EventBus使用以及源码处理

1. 使用EventBus

这部分之前学习过了，这里贴了我之前写的博客

2. EvetBus源码解析

1. EventBus的构建方法

不管我们进行什么操作，post或者register，都需要使用EventBus.getDefault()来获取实例，那么首先看一下这个方法内部是如何实现的

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

这里是一个单例模式，使用双重检查方法，现在进一步查看EventBus的构造方法

public EventBus() {
    this(DEFAULT_BUILDER);
}

private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();

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;
}

这里构建了一个EventBusBuilder来实现对EventBus的配置，这里使用了建造者模式

1. EventBus的订阅者注册

获得EventBus对象之后需要使用register方法来进行注册，现在看一下这个方法

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

首先它获取了订阅者的类对象，然后利用findSubscriberMethods方法获取了所有的订阅者方法（即那些应该被注册以响应特定事件的方法）的对象。最后在同步代码块中使用增强for遍历了每一个订阅者方法，这里有俩个方法需要注意一下分别是findSubscriberMethods和subscribe

查找订阅者的订阅方法

findSubscriberMethods：

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

从缓存中查找是否有订阅方法的集合，如果找到了就立马返回。

如果缓存中没有，则根据ignoreGeneratedIndex属性的值来选择采用何种方法查找订阅方法的集合。ignoreGeneratedIndex属性表示是否忽略注解器生成的MyEventBusIndex。如何生成ignoreGeneratedIndex属性表示是MyEventBusIndex类以及它的使用，HignoreGeneratedIndex的默认值是false,可以通过EventBusBuilder来设置