三种线程模式分别如下，DefaultEventHandler（在发布线程执行观察者放方法）：

public class DefaultEventHandler implements IEventHandler {

    @Override

    public void handleEvent(Subscription subscription, Object message) {

        if (subscription == null || subscription.subscriber.get() == null) {

            return;

        }

        try {

            subscription.subscriberMethod.method.invoke(subscription.subscriber.get(), message);

        } catch (IllegalAccessException | InvocationTargetException e) {

            e.printStackTrace();

        }

    }

}

MainEventHandler（在主线程执行）：

public class MainEventHandler implements IEventHandler {

    private Handler mMainHandler = new Handler(Looper.getMainLooper());

    DefaultEventHandler hanlder = new DefaultEventHandler();

    @Override

    public void handleEvent(final Subscription subscription, final Object message) {

        mMainHandler.post(new Runnable() {

            @Override

            public void run() {

                hanlder.handleEvent(subscription, message);

            }

        });

    }

}

AsyncEventHandler（新开一个线程执行）：

public class AsyncEventHandler implements IEventHandler {

    private DispatcherThread mDispatcherThread;

    private IEventHandler mEventHandler = new DefaultEventHandler();

    public AsyncEventHandler() {

        mDispatcherThread = new DispatcherThread(AsyncEventHandler.class.getSimpleName());

        mDispatcherThread.start();

    }

    @Override

    public void handleEvent(final Subscription subscription, final Object message) {

        mDispatcherThread.post(new Runnable() {

            @Override

            public void run() {

                mEventHandler.handleEvent(subscription, message);

            }

        });

    }

    private class DispatcherThread extends HandlerThread {

        // 关联了AsyncExecutor消息队列的Handle

        Handler mAsyncHandler;

        DispatcherThread(String name) {

            super(name);

        }

        public void post(Runnable runnable) {

            if (mAsyncHandler == null) {

                throw new NullPointerException("mAsyncHandler == null, please call start() first.");

            }

            mAsyncHandler.post(runnable);

        }

        @Override

        public synchronized void start() {

            super.start();

            mAsyncHandler = new Handler(this.getLooper());

        }

    }

}

以上便是发布消息的代码。

注销监听

最后一个对象被销毁还要注销监听，否则容易导致内存泄露，目前SimpleEventBus用的是WeakReference，能够通过GC自动回收，但不知道greenrobot/EventBus为什么没这样实现，待研究。注销监听其实就是遍历Map，拿掉该对象的订阅即可：

public class EventBus {

    ...

    public void unregister(Object subscriber) {

        if (subscriber == null) {

            return;

        }

        Iterator<CopyOnWriteArrayList<Subscription>> iterator = mSubscriptionsByEventtype.values().iterator();

        while (iterator.hasNext()) {

            CopyOnWriteArrayList<Subscription> subscriptions = iterator.next();

            if (subscriptions != null) {

                List<Subscription> foundSubscriptions = new LinkedList<>();

                for (Subscription subscription : subscriptions) {

                    Object cacheObject = subscription.subscriber.get();

                    if (cacheObject == null || cacheObject.equals(subscriber)) {

                        foundSubscriptions.add(subscription);

                    }

                }

                subscriptions.removeAll(foundSubscriptions);

            }

            // 如果针对某个Event的订阅者数量为空了,那么需要从map中清除

            if (subscriptions == null || subscriptions.size() == 0) {

                iterator.remove();

            }

        }

    }

    ...

}

以上便是事件总线最核心部分的代码实现，完整代码见vimerzhao/SimpleEventBus，后面发现问题更新或者进行升级也只会改动仓库的代码。

局限性

由于时间关系，目前只研究了EventBus的核心部分，还有几个值得深入研究的点，在此记录一下，也欢迎路过的大牛指点一二。

性能问题