EventBus3.1使用以及源码解析

概述

是一个发布/订阅事件。可以用来进行组件之间的通信，比如Activity与Activity,Activity与Fragment,Thread,Services通信。 常用的通信有

• Handler
• BroadcastReceiver/LocalBroadcastReceiver。
• 接口回调
• 文件I/O
• Sharepreference。

那么使用EventBus好处就是解藕，更优雅来实现通信,下面一起来分析EventBus的原理机制。 框架源码:github.com/greenrobot/… 通过链接下载

具体流程

1. 注册订阅者
2. 发布事件
3. 响应事件方法
4. 取消注册订阅者

在使用订阅之前通过build.gradle文件中添加依赖

dependencies {
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'

    })

    //添加EventBus 依赖
    compile 'org.greenrobot:eventbus:3.1.1'

}

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一.订阅

在Activity,或者Fragment,在onCreate()方法通过 EventBus.getDefault().register(this)完成注册 ,声明订阅事件onEvent和事件类型NormalMessage,onEvent和NormalMessageEventType可以自己定义，用来发布的时候来识别， 但注意的是必须是要声明@Subscribe和这个订阅者响应方法是public,方法里必须有参数，也就是事件类型

public class MainActivity extends FragmentActivity {


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //注册订阅者
        EventBus.getDefault().register(this);
    }

    /**
     *  订阅者方法
     * @type {[type]}
     */
    @Subscribe(threadMode = ThreadMode.MAIN)
    public void onEvent(NormalMessageEventType normalMessage){
        Log.i("test","onEvent");

    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        //取消注册订阅者
        EventBus.getDefault().unregister(this);

    }
}


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二.发布

通过 EventBus.getDefault().post(new NormalMessageEventType())来发布

/**
 * Created by longer on 2018/8/30.
 */

public class TwoActivity extends FragmentActivity {


    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_two);
        initView();
    }


    private void initView() {
        findViewById(R.id.btn).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                EventBus.getDefault().post(new NormalMessageEventType());
            }
        });
    }
}

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三.响应事件

也就是刚才订阅onEvent事件，通过NormalMessage事件类型来获取响应，在不同的Activity,或Fragment有多个NormalMessage事件类型也是会响应的，因为EventBus是通过事件类型来循环事件调用的，具体下面会源码分析

public class MainActivity extends FragmentActivity {
   /**
     *  订阅者方法
     * @type {[type]}
     */
    @Subscribe(threadMode = ThreadMode.MAIN)
    public void onEvent(NormalMessage normalMessage){
        Log.i("test","onEvent");

    }

}
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四.取消注册的订阅者

在Activity,Fragment的onDestroy()里取消注册，官网是在onStop()但会有问题

public class MainActivity extends FragmentActivity {


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //注册订阅者
        EventBus.getDefault().register(this);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        //取消注册订阅者
        EventBus.getDefault().unregister(this);

    }
}
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好了下面就开始进行源码分析吧。

源码解析

注册订阅者

先通过注册入口 EventBus.getDefault().register(this);着手

public class EventBus {


    /** Log tag, apps may override it. */
    public static String TAG = "EventBus";

    static volatile EventBus defaultInstance;

    private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();

    //
    private static final Map, List>> eventTypesCache = new HashMap<>();

    private final Map, CopyOnWriteArrayList> subscriptionsByEventType;
    
    //记录所有订阅者,用来取消注册使用
    private final Map>> typesBySubscriber;
    private final Map, Object> stickyEvent

    private final SubscriberMethodFinder subscriberMethodFinder;

    // @Nullable
    private final MainThreadSupport mainThreadSupport;
    // @Nullable
    private final Poster mainThreadPoster;
    private final BackgroundPoster backgroundPoster;
    private final AsyncPoster asyncPoster;


    /** Convenience singleton for apps using a process-wide EventBus instance. */
    public static EventBus getDefault() {
        if (defaultInstance == null) {
            synchronized (EventBus.class) {
                if (defaultInstance == null) {
                    defaultInstance = new EventBus();
                }
            }
        }
        return defaultInstance;
    }

   public EventBus() {
        this(DEFAULT_BUILDER);
    }

    EventBus(EventBusBuilder builder) {
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        mainThreadSupport = builder.getMainThreadSupport();
        mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);       
    }

        /**
     * Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they
     * are no longer interested in receiving events.
     * 

     * 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();
        //查找订阅者的所有订阅响应的事件方法,
        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

}
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EventBus是一个懒汉模式双重检索的单例,通过构造方法初始化成员变量，具体来看

1.Map, CopyOnWriteArrayList> subscriptionsByEventType; 所有事件类型和订阅者封装到Map，已事件类型为做key,value是事件类型对于的ArrayList响应事件列表，发布的时候通过事件类型进行回调 2.Map>> typesBySubscriber; 用Map记录所有订阅者，以订阅者作为key,value记录所有订阅者里面的响应的事件方法，比如onEvent()。在取消注册订阅者通过这个集合调用。 3.Map, Object> stickyEvent; 粘性订阅集合.粘性和非粘性的区别是前者是发送事件之后再订阅该事件也能收到该事件。

关于Poster,BackgroundPoster,AsyncPoster我们到分析发布订阅事件的时候再分析，好继续往下看，

发现EventBus.getDefault().register(this)来注册的时候实际是先获取订阅者的Class信息，也就是Activity,或者Fragment等Class订阅者的类消息，再使用SubscriberMethodFinder类里的findSubscriberMethods获取订阅的响应事件方法 ，那他具体是怎么去获取的？具体进一步看看

class SubscriberMethodFinder {

    private static final Map, List> METHOD_CACHE = new ConcurrentHashMap<>();


    List findSubscriberMethods(Class subscriberClass) {

        //获取订阅者缓存,如果有直接返回，没有先判断是否忽略生成下标
        List subscriberMethods = METHOD_CACHE.get(subscriberClass);
        if (subscriberMethods != null) {
            return subscriberMethods;
        }

        if (ignoreGeneratedIndex) {
            //通过反射技术
            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;
        }
    }

}
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我们先忽略ignoreGeneratedIndex，因为这个会默认显示false

class SubscriberMethodFinder {

    private List 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 {
                //最终会进入
                //
                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");
            }
        }
    }

}

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发现调用findUsingInfo(Class subscriberClass)方法，其他的不用管他，最终会进入findUsingReflectionInSingleClass(FindState findState)反射，然后获取@Subscribe注解，获取方法，用方法第一个参数获取事件类型，再封装SubscriberMethod，这下就真相大白了

先有必要理解下SubscriberMethod这个类是干啥的，在注册订阅的时候进行循环遍历的就是SubscriberMethod来订阅

   public void register(Object subscriber) {
        Class subscriberClass = subscriber.getClass();
        //查找订阅者的所有订阅响应的事件方法,
        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }
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package org.greenrobot.eventbus;

public class SubscriberMethod {
    final java.lang.reflect.Method method;
    final org.greenrobot.eventbus.ThreadMode threadMode;
    final java.lang.Class eventType;
    final int priority;
    final boolean sticky;
    java.lang.String methodString;

    public SubscriberMethod(java.lang.reflect.Method method, java.lang.Class eventType, org.greenrobot.eventbus.ThreadMode threadMode, int priority, boolean sticky) { /* compiled code */ }

    public boolean equals(java.lang.Object other) { /* compiled code */ }

    private synchronized void checkMethodString() { /* compiled code */ }

    public int hashCode() { /* compiled code */ }
}
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其实仔细看SubscriberMethod 就是自己通过@Subscribe定义事件（订阅响应的事件）,里面包含 1.method(Java反射的方法信息) 2.threadMode(线程模型), 下面会介绍到 3.eventType 事件类型 4.priority 优先级 5.sticky 是否为粘性事件

这次，已经分析EvenBus,SubscriberMethod,通过反射来找到订阅者的事件，下面分析如何订阅

如何去订阅？

上面知道是通过遍历SubscriberMethod再执行subscribe()方法，具体来看看做了什么

public class EventBus {

private final Map, CopyOnWriteArrayList> subscriptionsByEventType;
private final Map>> typesBySubscriber;
private final Map, Object> stickyEvents;


public void register(Object subscriber) {
        Class subscriberClass = subscriber.getClass();
        //查找订阅者的所有订阅响应的事件方法,
        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                //订阅
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

   // Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class eventType = subscriberMethod.eventType;
        //
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        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();
        for (int i = 0; i <= size; i++) {
            if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
                subscriptions.add(i, newSubscription);
                break;
            }
        }

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

            //是否为粘性事件
        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).
                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);
            }
        }
    }
}

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分三步来看

1. 通过Subscription 类把subscriber，subscriberMethod封装
2. 通过订阅者的事件类型eventType来作为value,CopyOnWriteArrayList保存所有的订阅，如果有相同事件类型，就直接copyOnWriteArrayList集合中add,否则创建一个CopyOnWriteArrayList,最终 subscriptionsByEventType.put(eventType, subscriptions);把事件类型映射的所有订阅关联
3. 把相关订阅者的事件类型加入到typesBySubscriber集合中，这个集合是用来取消注册的时候用到
4. 最后判断是否为粘性，如果为粘性直接回掉

如何订阅分析到这里。

如何发布订阅?

EventBus.getDefault().post(new NormalMessageEventType()); NormalMessageEventType这个类是自己定义的事件类型。

public class EventBus {


    private final Map, CopyOnWriteArrayList> subscriptionsByEventType;
    private final Map>> typesBySubscriber;

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