EventBus3.0源码分析

一、基本用法

添加依赖和注解处理器

android {
    defaultConfig {
        javaCompileOptions {
            annotationProcessorOptions {
                arguments = [eventBusIndex: 'org.greenrobot.eventbusperf.EventBusIndex']
            }
        }
    }

}
dependencies {
    implementation 'org.greenrobot:eventbus:3.1.1'
    annotationProcessor 'org.greenrobot:eventbus-annotation-processor:3.0.1'
}

在Application添加注解处理器自动生成的类

EventBus.builder().addIndex(new EventBusIndex()).installDefaultEventBus();

注册为事件观察者和声明接收事件的方法，注意@Subscribe注解的方法要为public，不然编译报错

 @Override
 public void onStart() {
     super.onStart();
     EventBus.getDefault().register(this);
 }

 @Override
 public void onStop() {
     super.onStop();
     EventBus.getDefault().unregister(this);
 }

 @Subscribe(threadMode = ThreadMode.MAIN)
   public void onEventMsg(String str) {
       Log.d(TAG, "onEventMsg: " + str);
   }

发送事件

EventBus.getDefault().post("发送事件");

二、扩展

看看@Subscribe 注解的定义

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface Subscribe {
    ThreadMode threadMode() default ThreadMode.POSTING;

    boolean sticky() default false;

    int priority() default 0;
}

可以看到使用该注解时我们有3个变量可以配置

 @Subscribe(threadMode = ThreadMode.MAIN, priority = -2, sticky = true)
   public void onEventMsg(String str) {
       Log.d(TAG, "onEventMsg: " + str);
 }

//发送粘性事件
EventBus.getDefault().postSticky("发送事件");

threadMode：用来声明接收事件的方法运行的线程，有5种取值
sticky：是否接收粘性事件
priority：事件优先级，priority越大优先级越高

ThreadMode有5种不同的取值：

POSTING：发送事件在哪个线程处理事件就在哪个线程
MAIN：处理事件在主线程
MAIN_ORDERED：和MAIN的区别就是即使发送事件是在主线程，也通过Handler发送消息来执行处理事件的方法，可以保证non-blocking
BACKGROUND：如果发送事件是子线程，那么处理事件就在该子线程，如果发送事件是主线程，那么处理事件在线程池的子线程，值得注意的是，所有事件都在同一子线程中执行
ASYNC：无论发送事件在哪个线程，处理事件都在线程池的子线程

三、源码分析

一、注解处理器生成类

编译阶段注解处理器会扫描@Subscribe注解，生成.java文件，我们需要在Application初始化时创建这些自动生成的类的对象，添加到EventBus对象中去。

生成的.java文件在build/generated/source/apt路径下，文件名为我们在gradle声明的名称，下面看看生成的文件内容

public class EventBusIndex implements SubscriberInfoIndex {
    private static final Map, SubscriberInfo> SUBSCRIBER_INDEX;

    static {
        SUBSCRIBER_INDEX = new HashMap, SubscriberInfo>();

        putIndex(new SimpleSubscriberInfo(com.yjx.myapplication.ScrollingActivity.class, true,
                new SubscriberMethodInfo[] {
            new SubscriberMethodInfo("onEventMsg", String.class, ThreadMode.MAIN),
            new SubscriberMethodInfo("onEventMsg2", String.class, ThreadMode.MAIN),
        }));

    }

    private static void putIndex(SubscriberInfo info) {
        SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);
    }

    @Override
    public SubscriberInfo getSubscriberInfo(Class subscriberClass) {
        SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);
        if (info != null) {
            return info;
        } else {
            return null;
        }
    }
}

可以看到EventBusIndex类维护了一个HashMap，key为订阅者，value为订阅者接收事件的方法信息。接下去看下在Application的操作：

EventBus.builder().addIndex(new EventBusIndex()).installDefaultEventBus();

public EventBusBuilder addIndex(SubscriberInfoIndex index) {
    if (subscriberInfoIndexes == null) {
        subscriberInfoIndexes = new ArrayList<>();
    }
    subscriberInfoIndexes.add(index);
    return this;
}

比较简单，不多说，主要是通过构建者模式创建一个EventBus单例对象，将生成的类的对象添加到subscriberInfoIndexes这个集合。

二、注册为观察者和反注册

直接看register方法

    public void register(Object subscriber) {
        Class subscriberClass = subscriber.getClass();
        // 关键方法1 查找该观察者所声明的所有接受事件的方法，即@Subscribe注解的方法
        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }
    
    // 关键方法1
    List findSubscriberMethods(Class subscriberClass) {
        // 先从缓存找
        List subscriberMethods = METHOD_CACHE.get(subscriberClass);
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
        // 3.0以上ignoreGeneratedIndex的值默认为false
        if (ignoreGeneratedIndex) {
            // 通过反射获取接收事件的方法的信息
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
            // 通过注解器生成的EventBusIndex类中获得该订阅者的接收事件的方法的信息
            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;
        }
    }

findSubscriberMethods这个方法可以看到，有两种方式获取该订阅者的接收事件的方法的信息，一种是通过反射，一种是使用注解处理器生成的类。还有一个小细节，如果最后没找到@Subscribe注解的方法，则会抛异常，所以，一旦你调用register方法，你就应该声明接收事件的方法。

先看看从注解处理器寻找的操作

    private List findUsingInfo(Class subscriberClass) {
        // FindState 此类可以理解为一个保存当前寻找状态的类
        // 因为在寻找订阅者接收事件的方法时，不仅在当前类，还在其父类找
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            // 关键方法1
            findState.subscriberInfo = getSubscriberInfo(findState);
            if (findState.subscriberInfo != null) {
                // 关键方法2
                SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
                for (SubscriberMethod subscriberMethod : array) {
                    if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                        // 将接收事件的方法添加到findState对象的List
                        findState.subscriberMethods.add(subscriberMethod);
                    }
                }
            } else {
                findUsingReflectionInSingleClass(findState);
            }
            // 指向父类，重新执行相同的操作
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

    // 关键方法1
    private SubscriberInfo getSubscriberInfo(FindState findState) {
        if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
            SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
            if (findState.clazz == superclassInfo.getSubscriberClass()) {
                return superclassInfo;
            }
        }
        if (subscriberInfoIndexes != null) {
            // 遍历我们在Application初始化时添加的EventBusIndex对象
            for (SubscriberInfoIndex index : subscriberInfoIndexes) {
                // 调用注解处理器帮我们生成的方法拿到接收事件方法的信息
                SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
                if (info != null) {
                    return info;
                }
            }
        }
        return null;
    }
    
    // 关键方法2，根据方法信息构造SubscriberMethod数组
    @Override
    public synchronized SubscriberMethod[] getSubscriberMethods() {
        int length = methodInfos.length;
        SubscriberMethod[] methods = new SubscriberMethod[length];
        for (int i = 0; i < length; i++) {
            SubscriberMethodInfo info = methodInfos[i];
            methods[i] = createSubscriberMethod(info.methodName, info.eventType, info.threadMode,
                    info.priority, info.sticky);
        }
        return methods;
    }

再看看通过反射的方式是怎么获取接收事件的方法信息

    private List findUsingReflection(Class subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            findUsingReflectionInSingleClass(findState);
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

主要是反射获取类里面的public的方法，获取方法的注解，将方法的信息保存到SubscriberMethod对象。

我们再回到register方法，findSubscriberMethods不管通过哪种方式，最终得到都是一个List集合，下面继续看

   public void register(Object subscriber) {
          Class subscriberClass = subscriber.getClass();
          // 关键方法1 查找该观察者所声明的所有接受事件的方法，即@Subscribe注解的方法
          List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
          synchronized (this) {
              for (SubscriberMethod subscriberMethod : subscriberMethods) {
                  // 关键方法2
                  subscribe(subscriber, subscriberMethod);
              }
          }
    }

    // 关键方法2
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class eventType = subscriberMethod.eventType;
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        // 1.获取订阅同一事件类型的所有@Subscribe的方法
        CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }
        
        int size = subscriptions.size();
        // 2.根据优先级的顺序插入到列表里
        for (int i = 0; i <= size; i++) {
            if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
                subscriptions.add(i, newSubscription);
                break;
            }
        }
        // 3.获取该订阅者的接收事件的集合，将当前接收事件方法添加到集合中
        List> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);
        // 4.是否是粘性事件
        if (subscriberMethod.sticky) {
            if (eventInheritance) {
                Set, Object>> entries = stickyEvents.entrySet();
                for (Map.Entry, 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);
            }
        }
    }

大致的流程引用EventBus 3.0 源码分析的图片，非常清晰

register流程图.png

接下来看反注册unregister方法

    public synchronized void unregister(Object subscriber) {
        // 获取该订阅者的接收事件的集合
        List> subscribedTypes = typesBySubscriber.get(subscriber);
        if (subscribedTypes != null) {
            for (Class eventType : subscribedTypes) {
                // 从订阅同一事件类型的列表中移除该订阅者
                unsubscribeByEventType(subscriber, eventType);
            }
            // 移除该订阅者
            typesBySubscriber.remove(subscriber);
        } else {
            logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
        }
    }

    private void unsubscribeByEventType(Object subscriber, Class eventType) {
        List subscriptions = subscriptionsByEventType.get(eventType);
        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--;
                }
            }
        }
    }

三、发送事件

    public void post(Object event) {
        // 1.通过ThreadLocal获取当前线程的PostingThreadState对象，该对象记录着当前线程发送事件的状态
        PostingThreadState postingState = currentPostingThreadState.get();
        // 2.将事件添加到事件队列里
        List