EvenBus的基本使用和源碼分析

前言

由於最近項目中頻繁使用EventBus，所以想了解下它內部原理，我們得"知其然知其所以然"。在這裏總結一下，方便自己複習。

歡迎start我的GitHub項目，裏面會不定時更細Android 、java、設計模式相關知識！

EventBus的使用

項目配置

很簡單只需要在build.gradle(app)下添加如下：

implementation 'org.greenrobot:eventbus:3.1.1'

項目中使用

普通使用：

1、發送消息：

EventBus.getDefault().post(事件);

2、在接受頁面註冊和反註冊，並接收。

//註冊
EventBus.getDefault().register(this);
//反註冊
EventBus.getDefault().unregister(this);
//接收事件(getdata這名字是隨便的，起什麼都可以)
@Subscribe(threadMode = ThreadMode.MAIN)
   public void getdata(事件) {
  //在這裏處理髮送的事件
 }

普通使用例子：

比如現在有這樣一個需求，有兩個頁面分別是MainActivity和Main2Activity。需要MainActivity跳轉到Main2Activity，點擊Main2Activity中的按鈕返回並攜帶數據。（當然我們也可以使用onActivityResult去實現 -。-）

MainActivity代碼:

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
      //註冊
        EventBus.getDefault().register(this);
    }
    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.main_tv:
                Intent intent = new Intent(this, Main2Activity.class);
                startActivity(intent);
                break;
        }
    }
  //訂閱者方法
    @Subscribe(threadMode = ThreadMode.MAIN)
    public void getdata(Message message) {
        Toast.makeText(this, "Type=" + message.getType(), Toast.LENGTH_SHORT).show();
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
      //反註冊
        EventBus.getDefault().unregister(this);
    }
}

Main2Activity代碼：

public class Main2Activity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
    }
    public void onClick2(View view) {
      //發送消息
        EventBus.getDefault().post(new Message(1));
        finish();
    }
}

Message事件實體:

public class Message {
    private int type;

    public Message(int type) {
        this.type = type;
    }
    public int getType() {
        return type;
    }
    public void setType(int type) {
        this.type = type;
    }
}

效果圖

可能有人會說，官網的是在onStart()/onStop()中註冊和反註冊，你怎麼在onCreate方法中註冊，在onDestory方法中反註冊？這樣的話有一些情況我們普通的方式去發送事件，是在需要接收消息頁面會收不到，因爲使用時需要往已經註冊的頁面發送消息，才能接受到。當我們發送消息時註冊方法還沒有被調用和關聯訂閱者，那麼就無法接受事件。

粘性事件使用：

1、發送消息：

EventBus.getDefault().postSticky(事件);

2、在接受頁面註冊和反註冊，並接收。

//註冊
EventBus.getDefault().register(this);
//反註冊
EventBus.getDefault().unregister(this);
//接收事件(getdata這名字是隨便的，起什麼都可以)
@Subscribe(threadMode = ThreadMode.MAIN,sticky = true)
   public void getdata(事件) {
  //在這裏處理髮送的事件
 }

粘性事件例子;

有這樣的一個需求，在MainActivity往Main2Activity（沒有創建過）頁面傳遞數據(當然我們也可以使用Intent攜帶Bundle傳遞)。

MainActivity代碼：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.main_tv:
            //發送消息
                EventBus.getDefault().postSticky(new Message(1));
                Intent intent = new Intent(this, Main2Activity.class);
                startActivity(intent);
                break;
        }
    }
}

Main2Activity代碼：

public class Main2Activity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
    }
    @Override
    protected void onStart() {
        super.onStart();
      //註冊
        EventBus.getDefault().register(this);
    }
    @Override
    protected void onStop() {
        super.onStop();
      //反註冊
        EventBus.getDefault().unregister(this);
    }
    public void onClick2(View view) {
        finish();
    }
  //訂閱者方法
    @Subscribe(threadMode = ThreadMode.MAIN,sticky = true)
    public void getdata(Message message) {
        Toast.makeText(this, "MainActivity發送Type爲" + message.getType(), Toast.LENGTH_SHORT).show();
    }
}

效果圖

EventBus源碼分析：

EventBus.getDefault()分析

我們在使用時不管操作什麼都需要調用EventBus.getDefault()方法。

public static EventBus getDefault() {
    if (defaultInstance == null) {
        synchronized (EventBus.class) {
            if (defaultInstance == null) {
                defaultInstance = new EventBus();
            }
        }
    }
    return defaultInstance;
}

我們看上面源碼可知，它用到了雙重檢鎖獲取到了EventBus的對象。

register註冊分析

public void register(Object subscriber) {
  //獲取註冊的對象
 Class<?> subscriberClass = subscriber.getClass();
  //找到訂閱方法集合
 List<SubscriberMethod>subscriberMethods=subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            //訂閱
            subscribe(subscriber, subscriberMethod);
        }
    }
}

解析1SubscriberMethod是存儲訂閱方法信息：

public class SubscriberMethod {
    final Method method;
    final ThreadMode threadMode;
    final Class<?> eventType;
    final int priority;
    final boolean sticky;
    /** Used for efficient comparison */
    String methodString;
    ......
    }

ThreadMode訂閱者線程

eventType事件

priority優先級

sticky 是否粘性事件

Method接受事件的方法(比如例子中的getdata())

解析2findSubscriberMethods查找訂閱者方法

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

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    //通過訂閱者對象查找緩存（以ConcurrentHashMap存儲的key爲訂閱者的類，value爲訂閱者方法集合）
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    //查到的緩存不爲空直接返回
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
   //是否忽略生成的索引 ，默認false
    if (ignoreGeneratedIndex) {
      //以反射的方式獲取訂閱者訂閱方法
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
      //以索引的形式獲取
        subscriberMethods = findUsingInfo(subscriberClass);
    }   
    //判斷內容是否爲空，爲空拋異常
    if (subscriberMethods.isEmpty()) {
        throw new EventBusException("Subscriber " + subscriberClass + " and its super classes have no public methods with the @Subscribe annotation");
    } else {
        //緩存到ConcurrentHashMap中，以便於下次查找
        METHOD_CACHE.put(subscriberClass, subscriberMethods);
        return subscriberMethods;
    }
}

解析2.1findUsingReflection

private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
   //創建FindState對象
  FindState findState = prepareFindState();
   //於訂閱者類關聯
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
         //使用反射獲取訂閱方法
        findUsingReflectionInSingleClass(findState);
        findState.moveToSuperclass();
    }
    //返回FindState中的訂閱方法集合
    return getMethodsAndRelease(findState);
} 

解析2.1.1findUsingReflectionInSingleClass

private void findUsingReflectionInSingleClass(FindState findState) {
    Method[] methods;
    try {
        // This is faster than getMethods, especially when subscribers are fat classes like Activities
        //獲取本類中的所有方法
        methods = findState.clazz.getDeclaredMethods();
    } catch (Throwable th) {
        // Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
        //獲取本類和父類的所有公共方法
        methods = findState.clazz.getMethods();
        //設置標記
        findState.skipSuperClasses = true;
    }
    //遍歷
    for (Method method : methods) {
        //返回方法的int值
        int modifiers = method.getModifiers();
         //忽略不是PUBLIC和static
        if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
          //獲取參數類型
            Class<?>[] parameterTypes = method.getParameterTypes();
            if (parameterTypes.length == 1) {
              //獲取Subscribe註解(裏面有線程模型、是否粘性、優先級)
                Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                if (subscribeAnnotation != null) {
                    Class<?> eventType = parameterTypes[0];
                    if (findState.checkAdd(method, eventType)) {
                      //獲取線程模型
                        ThreadMode threadMode = subscribeAnnotation.threadMode();
                      //添加到訂閱者集合
                        findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                    }
                }
              ......
}

解析2.2findUsingInfo

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        //第一次會爲空
        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 {
          //調用解析2.1.1反射獲取
            findUsingReflectionInSingleClass(findState);
        }
      //在父類中查找
        findState.moveToSuperclass();
    }
    //返回訂閱者集合
    return getMethodsAndRelease(findState);
}

解析3subscribe

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

// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    //獲取的事件
    Class<?> eventType = subscriberMethod.eventType;
    //存儲訂閱者和訂閱者方法信息
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    CopyOnWriteArrayList<Subscription>subscriptions = subscriptionsByEventType.get(eventType);
    //是否爲空，並存儲
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        //判斷是否已經註冊過，不能重複註冊，拋異常
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "+ eventType);
        }
    }
    
    int size = subscriptions.size();
    //優先級排序
    for (int i = 0; i <= size; i++) {
        if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
            subscriptions.add(i, newSubscription);
            break;
        }
    }
    //獲取當前訂閱者的事件
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    //爲空創建並添加緩存中
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    //添加當前事件
    subscribedEvents.add(eventType);
    //是否是粘性事件
    if (subscriberMethod.sticky) {
        //默認true
        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 {
            //獲取粘性事件
            Object stickyEvent = stickyEvents.get(eventType);
            //發送事件
            checkPostStickyEventToSubscription(newSubscription, stickyEvent);
        }
    }
}

註冊總結

註冊時發送事件checkPostStickyEventToSubscription會調用這個方法postSingleEventForEventType

post普通發送分析

private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
    @Override
    protected PostingThreadState initialValue() {
        return new PostingThreadState();
    }
};

使用ThreadLocal存儲PostingThreadState,線程安全的

/** For ThreadLocal, much faster to set (and get multiple values). */
fin al static class PostingThreadState {
   //事件隊列集合
    final List<Object> eventQueue = new ArrayList<>();
   //是否發送
    boolean isPosting;
   //是否是主線程
    boolean isMainThread;
   //存儲訂閱信息的
    Subscription subscription;
  //事件
    Object event;
  //是否取消
    boolean canceled;
}

/** Posts the given event to the event bus. */
public void post(Object event) {
    //
    PostingThreadState postingState = currentPostingThreadState.get();
   //獲取事件隊列集合
    List<Object> eventQueue = postingState.eventQueue;
   //添加當前事件
    eventQueue.add(event);
    //判斷是否發送
    if (!postingState.isPosting) {
        //是否是主線程
        postingState.isMainThread = isMainThread();
        //更改是否發送過默認值
        postingState.isPosting = true;
       //判斷是否取消
        if (postingState.canceled) {
            throw new EventBusException("Internal error. Abort state was not reset");
        }
        try {
            while (!eventQueue.isEmpty()) {
                //移除事件並傳遞
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
          //恢復狀態
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

postSingleEvent

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    //默認值爲true
    if (eventInheritance) {
        //查找事件所有Class對象，包括超類和接口內部使用hashmap緩存
        List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
        int countTypes = eventTypes.size();
        //遍歷
        for (int h = 0; h < countTypes; h++) {
            Class<?> clazz = eventTypes.get(h);
            //發送事件
            subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
        }
    } else {
        //發送事件
        subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
    }
    //subscriptionFound爲true是調用 也就是 subscriptions爲空或者size爲0時
    if (!subscriptionFound) { 
        if (logNoSubscriberMessages) {
            logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
        }
        if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                eventClass != SubscriberExceptionEvent.class) {
            post(new NoSubscriberEvent(this, event));
        }
    }
}

postSingleEventForEventType發送事件

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
    //存儲訂閱者信息
    CopyOnWriteArrayList<Subscription> subscriptions;
    synchronized (this) {
      //獲取緩存
        subscriptions = subscriptionsByEventType.get(eventClass);
    }
  //如果不爲空
    if (subscriptions != null && !subscriptions.isEmpty()) {
      //遍歷
        for (Subscription subscription : subscriptions) {
          //設置訂閱者的信息
            postingState.event = event;
            postingState.subscription = subscription;
            boolean aborted = false;
            try {
                postToSubscription(subscription, event, postingState.isMainThread);
                aborted = postingState.canceled;
            } finally {
               //清除狀態
                postingState.event = null;
                postingState.subscription = null;
                postingState.canceled = false;
            }
            if (aborted) {
                break;
            }
        }
        return true;
    }
    return false;
}

postToSubscription判斷哪個線程並通過反射調用訂閱方法

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
  //判斷獲取的線程模式
    switch (subscription.subscriberMethod.threadMode) {
        case POSTING:
            //直接調用
            invokeSubscriber(subscription, event);
            break;
        case MAIN:
            //判斷是否是主線程
            if (isMainThread) {
                invokeSubscriber(subscription, event);
            } else {
                mainThreadPoster.enqueue(subscription, event);
            }
            break;
        case MAIN_ORDERED:
            if (mainThreadPoster != null) {
                mainThreadPoster.enqueue(subscription, event);
            } else {
                // temporary: technically not correct as poster not decoupled from subscriber
                invokeSubscriber(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);
    }
}

1、PostThread 默認的線程模式，直接調用invokeSubscriber方法。

發佈事件和接收事件在同一個線程

2、MAIN判斷是否在主線程，true直接調用invokeSubscriber方法,false使用mainThreadPoster處理，他的視線類是HandlerPoster。加入隊列使用handler發送消息在handler的handleMessage中去執行invokeSubscriber方法。

事件的處理會在UI線程中執行

3、MAIN_ORDERED先判斷mainThreadPoster是否爲空，true調用mainThreadPoster排隊等待傳遞非阻塞的，否則直接調用invokeSubscriber方法。

事件的處理會在UI線程中執行

4、BACKGROUND判斷是否在主線程，true使用BackgroundPoster處理，false直接調用invokeSubscriber

事件是在UI線程中發佈出來的，那麼該事件處理函數就會在新的線程中運行，如果事件本來就是子線程中發佈出來的，那麼該事件處理函數直接在發佈事件的線程中執行

5、ASYNC直接交由AsyncPoster處理

事件處理函數都會在新建的子線程中執行

invokeSubscriber通過反射調用訂閱方法

void invokeSubscriber(Subscription subscription, Object event) {
    try {
        subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
    } catch (InvocationTargetException e) {
        handleSubscriberException(subscription, event, e.getCause());
    } catch (IllegalAccessException e) {
        throw new IllegalStateException("Unexpected exception", e);
    }
}

postSticky粘性事件

private final Map<Class<?>, Object> stickyEvents;
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);
}

先同步，緩存到stickyEvents（ConcurrentHashMap緩存的），之後調用post方法。之後流程跟普通發送事件一樣。

unregister 反註冊分析

/** Unregisters the given subscriber from all event classes. */
public synchronized void unregister(Object subscriber) {
  //獲取訂閱者集合
    List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
    if (subscribedTypes != null) {
      //遍歷
        for (Class<?> eventType : subscribedTypes) {
            unsubscribeByEventType(subscriber, eventType);
        }
      //移除訂閱者
        typesBySubscriber.remove(subscriber);
    } else {
        logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

unsubscribeByEventType

/** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
  //獲取訂閱者信息集合
    List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    if (subscriptions != null) {
        int size = subscriptions.size();
      //遍歷
        for (int i = 0; i < size; i++) {
            Subscription subscription = subscriptions.get(i);
          //判斷是否是當前訂閱者
            if (subscription.subscriber == subscriber) {
                subscription.active = false;
              //移除
                subscriptions.remove(i);
                i--;
                size--;
            }
        }
    }
}