源码阅读系列:EventBus


title: 源码阅读系列:EventBus
date: 2016-12-22 16:16:47
tags: 源码阅读
---

EventBus 是人们在日常开发中经常会用到的开源库,即使是不直接用的人,也多少借鉴过事件总线的用法。而且EventBus的代码其实是非常简单的,可以试着阅读一下。

源码阅读系列不采用对功能进行归类的方法进行阅读,而是采用一个刚开始阅读源码的视角,从我们平时的API调用,一步步的去理解设计意图和实现原理。

从这里开始

从这里开始吧,我们最常用的地方就是给一个函数添加上注解,我们先抛开apt生成的table,只看这个运行时版本的订阅设定。

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

    /**
     * If true, delivers the most recent sticky event (posted with
     * {@link EventBus#postSticky(Object)}) to this subscriber (if event available).
     */
    boolean sticky() default false;

    /** Subscriber priority to influence the order of event delivery.
     * Within the same delivery thread ({@link ThreadMode}), higher priority subscribers will receive events before
     * others with a lower priority. The default priority is 0. Note: the priority does *NOT* affect the order of
     * delivery among subscribers with different {@link ThreadMode}s! */
    int priority() default 0;
}

这个设定还是非常简单的,而且都是我们熟悉的东西,线程类型(默认的是抛出线程),是否是粘性事件,时间的优先级。经过这个类的出现,我们就可以在类里面写我们经常写的某个函数是订阅函数了。

@Subscribe (...)
public void getMessage(Event event) { ... }

下面的问题是我们改怎么让EventBus找到这些方法呢?通过apt的版本我们知道这里面肯定有一个map或者是table的东西记录了Object和Method之间的订阅关系,而且还是一对多的。这个地方就是从每个我们进行register的地方进行的。

register & unregister

// eventbus/EventBus 
  /**
     * Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they
     * are no longer interested in receiving events.
     * <p/>
     * Subscribers have event handling methods that must be annotated by {@link Subscribe}.
     * The {@link Subscribe} annotation also allows configuration like {@link
     * ThreadMode} and priority.
     */
    public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

我们在Activity/Fragment中都有可能会调用这个方法,如果是Fragment里面我们还会在onDestoryView()中进行unregister(...)。在这段函数里我们发现使用反射从这个Class中找到了所有的订阅者函数了,然后对每个订阅者函数进行注册。

这里我们看看我们的SubribeMethod被包装成了什么样子:

/** Used internally by EventBus and generated subscriber indexes. */
public class SubscriberMethod {
    final Method method;
    final ThreadMode threadMode;
    final Class<?> eventType;
    final int priority;
    final boolean sticky;
    /** Used for efficient comparison */
    String methodString;

    public SubscriberMethod(Method method, Class<?> eventType, ThreadMode threadMode, int priority, boolean sticky) {
        this.method = method;
        this.threadMode = threadMode;
        this.eventType = eventType;
        this.priority = priority;
        this.sticky = sticky;
    }

    @Override
    public boolean equals(Object other) {
        if (other == this) {
            return true;
        } else if (other instanceof SubscriberMethod) {
            checkMethodString();
            SubscriberMethod otherSubscriberMethod = (SubscriberMethod)other;
            otherSubscriberMethod.checkMethodString();
            // Don't use method.equals because of http://code.google.com/p/android/issues/detail?id=7811#c6
            return methodString.equals(otherSubscriberMethod.methodString);
        } else {
            return false;
        }
    }

    private synchronized void checkMethodString() {
        if (methodString == null) {
            // Method.toString has more overhead, just take relevant parts of the method
            StringBuilder builder = new StringBuilder(64);
            builder.append(method.getDeclaringClass().getName());
            builder.append('#').append(method.getName());
            builder.append('(').append(eventType.getName());
            methodString = builder.toString();
        }
    }

    @Override
    public int hashCode() {
        return method.hashCode();
    }
}

SubscribeMethod 携带了Method函数原型,还有就是我们在注解类里面提供的所有信息。还有一个Class<?>类型的EventType是指我们的事件类所对应的Class,其余的方法都是为了比较和判断是否相等来做的,equal/checkMethodString都是各种的拼字串来进行存储和判断。

下面我们来看register里面调用的这段subscribe,这段非常的重要涉及了EventBus运行时处理的绝大多数部分,还有就是粘性事件的分发。这段使用了大量的JDK的反射包的API,本身注释也提醒我们了这段代码需要加锁,毕竟里面这一堆并发容器。所以我们最好先明确这段里面用的并发容器到底都是什么,这段代码才好继续看的下去。

    private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
    private final Map<Object, List<Class<?>>> typesBySubscriber;
    private final Map<Class<?>, Object> stickyEvents;

主要的有这几个:

  1. 第一个Map存储的Key是Class类型,Value是一个并发的ArrayList里面存的是对订阅者和订阅函数的一种绑定类Subscription从名字上也能看出Key是Event的Class对象。
  2. 第二个存储的是订阅者(Activity什么的啊)和Event类型的List。
  3. 第三个Map存储的是粘性事件,Key是Event类型,Value是真实存在的StickyEvent对象。

知道这三个都是什么之后,这段代码就好看了。我们来看前一部分。

// Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class<?> eventType = subscriberMethod.eventType;
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
      // Map<Class<?>, CopyOnWriteArrayList<Subscription>> 
        CopyOnWriteArrayList<Subscription> 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();
        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);

这段写的虽然有点乱套,但实际上写的挺简单的,而且一堆堆的O(n)遍历,性能也就那样(?)。

首先这里面出现了Subscription:

final class Subscription {
    final Object subscriber;
    final SubscriberMethod subscriberMethod;
    /**
     * Becomes false as soon as {@link EventBus#unregister(Object)} is called, which is checked by queued event delivery
     * {@link EventBus#invokeSubscriber(PendingPost)} to prevent race conditions.
     */
    volatile boolean active;

    Subscription(Object subscriber, SubscriberMethod subscriberMethod) {
        this.subscriber = subscriber;
        this.subscriberMethod = subscriberMethod;
        active = true;
    }

    @Override
    public boolean equals(Object other) {
        if (other instanceof Subscription) {
            Subscription otherSubscription = (Subscription) other;
            return subscriber == otherSubscription.subscriber
                    && subscriberMethod.equals(otherSubscription.subscriberMethod);
        } else {
            return false;
        }
    }

    @Override
    public int hashCode() {
        return subscriber.hashCode() + subscriberMethod.methodString.hashCode();
    }
}

我们发现了这是订阅者和订阅方法类的一个契约关系类。

所以说上面subscribe函数主要做了,

  • 创建了订阅者和方法类的绑定,然后存进了subscriptionsByEventType
  • 对每个类型重新排列了一次优先级
  • typesBySubscriber添加了对应的类型

然后我们可以看一下这个函数的下一半,我们会惊奇地发现,StickyEvent的发送时机居然是在register的时候:

...       
if (subscriberMethod.sticky) {
            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);
            }
        }

这时候轮了一遍所有的粘性事件。isAssignableFrom类似于使用在Class之间的instance of 就是判断两个类是否有相同的接口关系,也就是说有继承和实现关系的事件类,都会被判断处理。

  private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
        if (stickyEvent != null) {
            // If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
            // --> Strange corner case, which we don't take care of here.
            postToSubscription(newSubscription, stickyEvent, Looper.getMainLooper() == Looper.myLooper());
        }
    }
 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 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);
        }
    }

之后就是针对各种的ThreadMode进行了处理,同一线程的直接依赖Java的反射invoke执行了,各种不可以的情况,比如说发到主线程但还没在主线程的时候,都是用队列进行发送到对应线程。

接下来我们看看这里面在各线程之间的发送是怎么实现的。

消息转换线程

我们发现在Subscription和event入队的时候我们把他们封装成了一个PendingPost类:

// HandlePoster    
void enqueue(Subscription subscription, Object event) {
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
            queue.enqueue(pendingPost);
            if (!handlerActive) {
                handlerActive = true;
                if (!sendMessage(obtainMessage())) {
                    throw new EventBusException("Could not send handler message");
                }
            }
        }
    }

然后才进行的入队和发送,这个PendingPost就是一个带有回收池的掩饰传送类:

final class PendingPost {
    private final static List<PendingPost> pendingPostPool = new ArrayList<PendingPost>();

    Object event;
    Subscription subscription;
    PendingPost next;

    private PendingPost(Object event, Subscription subscription) {
        this.event = event;
        this.subscription = subscription;
    }

    static PendingPost obtainPendingPost(Subscription subscription, Object event) {
        synchronized (pendingPostPool) {
            int size = pendingPostPool.size();
            if (size > 0) {
                PendingPost pendingPost = pendingPostPool.remove(size - 1);
                pendingPost.event = event;
                pendingPost.subscription = subscription;
                pendingPost.next = null;
                return pendingPost;
            }
        }
        return new PendingPost(event, subscription);
    }

    static void releasePendingPost(PendingPost pendingPost) {
        pendingPost.event = null;
        pendingPost.subscription = null;
        pendingPost.next = null;
        synchronized (pendingPostPool) {
            // Don't let the pool grow indefinitely
            if (pendingPostPool.size() < 10000) {
                pendingPostPool.add(pendingPost);
            }
        }
    }
}

这里的设计其实挺不错的,一个静态的回收池,初始化靠一个静态方法,优先使用被回收的对象,实现和Message其实很像。另一个release方法就是把用完的对象回收起来。

PendingPostQueue 就是一个PendingPost的队列,里面的操作基本上就是入队出队之类的,有点特殊的是入队和出队都有一把锁。

接着这个队列被用在了好几个Poster类中,实现了向各个线程的消息转换,首先我们来看向主线程发送数据的:

HandlePoster

final class HandlerPoster extends Handler {

    private final PendingPostQueue queue;
    private final int maxMillisInsideHandleMessage;
    private final EventBus eventBus;
    private boolean handlerActive;

    HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {
        super(looper);
        this.eventBus = eventBus;
        this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage;
        queue = new PendingPostQueue();
    }

    void enqueue(Subscription subscription, Object event) {
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
            queue.enqueue(pendingPost);
            if (!handlerActive) {
                handlerActive = true;
                if (!sendMessage(obtainMessage())) {
                    throw new EventBusException("Could not send handler message");
                }
            }
        }
    }

    @Override
    public void handleMessage(Message msg) {
        boolean rescheduled = false;
        try {
            long started = SystemClock.uptimeMillis();
            while (true) {
                PendingPost pendingPost = queue.poll();
                if (pendingPost == null) {
                    synchronized (this) {
                        // Check again, this time in synchronized
                        pendingPost = queue.poll();
                        if (pendingPost == null) {
                            handlerActive = false;
                            return;
                        }
                    }
                }
                eventBus.invokeSubscriber(pendingPost);
                long timeInMethod = SystemClock.uptimeMillis() - started;
                if (timeInMethod >= maxMillisInsideHandleMessage) {
                    if (!sendMessage(obtainMessage())) {
                        throw new EventBusException("Could not send handler message");
                    }
                    rescheduled = true;
                    return;
                }
            }
        } finally {
            handlerActive = rescheduled;
        }
    }
}

HandlePoster 继承自 Handler 再加上初始化的时候传进去的是Looper.getMainThread();所以能向主线程发送消息。每次入队之后都会发送一条空消息去通知handleMessage函数处理队列数据,使用handlerActive作为控制标记位。handleMessage是个死循环两段的if判断用来处理多线程的情况,invokeSubscriber的方式和之前类似。之后就是有一个阀值,当时间超过10ms的时候就会发一个消息重入,并且退出这次循环,这是防止时间太长阻塞主线程。

BackgroundPoster

final class BackgroundPoster implements Runnable {

    private final PendingPostQueue queue;
    private final EventBus eventBus;

    private volatile boolean executorRunning;

    BackgroundPoster(EventBus eventBus) {
        this.eventBus = eventBus;
        queue = new PendingPostQueue();
    }

    public void enqueue(Subscription subscription, Object event) {
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
            queue.enqueue(pendingPost);
            if (!executorRunning) {
                executorRunning = true;
                eventBus.getExecutorService().execute(this);
            }
        }
    }

    @Override
    public void run() {
        try {
            try {
                while (true) {
                    PendingPost pendingPost = queue.poll(1000);
                    if (pendingPost == null) {
                        synchronized (this) {
                            // Check again, this time in synchronized
                            pendingPost = queue.poll();
                            if (pendingPost == null) {
                                executorRunning = false;
                                return;
                            }
                        }
                    }
                    eventBus.invokeSubscriber(pendingPost);
                }
            } catch (InterruptedException e) {
                Log.w("Event", Thread.currentThread().getName() + " was interruppted", e);
            }
        } finally {
            executorRunning = false;
        }
    }
}

BackgroundPoster 自身是一个Runnable ,入队之后就调用EventBus携带的一个线程池进行运行,同样也是一个死循环,用了一个生产者 vs 消费者模式 进行了有限等待,这1000ms内入队的消息都会被弹出处理。

    synchronized PendingPost poll(int maxMillisToWait) throws InterruptedException {
        if (head == null) {
            wait(maxMillisToWait);
        }
        return poll();
    }

PendingPostQueue的poll(int)方法对队列为空的情况进行了等待,唤醒则出现在enqueue:

    synchronized void enqueue(PendingPost pendingPost) {
        if (pendingPost == null) {
            throw new NullPointerException("null cannot be enqueued");
        }
        if (tail != null) {
            tail.next = pendingPost;
            tail = pendingPost;
        } else if (head == null) {
            head = tail = pendingPost;
        } else {
            throw new IllegalStateException("Head present, but no tail");
        }
        notifyAll(); // 在这进行了唤醒
    }

AsyncPoster

如果说Background尚且能保证在同一个线程内完成,AsyncPoster就完全进行了异步操作。

class AsyncPoster implements Runnable {
    private final PendingPostQueue queue;
    private final EventBus eventBus;
    AsyncPoster(EventBus eventBus) {
        this.eventBus = eventBus;
        queue = new PendingPostQueue();
    }
    public void enqueue(Subscription subscription, Object event) {
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        queue.enqueue(pendingPost);
        eventBus.getExecutorService().execute(this);
    }
    @Override
    public void run() {
        PendingPost pendingPost = queue.poll();
        if(pendingPost == null) {
            throw new IllegalStateException("No pending post available");
        }
        eventBus.invokeSubscriber(pendingPost);
    }
}

这里面基本上什么都不控制,直接就来一个运行一次,也不会有什么问题。。。

到这为止我们不但知道了方法是怎么注册和绑定的,我们甚至还知道了粘性事件是怎么发送的了,接着我们来看方法查找和普通事件的发送是怎么进行的。

方法查找

// package org.greenrobot.eventbus.meta;
/** Base class for generated index classes created by annotation processing. */
public interface SubscriberInfo {
    // 获取订阅的类
    Class<?> getSubscriberClass();
    // 所有的method
    SubscriberMethod[] getSubscriberMethods();
    // 获取父类的info
    SubscriberInfo getSuperSubscriberInfo();
    // 是否检查父类
    boolean shouldCheckSuperclass();
}

SubscriberInfo 描述了能通过注解类生成的Index的方法(具体功能我加了主食)。

/**
 * Interface for generated indexes.
 */
public interface SubscriberInfoIndex {
    SubscriberInfo getSubscriberInfo(Class<?> subscriberClass);
}

这个接口是查找info的。

另外可以说这其中的SubscriberMethodInfo存储着SubscriberMethod所需的元信息:

public class SubscriberMethodInfo {
    final String methodName;
    final ThreadMode threadMode;
    final Class<?> eventType;
    final int priority;
    final boolean sticky;
  ...

AbstractSubscriberInfo是一个抽象类,主要负责从Info创建出Method,又是一个反射:

    protected SubscriberMethod createSubscriberMethod(String methodName, Class<?> eventType, ThreadMode threadMode,
                                                      int priority, boolean sticky) {
        try {
            Method method = subscriberClass.getDeclaredMethod(methodName, eventType);
            return new SubscriberMethod(method, eventType, threadMode, priority, sticky);
        } catch (NoSuchMethodException e) {
            throw new EventBusException("Could not find subscriber method in " + subscriberClass +
                    ". Maybe a missing ProGuard rule?", e);
        }
    }

另外还有一个SimpleSubscriberInfo作为他的子类。

接下来的SubscriberMethodFinder也非常重要运行时的方法查找都来自这里:

刚才我们在EventBus.register(...)中调用了这个函数:

    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;
        }
    }

其中的METHOD_CACHE是对每个类方法进行缓存,防止多次查找,毕竟运行时查找还是个复杂的操作,根据是否忽略生成Index。

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

    private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // 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");
            }
        }
    }

findUsingReflectionInSingleClass对反射类进行了处理,这里面通过掩模运算检查了访问权限, 检查了参数个数。

        boolean checkAdd(Method method, Class<?> eventType) {
            // 2 level check: 1st level with event type only (fast), 2nd level with complete signature when required.
            // Usually a subscriber doesn't have methods listening to the same event type.
            Object existing = anyMethodByEventType.put(eventType, method);
            if (existing == null) {
                return true;
            } else {
                if (existing instanceof Method) {
                    if (!checkAddWithMethodSignature((Method) existing, eventType)) {
                        // Paranoia check
                        throw new IllegalStateException();
                    }
                    // Put any non-Method object to "consume" the existing Method
                    anyMethodByEventType.put(eventType, this);
                }
                return checkAddWithMethodSignature(method, eventType);
            }
        }

其中的checkAdd检查了类型和方法签名,每次轮转完成之后都会进行一次findState.moveToSuperclass();对父类进行处理。

使用索引

因为反射所使用的运行时查找速度缓慢,所以我们也经常会通过apt使用已经创建好的Index。

刚才另一个分支的findUsingInfo就是使用已有的Index:

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

这段非常简单,几乎就是刚才的验证而已,如果没拿到数据的话,还会进行正常的反射查找。

 // EventBusAnnotationProcessor 负责生成注解路由表
    private void createInfoIndexFile(String index) {
        BufferedWriter writer = null;
        try {
            JavaFileObject sourceFile = processingEnv.getFiler().createSourceFile(index);
            int period = index.lastIndexOf('.');
            String myPackage = period > 0 ? index.substring(0, period) : null;
            String clazz = index.substring(period + 1);
            writer = new BufferedWriter(sourceFile.openWriter());
            if (myPackage != null) {
                writer.write("package " + myPackage + ";\n\n");
            }
            writer.write("import org.greenrobot.eventbus.meta.SimpleSubscriberInfo;\n");
            writer.write("import org.greenrobot.eventbus.meta.SubscriberMethodInfo;\n");
            writer.write("import org.greenrobot.eventbus.meta.SubscriberInfo;\n");
            writer.write("import org.greenrobot.eventbus.meta.SubscriberInfoIndex;\n\n");
            writer.write("import org.greenrobot.eventbus.ThreadMode;\n\n");
            writer.write("import java.util.HashMap;\n");
            writer.write("import java.util.Map;\n\n");
            writer.write("/** This class is generated by EventBus, do not edit. */\n");
            writer.write("public class " + clazz + " implements SubscriberInfoIndex {\n");
            writer.write("    private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;\n\n");
            writer.write("    static {\n");
            writer.write("        SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();\n\n");
            writeIndexLines(writer, myPackage);
            writer.write("    }\n\n");
            writer.write("    private static void putIndex(SubscriberInfo info) {\n");
            writer.write("        SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);\n");
            writer.write("    }\n\n");
            writer.write("    @Override\n");
            writer.write("    public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {\n");
            writer.write("        SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);\n");
            writer.write("        if (info != null) {\n");
            writer.write("            return info;\n");
            writer.write("        } else {\n");
            writer.write("            return null;\n");
            writer.write("        }\n");
            writer.write("    }\n");
            writer.write("}\n");
        } catch (IOException e) {
            throw new RuntimeException("Could not write source for " + index, e);
        } finally {
            if (writer != null) {
                try {
                    writer.close();
                } catch (IOException e) {
                    //Silent
                }
            }
        }
    }

 private void writeIndexLines(BufferedWriter writer, String myPackage) throws IOException {
        for (TypeElement subscriberTypeElement : methodsByClass.keySet()) {
            if (classesToSkip.contains(subscriberTypeElement)) {
                continue;
            }

            String subscriberClass = getClassString(subscriberTypeElement, myPackage);
            if (isVisible(myPackage, subscriberTypeElement)) {
                writeLine(writer, 2,
                        "putIndex(new SimpleSubscriberInfo(" + subscriberClass + ".class,",
                        "true,", "new SubscriberMethodInfo[] {");
                List<ExecutableElement> methods = methodsByClass.get(subscriberTypeElement);
                writeCreateSubscriberMethods(writer, methods, "new SubscriberMethodInfo", myPackage);
                writer.write("        }));\n\n");
            } else {
                writer.write("        // Subscriber not visible to index: " + subscriberClass + "\n");
            }
        }
    }

有了这两个方法之后我们就知道,平常的index就是通过这种方式拼接出来的。

Post消息

    /** Posts the given event to the event bus. */
    public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);

        if (!postingState.isPosting) {
            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()) {
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }

PostingThreadState是一个存储在ThreadLocal中的对象,包含有以下各种内容,线程信息,是否是主线程,是否取消,还有一个相应的事件队列。

  private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        if (eventInheritance) {
              /** Looks up all Class objects including super classes and interfaces. Should also work for interfaces. */
            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
            int countTypes = eventTypes.size();
            // 对所有的订阅函数,都调用发送数据
            for (int h = 0; h < countTypes; h++) {
                // 所有的订阅类
                Class<?> clazz = eventTypes.get(h);
                subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
            }
        } else {
          // 只发送一次
            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) {
                // 无订阅者的处理
                post(new NoSubscriberEvent(this, event));
            }
        }
    }

之后:

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;
                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;
    }

之后对所有的订阅类的所有订阅者都发送一次数据,发送数据方法和上文相同。

发送粘性数据就是拿锁然后保存到队列中去,这样就可以在重新发送:

    public void postSticky(Object event) {
        synchronized (stickyEvents) {
            stickyEvents.put(event.getClass(), event);
        }
        // Should be posted after it is putted, in case the subscriber wants to remove immediately
        post(event);
    }

因为我们无法确定什么时候粘性事件应该停止继续传播,这取决于我们应用的需要,所以我们应当手动remove掉Sticky Event :

// 系统提供了如下方法
 public <T> T removeStickyEvent(Class<T> eventType) {
        synchronized (stickyEvents) {
            return eventType.cast(stickyEvents.remove(eventType));
        }
    }
    public boolean removeStickyEvent(Object event) {
        synchronized (stickyEvents) {
            Class<?> eventType = event.getClass();
            Object existingEvent = stickyEvents.get(eventType);
            if (event.equals(existingEvent)) {
                stickyEvents.remove(eventType);
                return true;
            } else {
                return false;
            }
        }
    }
    public void removeAllStickyEvents() {
        synchronized (stickyEvents) {
            stickyEvents.clear();
        }
    }

至此我们就分析完了EventBus的基本上所有的代码(处理util包下的错误日志),EventBus本身的实现并不复杂,使用运行时的反射技巧也很简单,单纯的使用注解类可能会拖慢速度,但是通过apt生成的静态表把速降提升到了一个新的高度,apt的生成大家也看到了并不是很复杂,几乎就是类型检查和拼接字串,不过想法决定了EventBus仍然是一个优秀的开源库,希望我们在使用的同时,仍能对实现原理有所了解。

posted @ 2016-12-26 21:48 刘丰恺 阅读(...) 评论(...) 编辑 收藏