<?> clazz = eventTypes.get(h);
                  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) {
                  post(new NoSubscriberEvent(this, event));
              }
          }
      }
  

  这里可以看到最终都会回调postSingleEventForEventType()这个方法，我们继续看这个方法：

  private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
          CopyOnWriteArrayList<Subscription> subscriptions;
          synchronized (this) {
              subscriptions = subscriptionsByEventType.get(eventClass);
          }
          if (subscriptions != null && !subscriptions.isEmpty()) {
              for (Subscription subscription : subscriptions) {
                  postingState.event = event;//当前正在处理的事件
                  postingState.subscription = subscription;//当前正在轮询的订阅事件
                  boolean aborted = false;
                  try {
                      postToSubscription(subscription, event, postingState.isMainThread);//事件处理方法
                      aborted = postingState.canceled;
                  } finally {
                      postingState.event = null;
                      postingState.subscription = null;
                      postingState.canceled = false;
                  }
                  if (aborted) {
                      break;
                  }
              }
              return true;
          }
          return false;
      }
  

  我们接着看：

  private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
          switch (subscription.subscriberMethod.threadMode) {//这里就是处理线程模式的不同策略
              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);
          }
      }
  

  我们从默认的POSTING模式往下看：

      void invokeSubscriber(PendingPost pendingPost) {
          Object event = pendingPost.event;
          Subscription subscription = pendingPost.subscription;
          PendingPost.releasePendingPost(pendingPost);
          if (subscription.active) {
              invokeSubscriber(subscription, event);
          }
      }
  

  我们接着看：

  void invokeSubscriber(Subscription subscription, Object event) {
          try {
              subscription.subscriberMethod.method.invoke(subscription.subscriber, event);//最终是通过反射的方法调用事件的处理
          } catch (InvocationTargetException e) {
              handleSubscriberException(subscription, event, e.getCause());
          } catch (IllegalAccessException e) {
              throw new IllegalStateException("Unexpected exception", e);
          }
      }
  

  这样我们可以知道，最后调用方法是通过反射来调用与之对应的消费事件的方法。