Android EventBus源码解析

 项目地址 ：https://github.com/greenrobot/EventBus

这个项目个人感觉就是为了解决回调事件过多的，比方说A函数在做完以后 要调用b类的c函数，那我们通常的做法就是 定义一个接口 然后再A函数所属的类里面注册这个接口。

然后a函数做完以后 直接调用这个接口即可。但是这种方法写多了以后确实很麻烦，于是EventBus就是用来解决这种场景的。

和以往一样，我们只解析他的源码，如果你要学习他的用法请自行谷歌。

我们就从register函数开始说起。

 private synchronized void register(Object subscriber, boolean sticky, int priority) {
        //这个list就是方法的集合
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriber.getClass());
        //这个循环的目的就是为了用  方法对象 来构造 Subscription对象
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod, sticky, priority);
        }
    }

首先来看一下SubscriberMethod这个类，

/*
 * Copyright (C) 2012 Markus Junginger, greenrobot (http://greenrobot.de)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package de.greenrobot.event;

import java.lang.reflect.Method;

import android.util.Log;

/**
 * 这个类就是描述方法用的
 */
final class SubscriberMethod {
    final Method method;
    final ThreadMode threadMode;
    final Class<?> eventType;
    /** Used for efficient comparison */
    /**
     * 这个methodString 实际上就是用来描述SubscriberMethod对象的，尤其是在重写的equals方法里 起到关键的作用
     * 就类似于这种结构
     * com.example.administrator.eventbustest.ItemDetailFragment#onEventMainThread(com.example.administrator.eventbustest.Item
     *其实也很好理解就是 包名+类名#方法名(参数的类型
     * 注意这里参数的类型也是全路径名 包名+类名
     */
    String methodString;

    SubscriberMethod(Method method, ThreadMode threadMode, Class<?> eventType) {
        this.method = method;
        this.threadMode = threadMode;
        this.eventType = eventType;
    }

    @Override
    public boolean equals(Object other) {
        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();
    }
}

然后我们来看看这个SubscriberMethod对象组成的集合 是怎么构造出来的。

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        String key = subscriberClass.getName();
        List<SubscriberMethod> subscriberMethods;
        synchronized (methodCache) {
            subscriberMethods = methodCache.get(key);
        }
      
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
        subscriberMethods = new ArrayList<SubscriberMethod>();
        Class<?> clazz = subscriberClass;
        HashSet<String> eventTypesFound = new HashSet<String>();
        StringBuilder methodKeyBuilder = new StringBuilder();
        while (clazz != null) {
            String name = clazz.getName();
            //这个地方判断如果是这些类，那么就直接跳出这个while循环
            //注意name的值 也是包名+类名，所以这里就是过滤掉基础的sdk的那些方法
            //如果你有引用其他公共lib库的话 你也可以过滤他们的包，
            if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {
                // Skip system classes, this just degrades performance
                break;
            }

            // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
            Method[] methods = clazz.getDeclaredMethods();
            for (Method method : methods) {
                String methodName = method.getName();
                Log.e("burning", "methodName == " + methodName);
                //只有那些以onEvent开头的方法才是我们需要的
                if (methodName.startsWith(ON_EVENT_METHOD_NAME)) {
                    int modifiers = method.getModifiers();
                    //注意这个地方判断方法属性的技巧 与 操作
                    if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                        Class<?>[] parameterTypes = method.getParameterTypes();
                        //如果参数只有一个
                        if (parameterTypes.length == 1) {
                            String modifierString = methodName.substring(ON_EVENT_METHOD_NAME.length());
                            //取ThreadMode
                            ThreadMode threadMode;
                            if (modifierString.length() == 0) {
                                threadMode = ThreadMode.PostThread;
                            } else if (modifierString.equals("MainThread")) {
                                threadMode = ThreadMode.MainThread;
                            } else if (modifierString.equals("BackgroundThread")) {
                                threadMode = ThreadMode.BackgroundThread;
                            } else if (modifierString.equals("Async")) {
                                threadMode = ThreadMode.Async;
                            } else {
                                if (skipMethodVerificationForClasses.containsKey(clazz)) {
                                    continue;
                                } else {
                                    throw new EventBusException("Illegal onEvent method, check for typos: " + method);
                                }
                            }
                            Class<?> eventType = parameterTypes[0];
                            methodKeyBuilder.setLength(0);
                            methodKeyBuilder.append(methodName);
                            methodKeyBuilder.append('>').append(eventType.getName());
                            //onEventMainThread>java.lang.String
                            //methodKey就是上面的形式，可以看出来是方法名>参数  的格式
                            String methodKey = methodKeyBuilder.toString();
                            //这个地方先去这个hashset里面add这个key，当然了，如果你这个hashset里面已经有这个key
                            //那必然是add不成功的，只有add成功返回true以后括号内的代码才会得到执行
                            if (eventTypesFound.add(methodKey)) {
                                // Only add if not already found in a sub class
                                //这个地方就是构造SubscriberMethod对象放到list里 准备返回
                                subscriberMethods.add(new SubscriberMethod(method, threadMode, eventType));
                            }
                        }
                    } else if (!skipMethodVerificationForClasses.containsKey(clazz)) {
                        Log.d(EventBus.TAG, "Skipping method (not public, static or abstract): " + clazz + "."
                                + methodName);
                    }
                }
            }
            //这里注意还在大的while循环内，所以你传进去的类自己查完一遍方法以后 还会去找他的父类继续查询方法
            //一直遍历到父类为 那些java android开头的基类为止！
            clazz = clazz.getSuperclass();

        }
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass + " has no public methods called "
                    + ON_EVENT_METHOD_NAME);
        } else {
            synchronized (methodCache) {
                methodCache.put(key, subscriberMethods);
            }
            return subscriberMethods;
        }
    }

然后我们来看看register函数里面 5-6行 那个循环遍历做了什么

首先我们看看这个循环调用的方法：

/**
     * @param subscriber       方法所述的类的 包名+类名
     * @param subscriberMethod
     * @param sticky
     * @param priority
     */
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod, boolean sticky, int priority) {
        //这个eventtype就是方法的参数的类名
        Class<?> eventType = subscriberMethod.eventType;
 
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod, priority);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<Subscription>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }

        // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
        // subscriberMethod.method.setAccessible(true);
        //这个就是优先级高的位置在前面
        int size = subscriptions.size();
        for (int i = 0; i <= size; i++) {
            if (i == size || newSubscription.priority > subscriptions.get(i).priority) {
                subscriptions.add(i, newSubscription);
                break;
            }
        }

        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<Class<?>>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);

        if (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);
            }
        }
    }

10-13行 我们可以看出来 这个函数 主要是为了构造subscription这个list对象。

  /**
     * 这个map 存储方法的地方 key就是eventType，value就是copyOnWriteArrayList value就是方法的一切
     */
    private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;

我们可以看看这个类是什么

/**
 * 这个类里面包含有SubscriberMethod类对象，
 * subscriber
 */
final class Subscription {
    //这个实际上就是描述方法所属的类的
    final Object subscriber;
    //描述方法的类
    final SubscriberMethod subscriberMethod;
    //优先级
    final int priority;
    /**
     * 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, int priority) {
        this.subscriber = subscriber;
        this.subscriberMethod = subscriberMethod;
        this.priority = priority;
        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();
    }
}

所以总结起来，SubscriberMethod 就是对方法的描述，而我们的SubscriberMethod 实际上就是Subscription的子集，Subscription除了有描述方法的对象以外，还有这个方法所属的类，

而我们的register方法总体来说 就是先通过findSubscriberMethods方法 取得我们注册类（就是你register调用的时候传的this）所需要的的那些方法（注意不是每个方法都需要 只选择自己需要的）

然后把这些方法 做一个list，最后再通过便利这个list ： 用每一个SubscriberMethod 对象和这个方法所需的类（包名+类名） 来构造出一个Subscription对象，然后把这个对象

存储在SubscriptionsByEventType里，注意这个map的key 实际上就是eventType，而value则代表方法的list，换句话说。

这个SubscriptionsByEventType 是一个键值对，它的key 实际上就是我们的类名，value则是这个类里面我们需要存储的方法的list！

这就是register的大致流程，我们再来看看post 流程即可。

 /**
     * Posts the given event to the event bus.
     */
    public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        //这个地方可以看出来是每次有人调用post方法的时候 都会从postingState取出这个队列，然后把这个事件放到这个队列里
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);

        //这个判断isPosting 主要是为了保证同一时间只能有一个线程在处理括号体里的内容
        //currentPostingThreadState 是用threadlocal来构造的 所以保证了同步性
        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()) {
                    //队列不为空就处理
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }

先看看第5行的postingState是什么

 /**
     * 静态类,里面除了有一个队列以外,还有几个标志位，以及一个Subscription
     */
    final static class PostingThreadState {
        final List<Object> eventQueue = new ArrayList<Object>();
        boolean isPosting;
        boolean isMainThread;
        Subscription subscription;
        Object event;
        boolean canceled;
    }

这个地方就能看出来，我们每次调用post 都是往eventQueue里面添加一个事件，而12行开始则是从队列里面

取事件来处理，注意12行开始 一次性只能允许一个线程使用~同步的

然后继续看是怎么处理的。

 /**
     * @param event        方法的参数的类名
     * @param postingState
     * @throws Error
     */
    private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
      
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
       
        if (eventInheritance) {
            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
            int countTypes = eventTypes.size();
            for (int h = 0; h < countTypes; h++) {
                Class<?> clazz = eventTypes.get(h);
                //这个地方就是取出Subscription对象的的所有信息！发消息也是在这个函数里发送的
                subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
            }
        } else {
            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) {
                post(new NoSubscriberEvent(this, event));
            }
        }
    }

继续跟进去

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 {
                    //这个地方就是真正发消息的地方了
                    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;
    }

可以看出来 2-6行 就是从我们register流程里存储的键值对里 把我们存放的方法给取出来。，

取出来以后 就可以反射调用他们的方法了

  /**
     * 这个类就是反射执行方法 并且是最终执行回调方法的地方
     *
     * @param subscription
     * @param event
     * @param isMainThread
     */
    private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        switch (subscription.subscriberMethod.threadMode) {
            case PostThread:
                invokeSubscriber(subscription, event);
                break;
            case MainThread:
                if (isMainThread) {
                    invokeSubscriber(subscription, event);
                } else {
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
            case BackgroundThread:
                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);
        }
    }

13-18行 这个case 如果是主线程，那么就直接反射方法，如果不是的话 则要放到主线程handler里执行。

 private final HandlerPoster mainThreadPoster;

 //这个就是主线程handler初始化
        mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);

/*
 * Copyright (C) 2012 Markus Junginger, greenrobot (http://greenrobot.de)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package de.greenrobot.event;

import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.os.SystemClock;

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;
        }
    }
}

51-77行 就是我们实际最终调用的地方。

同样的 我们在看看20-26行的这个background这个case

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;
        }
    }

一看就知道 他是runnable对象 必然是在后台 在子线程内执行，同时他也是一次性只能做一次操作，完成一个事件，

最后我们来看看Async这个case：

/*
 * Copyright (C) 2012 Markus Junginger, greenrobot (http://greenrobot.de)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package de.greenrobot.event;


/**
 * Posts events in background.
 * 
 * @author Markus  并发执行任务，在线程池内执行
 */
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);
    }

}

这个地方和background相同的就是也是在非主线程，在子线程内执行，但是这个地方是在线程池内执行，可以并发执行多个任务，

而我们的background 则一次性只能执行一个任务，这是2者之间的区别。