Android发布/订阅事件总线框架之EventBus使用详解

1.前言

曾经，一层又一层的业务逻辑让我不知所措，一个又一个的回调让你头晕眼花，一个又一个的参数让你混乱不堪。EventBus，，一个耦合度低到令你害怕的框架。

2.什么是EventBus

EventBus是一个消息总线，以观察者模式实现，用于简化程序的组件、线程通信，可以轻易切换线程、开辟线程。EventBus3.0跟先前版本的区别在于加入了annotation @Subscribe，取代了以前约定命名的方式。

3.相似产品比较

产品名	开发者	备注
EventBus	greenrobot	用户最多，简洁，方便，小巧，文档简洁明了
Guava	google	一个庞大的工具类库，EventBus只是一个小功能
otto	square	fork guava ，用的人不少
AndroidEventBus	何红辉	模仿EventBus开发的

使用EventBus3.0三部曲

1.定义事件

public class MessageEvent {
    public final String message;

    public MessageEvent(String message) {
        this.message = message;
    }
}

2.准备订阅者

    // This method will be called when a MessageEvent is posted
    @Subscribe
    public void onMessageEvent(MessageEvent event){
        Toast.makeText(getActivity(), event.message, Toast.LENGTH_SHORT).show();
    }
    
    // This method will be called when a SomeOtherEvent is posted
    @Subscribe
    public void handleSomethingElse(SomeOtherEvent event){
        doSomethingWith(event);
    }

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

3.发送事件

    EventBus.getDefault().post(new MessageEvent("Hello everyone!"));

深入了解

1.ThreadMode线程通信

EventBus可以很简单的实现线程间的切换，包括后台线程、UI线程、异步线程

ThreadMode.POSTING

    //默认调用方式，在调用post方法的线程执行，避免了线程切换，性能开销最少    
    // Called in the same thread (default)
    @Subscribe(threadMode = ThreadMode.POSTING) // ThreadMode is optional here
    public void onMessage(MessageEvent event) {
        log(event.message);
    }

ThreadMode.MAIN

    // Called in Android UI's main thread
    @Subscribe(threadMode = ThreadMode.MAIN)
    public void onMessage(MessageEvent event) {
        textField.setText(event.message);
    }

ThreadMode.BACKGROUND

    // 如果调用post方法的线程不是主线程，则直接在该线程执行
    // 如果是主线程，则切换到后台单例线程，多个方法公用同个后台线程，按顺序执行，避免耗时操作
    // Called in the background thread
    @Subscribe(threadMode = ThreadMode.BACKGROUND)
    public void onMessage(MessageEvent event){
        saveToDisk(event.message);
    }

ThreadMode.ASYNC

    //开辟新独立线程，用来执行耗时操作，例如网络访问
    //EventBus内部使用了线程池，但是要尽量避免大量长时间运行的异步线程，限制并发线程数量
    //可以通过EventBusBuilder修改，默认使用Executors.newCachedThreadPool()
    // Called in a separate thread
    @Subscribe(threadMode = ThreadMode.ASYNC)
    public void onMessage(MessageEvent event){
        backend.send(event.message);
    }

2.配置EventBusBuilder

EventBus提供了很多配置，一般的情况下我们可以不用配置.但是，如果你有一些其他要求,比如控制日志在开发的时候输出,发布的时候不输出，在开发的时候错误崩溃，而发布的时候不崩溃...等情况。
EventBus提供了一个默认的实现，但不是单例。

    EventBus eventBus = new EventBus();
    //下面这一条的效果是完全一样的
    EventBus eventBus = EventBus.builder().build();
    //修改默认实现的配置，记住，必须在第一次EventBus.getDefault()之前配置，且只能设置一次。建议在application.onCreate()调用
    EventBus.builder().throwSubscriberException(BuildConfig.DEBUG).installDefaultEventBus();

3.StickyEvent

StickyEvent在内存中保存最新的消息，取消原有消息，执行最新消息，只有在注册后才会执行，如果没有注册，消息会一直保留来内存中

    //在注册之前发送消息
    EventBus.getDefault().postSticky(new MessageEvent("Hello everyone!"));

    //限制，新界面启动了
   @Override
    public void onStart() {
        super.onStart();
        EventBus.getDefault().register(this);
    }
    //在onStart调用register后，执行消息
    @Subscribe(sticky = true, threadMode = ThreadMode.MAIN)
    public void onEvent(MessageEvent event) {
        // UI updates must run on MainThread
        textField.setText(event.message);
    }

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

你也可以手动管理StickyEvent

    MessageEvent stickyEvent = EventBus.getDefault().getStickyEvent(MessageEvent.class);
    // Better check that an event was actually posted before
    if(stickyEvent != null) {
        // "Consume" the sticky event
        EventBus.getDefault().removeStickyEvent(stickyEvent);
        //or
         EventBus.getDefault().removeAllStickyEvents();
        // Now do something with it
    }

在这里，或许你会有个疑问，
StickyEvent=true的订阅者能否接收post的事件？
StickyEvent=false的订阅者能否接收postSticky的事件？
查看源码发现

   /**
     * Posts the given event to the event bus and holds on to the event (because it is sticky). The most recent sticky
     * event of an event's type is kept in memory for future access by subscribers using {@link Subscribe#sticky()}.
     */
    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);
    }

    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        ...省略部分代码
        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);
            }
        }
    }

        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);
            }
        }

发现，post方法没有过滤StickyEvent，而postSticky是调用post方法的，所以，无论post还是postSticky，StickyEvent是true或false，都会执行

4.priority事件优先级

    //priority越大，级别越高
    @Subscribe(priority = 1);
    public void onEvent(MessageEvent event) {
    …
    }

//优先级实现方式，遍历当前列表，把当前
    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;
        }
    }

5.中止事件传递

    // 中止事件传递，后续事件不在调用,注意，只能在传递事件的时候调用
    @Subscribe
    public void onEvent(MessageEvent event){
        …
        EventBus.getDefault().cancelEventDelivery(event) ;
    }

6.index索引加速

EventBus使用了annotation，默认在编译时生成代码，生成索引，
添加index后会在编译时运行，自动生成相应代码。
ps:由于apt的限制，匿名内部类中的annotation不会被识别，会自动降级在运行时反射，此时，效率会降低
EventBus官网地址

buildscript {
    dependencies {
        classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'
    }
}
apply plugin: 'com.neenbedankt.android-apt'
dependencies {
    compile 'org.greenrobot:eventbus:3.0.0'
    apt 'org.greenrobot:eventbus-annotation-processor:3.0.1'
}
apt {
    arguments {
        eventBusIndex "com.example.myapp.MyEventBusIndex"
    }
}

由于kotlin的使用annotation处理方式不同，需要使用kapt

apply plugin: 'kotlin-kapt' // ensure kapt plugin is applied

dependencies {
    compile 'org.greenrobot:eventbus:3.1.1'
    kapt 'org.greenrobot:eventbus-annotation-processor:3.1.1'
}

kapt {
    arguments {
        arg('eventBusIndex', 'com.example.myapp.MyEventBusIndex')
    }
}

ps:如果是多个模块都需要加索引，则每个模块都要加上上面三个apt，包引入可以不需要重复

这样，编译后就会自动生成代码


/** This class is generated by EventBus, do not edit. */
public class MyEventBusIndex implements SubscriberInfoIndex {
    private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;

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

        putIndex(new SimpleSubscriberInfo(com.zhiyecn.demoeventbus.ScrollingActivity.class, true,
                new SubscriberMethodInfo[] {
            new SubscriberMethodInfo("eventBus", com.zhiyecn.demoeventbus.StickEvent.class),
            new SubscriberMethodInfo("eventBusStick", com.zhiyecn.demoeventbus.StickEvent.class, ThreadMode.BACKGROUND,
                    0, true),
        }));

    }

    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;
        }
    }
}

在编译时通过反射生成event方法存在map中，在发送事件的时候，调用getSubscriberInfo获取该map。

private SubscriberInfo getSubscriberInfo(FindState findState) {
    ...省略
    if (subscriberInfoIndexes != null) {
        for (SubscriberInfoIndex index : subscriberInfoIndexes) {
            SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
            if (info != null) {
                return info;
            }
        }
    }
    return null;
}

使用方式

//给自定义eventbus添加索引
EventBus eventBus = EventBus.builder().addIndex(new MyEventBusIndex()).build();

//给默认eventbus添加索引
EventBus.builder().addIndex(new MyEventBusIndex()).installDefaultEventBus();
// Now the default instance uses the given index. Use it like this:
EventBus eventBus = EventBus.getDefault();

//多个库需要添加索引
EventBus eventBus = EventBus.builder()
    .addIndex(new MyEventBusAppIndex())
    .addIndex(new MyEventBusLibIndex()).build();

7.NoSubscriberEvent

如果没找到订阅者事件，可以通过EventBusBuilder设置是否默认发送NoSubscriberEvent，默认是打开的


/**
 * This Event is posted by EventBus when no subscriber is found for a posted event.
 * 
 * @author Markus
 */
public final class NoSubscriberEvent {
    /** The {@link EventBus} instance to with the original event was posted to. */
    public final EventBus eventBus;

    /** The original event that could not be delivered to any subscriber. */
    public final Object originalEvent;

    public NoSubscriberEvent(EventBus eventBus, Object originalEvent) {
        this.eventBus = eventBus;
        this.originalEvent = originalEvent;
    }

}

    private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        ....
        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));
            }
        }
    }

8.混淆

-keepattributes *Annotation*
-keepclassmembers class ** {
    @org.greenrobot.eventbus.Subscribe <methods>;
}
-keep enum org.greenrobot.eventbus.ThreadMode { *; }

# Only required if you use AsyncExecutor
-keepclassmembers class * extends org.greenrobot.eventbus.util.ThrowableFailureEvent {
    <init>(java.lang.Throwable);
}

9.利弊

好处

简单，方便，小巧，文档清晰，性能消耗少，可定制行强，耦合度低

坏处

耦合度太低

这绝对不是个笑话，，EventBus的耦合太低了，如果不加以控制管理，你会不知道，你发的消息到跑哪里去了。也不知道你的这条消息，会在哪里发出。如果你没有很好的方法解决这个问题，建议不好用太多。

使用建议

1、EventBus管理

EventBus运行创建多个，那么，明确事件的生命周期，根据不同生命周期使用不同的EventBus？

/**
 * 方法1
 * 用annotation配合使用工厂
 * EventBusFactory.getBus(EventBusFactory.START);
 * EventBusFactory.getBus();
 */
public class EventBusFactory {
    private static SparseArray<EventBus> mBusSparseArray = new SparseArray<>(2);

    @IntDef({CREATE, START})
    @Retention(RetentionPolicy.SOURCE)
    public @interface BusType {
    }

    public static final int CREATE = 0;
    public static final int START = 1;

    static {
        mBusSparseArray.put(CREATE, EventBus.builder().build());
        mBusSparseArray.put(START, EventBus.getDefault());
    }

    public static EventBus getBus() {
        return getBus(START);
    }

    public static EventBus getBus(@BusType int type) {
        return mBusSparseArray.get(type);
    }

}

/**
 * 方法2
 * 用枚举工厂
 * EventBusFactory.START.getBus();
 */
public enum EventBusFactory {
    CREATE(0),
    START(1);

    private int mType;

    EventBusFactory(int type) {
        mType = type;
    }

    public EventBus getBus() {
        return mBusSparseArray.get(mType);
    }

    private static SparseArray<EventBus> mBusSparseArray = new SparseArray<>(2);

    static {
        mBusSparseArray.put(CREATE.mType, EventBus.builder().build());
        mBusSparseArray.put(START.mType, EventBus.getDefault());
    }
}

2、以事件为对象

将数据封装到一个事件类。所有事件放到一个包下。如果事件太多，同个模块的事件可以考虑使用静态内部类，或者再分包。

public class Event  {  
    public static class UserListEvent {  
        public List<User> users ;  
    }
    public static class ItemListEvent {  
        public List<Item> items;  
    }    
}

注意，不是相同类型就一定要作为一个事件封装，具体需要考虑业务情景跟代码情况，比如事件行为不同、事件生命周期不同，如果有必要，写封装成两个Event可能是更好的选择。

public class Event  { 
    public static class UserListUpdateEventOnCreate {  
        public List<User> users;  
    } 
    public static class UserListUpdateEventOnStart {  
        public List<User> users ;  
    }
    public static class UserListRemoveEventOnStart {  
        public List<User> users;  
    } 
}