EventBus - 3 源码解析-1：EventBus对象构建／如何进行线程调度（整理中）

 EventBus.getDefault().register(this);

/** Convenience singleton for apps using a process-wide EventBus instance. */
  public static EventBus getDefault() {
        EventBus instance = defaultInstance;
        if (instance == null) {
            synchronized (EventBus.class) {
                instance = EventBus.defaultInstance;
                if (instance == null) {
                    instance = EventBus.defaultInstance = new EventBus();
                }
            }
        }
        return instance;
   }

一个很经典的单例模式，调用EventBus的构造器创建EventBus对象；

/**
     * Creates a new EventBus instance; 
     * each instance is a separate scope in which events are delivered. To use a
     * central bus, consider {@link #getDefault()}.
     */
    public EventBus() {
        this(DEFAULT_BUILDER);
    }

public EventBus() {
        this(DEFAULT_BUILDER);
}
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; //一般情况为0
        subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,builder.strictMethodVerification, builder.ignoreGeneratedIndex);
        //一般情况参数为null、false、false；该对象我们后面会介绍
        logSubscriberExceptions = builder.logSubscriberExceptions; //一般情况为true
        logNoSubscriberMessages = builder.logNoSubscriberMessages; //一般情况为true
        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent; //一般情况为true
        sendNoSubscriberEvent = builder.sendNoSubscriberEvent;//一般情况为true
        throwSubscriberException = builder.throwSubscriberException; //一般情况为false
        eventInheritance = builder.eventInheritance; //一般情况为true
        executorService = builder.executorService; //一个newCachedThreadPool()
}

private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();    
List<SubscriberInfoIndex> subscriberInfoIndexes;  
boolean strictMethodVerification;  
boolean ignoreGeneratedIndex;  
boolean logSubscriberExceptions = true;  
boolean logNoSubscriberMessages = true;  
boolean sendSubscriberExceptionEvent = true;  
boolean sendNoSubscriberEvent = true;  
boolean throwSubscriberException;  
boolean eventInheritance = true;  
ExecutorService executorService = DEFAULT_EXECUTOR_SERVICE;  
//总结：eventInheritance、logXX和sendXX全为true；其它全为false；

到此为止我们分析了调用getDefault方法得到的EventBus对象；

该对象存储有如下两个集合

Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType; 

• 以事件类型为键，每个事件对应一个Subscription对象的集合。EventBus一旦收到一个事件就会遍历它对应的Subscription集合，将该事件推送给Subscription处理。对于Subscription对象的如何构建和使用后文将会给出。

Map<Object, List<Class<?>>> typesBySubscriber; 

• 以事件订阅者为键（事件订阅者就是register方法的参数），每个订阅者对应一组它能接收的事件类型集合。一旦订阅者和EventBus解除注册，即调用unregister方法时，EventBus就会取出订阅者对应的所有事件类型，去subscriptionsByEventType中移除该事件对应的Subscription对象集合中的和订阅者关联的Subscription。

该对象存储有如下四个执行器

ExecutorService executorService = Executors.newCachedThreadPool();  

• 是一个线程池，ThreadPoolExecutor(0.Integer.MAX_VALUE,....)

HandlerPoster mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);

• 对应ThreadMode.MAIN模式，是一个继承Handler的类；事件的推送在其HandlerMessage方法中执行

private final BackgroundPoster backgroundPoster;

• 对应ThreadMode.BACKGROUND模式，是一个实现了Runnable方法的类；事件的推送就是将自己扔给executorService，如果当前对象已经在执行了，则只是将事件放入到BackgroundPoster中的一个队列中；否则将自己交给线程池执行其run方法。BackgroundPoster对象的run方法会一直遍历这个队列，直到队列没有数据则退出run方法。

private final AsyncPoster asyncPoster;   

• 对应ThreadMode.ASYNC模式，是一个实现了Runnable方法的类；相对于BackgroundPoster这里一旦有事件需要分发则立即将自己扔给线程池，而不是直接将事件扔给队列就完了。