EventBus浅析

事件总线EventBus

为了简化并提高在activity、Fragment、Thread和Service之间的通信，同时解决组件之间的高耦合的同时仍然能继续高效地通信，事件总线设计出现了。著名的开源框架有EventBus和otto，我们先来解析EventBus。

一、EventBus简介

EventBus是针对Android优化的发布–订阅事件总线。它简化了应用程序内各组件、组件与后台线程间的通信。优点是开销小、代码优雅。以及将发布者和订阅者解耦。我们知道在平时的开发中Activity和Activity的交互还好说，但是Fragment与Fragment之间的交互就复杂的多。这时候我们会用广播来处理，但是使用广播效率也不高。

二、使用EventBus

EventBus的三要素和它的四中ThreadMode。
三要素：

• a,Event：事件，可以是任意类型的对象
• b,Subscriber:事件订阅者。在EventBus 3.0之前消息处理方式只能限定于onEvent、onEventMainThread、onEventBackgroundThread和onEventAsync，分别代表四种线程模型。在EventBus 3.0之后，事件处理的方法可以随便取名，但是需要添加一个注解@Subscribe，并且要指定线程模型(默认为POSTING)。
• c,Publisher:事件发布者。可以在任意线程任意位置发送事件，直接调用EventBus的post(Object)方法，可以自己实例化EventBus对象，但一般使用EventBus.getDefault()就可以。根据post函数参数的类型，会自动调用订阅相应类型事件的函数。

四种ThreadMode(线程模型)。
四模型：

• 1,POSTING(默认)：该模型下，该事件是在哪个线程发布出来的，事件处理函数就会在哪个线程中运行，也就是说事件发布和订阅事件在同一个线程中。在此模型状态下应该注意尽量避免处理函数中执行耗时操作，因为它会阻塞事件的传递，甚至引起ANR。
• 2，MAIN:事件处理在UI线程，事件处理事件不能太长，否则会引起ANR。
• 3，BACKGROUND:此模式下，若事件是从UI线程发出来的，那么事件处理函数就会在新的线程中运行：若本来就是在子线程中发布出来的，那么该事件处理函数就直接在发布事件的线程执行。在此事件处理函数中禁止进行UI操作。
• 4，ASYNC：此模式下，无论在哪个线程发布事件，该事件处理函数都会在新建的子线程中进行，同样，事件处理函数也禁止进行UI更新操作。

2.1 EventBus的用法

分为步骤：

• 1，自定义一个事件类

  public class MessageEvent {
  
  }

• 2，在需要订阅的地方注册事件：

  Event.getDefault().rigister(this);

• 3,发送事件

  EventBus.getDefault().post(messageEvent);

• 4,处理事件

  @Subscribe (tthreadMode = Thread.MAIN)
  public void XXX (MessageEvent messageEvent) {
      //code
  }

• 5，取消订阅事件

  Event.getDefault().unregister(this);

注：消息方法可以随意取名，但是需要添加一个注解@Subscribe,并且要指定线程模型(默认为POSTING)。

2.2 EventBus应用举例

2.2.1 添加依赖库

implementation 'org.greenrobot:eventbus:3.1.1'

现在已经更新到3.1.1版本。
2.2.1 定义消息类事件

public class MessageEvent {

    private String message;

    public MessageEvent(String message) {
        this.message = message;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

2.2.3 注册和取消订阅

public class MainActivity extends AppCompatActivity {
    private TextView tv_message;
    private Button bt_message;
    private Button bt_subscription;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        tv_message = this.findViewById(R.id.tv_message);
        tv_message.setText("MainActivity");

        bt_subscription = this.findViewById(R.id.bt_subscription);
        bt_subscription.setText("注册事件");

        bt_message = this.findViewById(R.id.bt_message);
        bt_message.setText("跳转到secondactivity");
        bt_message.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(MainActivity.this,SecondActivity.class));
            }
        });
        bt_subscription.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //注册事件
                EventBus.getDefault().register(MainActivity.this);
            }
        });
    }
    //事件处理者处理事件
    @Subscribe(threadMode = ThreadMode.MAIN)
    public void onMoonEvent(MessageEvent messageEvent){
        tv_message.setText(messageEvent.getMessage());
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        EventBus.getDefault().unregister(this);
    }  
}

我们定义了两个Button和TextView，Button用来注册和跳转到另一个Activity，TextView用来显示内容。
2.2.4 事件订阅者处理事件
在MainActivity中定义方法来处理事件，在这里因为ThreadMode为MAIN，事件的处理会在UI线程中执行，用TextView来展示收到的信息。

@Subscribe(threadMode = ThreadMode.MAIN)
    public void onMoonEvent(MessageEvent messageEvent){
        tv_message.setText(messageEvent.getMessage());
    }

2.2.5 事件发布者发布事件
SecondActivity中的代码如下所示：

public class SecondActivity extends AppCompatActivity {
    private Button bt_message;
    private Button bt_message1;
    private TextView tv_message;

    @Override
    public void onCreate( Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sec);

        tv_message = findViewById(R.id.tv_message);
        tv_message.setText("SecondActivity");

        bt_message = findViewById(R.id.bt_message);
        bt_message.setText("发送事件");
        bt_message.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                EventBus.getDefault().post(new MessageEvent("欢迎"));
                finish();
            }
        });
    }
}

在SecondActivity中，定义”发送事件”来发送事件并将SecondActivity finish掉，运行程序，如下图所示。

接下来点击”注册事件”按钮来注册事件，然后点击下面的按钮跳转到SEcondActivity，这时候跳转到SecondActivity，如下图所示，接下来点发送事件按钮，这是SecondActivity会被finish掉，这时候MainActivity的TextView会显示”欢迎”。


注：以上代码是这个程序后续都写好了运行截图的，所以后续的黏性事件的代码也显示了，懒得改了

2.3 EventBus的粘性事件

EventBus还支持发送黏性事件，就是在发送事件之后再订阅该事件也能收到该事件，这类似黏性广播。我们看怎么修改代码。

2.3.1 订阅者处理黏性事件

在MainActivity中写一个方法来处理黏性事件。

代码位于MainActivity中
@Subscribe(threadMode = ThreadMode.POSTING,sticky = true)
    public void ononMoonSticky (MessageEvent messageEvent) {
        tv_message.setText(messageEvent.getMessage());
    }

2.3.2 发送黏性事件

代码位于SecondActivity中
bt_message1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                EventBus.getDefault().postSticky(new MessageEvent("粘性事件"));
                finish();
            }
});

运行代码，我们在MainActivity没有点注册按钮，而是直接进入到SecondActivity中，然后选择发送黏性事件，这时回到MainActivity，我们看到在MainActivity仍然显示”MainActivity”，点击”注册事件”，TextView内容发生了变化，显示”黏性事件”，说明黏性事件被接收到了。

三、源码解析EventBus

3.1 EventBus构造方法

EventBus.getDefault()方法，可以看到这是一个经典的单例模式，还是DCL单例。

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

再看EventBus()构造函数：

public EventBus() {
        this(DEFAULT_BUILDER);
}

DEFAULT_BUILDER是默认的EventBusBuilder，用来构造EventBus。这里this是调用了Eventbus的另一个有一个参数的构造方法。我们再看这个有一个DEFAULT_BUILDER参数的构造方法。

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;
   }

可以看到，我们可以通过一个EventBusBuilder来构造EventBus，这里应用了建造者模式。

3.2 订阅者注册

获取EventBus实例后，便可以将订阅者注册到EventBus中，看rigister方法。

public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);//1
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);//2
            }
        }
}

对于以上代码解析：

• 1，查找订阅者的订阅方法
  在注释1处的findSubscriberMethods方法找出一个SubscribeMethod集合，也就是传进来的订阅者的所有订阅方法，接下来遍订阅者历所有的订阅方法来完成订阅者的注册操作。register做了两件事：一是查找订阅者的订阅方法，二是订阅者的注册，SubscriberMethod类用来保存订阅方法的Method对象、线程模型、事件类型、优先级、是否是黏性事件一系列属性。findSubscriberMethods代码：

  List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
       List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);//1
       if (subscriberMethods != null) {
           return subscriberMethods;
       }
  
       if (ignoreGeneratedIndex) {
           subscriberMethods = findUsingReflection(subscriberClass);
       } else {
           subscriberMethods = findUsingInfo(subscriberClass);//3
       }
       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);//2
           return subscriberMethods;
       }
   }

  findUsingInfo方法,在项目中通常是通过EventBus单例模式来获取默认的EventBus对象，也就是ignoreGeneratedIndex为false的情况，这时候就会调用findUsingInfo方法。

  private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
      FindState findState = prepareFindState();
      findState.initForSubscriber(subscriberClass);
      while (findState.clazz != null) {
          findState.subscriberInfo = getSubscriberInfo(findState);//1
          if (findState.subscriberInfo != null) {
              SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();//2
              for (SubscriberMethod subscriberMethod : array) {
                  if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                      findState.subscriberMethods.add(subscriberMethod);
                  }
              }
          } else {
              findUsingReflectionInSingleClass(findState);//3
          }
          findState.moveToSuperclass();
      }
      return getMethodsAndRelease(findState);
  }

  注释1处：通过getSubscriberInfo方法获取订阅者的信息，在开始查找订阅方法的时候并没有忽略注释器为我们生成的索引MyEvenBusIndex。如果我们通过EventBusBuilder配置了MyEvenBusIndex，便会获取subscriberInfo。就会调用subscriberInfo的getSubscriberMethods方法，获取订阅方法相关的信息；若没有配置MyEvenBusIndex，就执行3处的findUsingReflectionInSingleClass方法，将订阅方法保存到findState中。最后通过getMethodsAndRelease方法对dindState做回收处理并返回订阅的List集合。默认是没有配置MyEvenBusIndex的，所以看看findUsingReflectionInSingleClass的代码

  private void findUsingReflectionInSingleClass(FindState findState) {
       Method[] methods;
       try {
           //通过反射来获取订阅者中所有方法
           // This is faster than getMethods, especially when subscribers are fat classes like Activities
           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) {
           int modifiers = method.getModifiers();
           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");
           }
       }
   }

• 2，订阅者的注册过程
  在rigister方法中，在注释2处调用了subscribe方法来对订阅方法进行注册，如下：

   // Must be called in synchronized block
  private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
       Class<?> eventType = subscriberMethod.eventType;
       Subscription newSubscription = new Subscription(subscriber, subscriberMethod);//1
       CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);//2
       if (subscriptions == null) {
           subscriptions = new CopyOnWriteArrayList<>();
           subscriptionsByEventType.put(eventType, subscriptions);
       } else {
          //判断订阅者是否已经注册
           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);
               break;
           }
       }
  
       List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
       if (subscribedEvents == null) {
           subscribedEvents = new ArrayList<>();
           typesBySubscriber.put(subscriber, subscribedEvents);
       }
       subscribedEvents.add(eventType);
  
       if (subscriberMethod.sticky) {
           if (eventInheritance) {
              //对黏性事件的处理
               // 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<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);
           }
       }
   }

3.3 事件的发送

获取EventBus对象后，可以通过post方法来进行事件的提交。post源码：

public void post(Object event) {
        //PostingThreadState保存着事件队列和线程状态信息
        PostingThreadState postingState = currentPostingThreadState.get();
        //获取事件队列，并将当前事件插入事件队列
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);

        if (!postingState.isPosting) {
            postingState.isMainThread = isMainThread();
            postingState.isPosting = true;
            if (postingState.canceled) {
                throw new EventBusException("Internal error. Abort state was not reset");
            }
            try {
               //处理队列中的所有事件
                while (!eventQueue.isEmpty()) {
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
}

分析：首先从PostingThreadState对象中取出事件队列，然后再讲当前的事件插入事件队列。最后将队列中的事件一次交由postSingleEvent方法处理，并移除该事件。之后看postSingleEvent方法里做了什么：
postSingleEvent

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        //eventInheritance表示是否向上查找事件的父类，默认为true
        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);
        }
        //找不到该事件时的异常
        if (!subscriptionFound) {
            if (logNoSubscriberMessages) {
                logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
            }
            if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                    eventClass != SubscriberExceptionEvent.class) {
                post(new NoSubscriberEvent(this, event));
            }
        }
    }

eventInheritance表示是否向上查找事件的父类，它的默认值是true，可以通过在EventBusBuilder中进行配置。当eventInheritance为true，则通过lookupAllEventTypes找到所有父类事件并存在List中，然后通过postSingleEventForEventType方法逐一处理，postSingleEventForEventType代码：

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
           //同步取出该事件对应的Subscription(订阅对象集合)
            subscriptions = subscriptionsByEventType.get(eventClass);//1
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
            for (Subscription subscription : subscriptions) {//2
                //处理Subscription，将事件event和Subscription(订阅对象)传递给postingState并调用postToSubscription方法对事件进行///处理
                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;
                }
            }
            return true;
        }
        return false;
   }

注释1处同步取出该事件对应的Subscription(订阅对象集合)，注释2处处理Subscription，将事件event和Subscription(订阅对象)传递给postingState并调用postToSubscription方法对事件进行，接下来看postToSubscription源码：
postToSubscription：

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        switch (subscription.subscriberMethod.threadMode) {
            case POSTING:
                invokeSubscriber(subscription, event);
                break;
            case MAIN:
                if (isMainThread) {
                    invokeSubscriber(subscription, event);
                } else {
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
            case MAIN_ORDERED:
                if (mainThreadPoster != null) {
                    mainThreadPoster.enqueue(subscription, event);
                } else {
                    // temporary: technically not correct as poster not decoupled from subscriber
                    invokeSubscriber(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);
        }
}

取出订阅者的threadMode之后,根据threadMode来分别处理，若threadMode是MAIN，则通过反射直接运行订阅的方法；若不是主线程，则需要mainThreadPoster将我们的订阅事件添加到主线程队列中。mainThreadPoster是HandlerPoster类型的，继承自Handler，通过Handler将订阅的方法切换到主线程。

3.4 订阅者取消注册

unrigister方法：

/** Unregisters the given subscriber from all event classes. */
   public synchronized void unregister(Object subscriber) {
       List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);//1
       if (subscribedTypes != null) {
           for (Class<?> eventType : subscribedTypes) {
               unsubscribeByEventType(subscriber, eventType);//2
           }
           typesBySubscriber.remove(subscriber);//3
       } else {
           logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
       }
  }

typesBySubscriber是一个map集合，注释1处通过subscriber找到subscribedTypes(事件类型集合)。注释3处将subscriber对应的eventType从typesBySubscriber中移除。注释2处遍历subscribedTypes，并调用unsubscribeByEventType方法：
unsubscribeByEventType源码：

/** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */
    private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
        List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);//1
        if (subscriptions != null) {
            int size = subscriptions.size();
            for (int i = 0; i < size; i++) {
                Subscription subscription = subscriptions.get(i);
                if (subscription.subscriber == subscriber) {
                    subscription.active = false;
                    subscriptions.remove(i);
                    i--;
                    size--;
                }
            }
        }    
    }

注释1处通过eventType来得到对应的Subscription(订阅对象集合)，并在for循环中判断如果Subscription(订阅对象)的subscriber属性等于传进来的subscriber，则从Subscription中移除该Subscription。
这就是EventBus的源码