2018-09-17

Android源码解析-EventBus

本篇主要介绍eventbus的使用以及源码分析

####Eventbus是什么

专门为Android设计的用于订阅，发布总线的库
andorid组件之间通信
。。。

现有的通信方式

同一进程线程间：

handler
共享静态变量
eventbus

进程间：

aidl
　socket
广播
contentProvider
Messager

以上aidl跟广播也可以在同一个进程中使用，但一般同一个进程通信的话不太会用到

####为什么使用eventbus

相比于handler，handler通信是单向的，没有handler的线程可以向有handler的一方发消息。常见的是子线程d可以给主线程发消息（子线程做完耗时操作之后通知主线程修改ui）。如果主线程要给子线程发消息的话，需要子线程new looper，实现handler，并让主线程持有该handler，推荐直接使用HandlerThread（包含looper的thread）。相对来说代码量不小。

共享静态变量这种方法耗内存不推荐。

eventBus中子线程传递消息给主线程实质使用的还是handler，但是它比使用handler的方式简单很多。下面会给出详细分析。

使用方法

定义一个pojo：message的实体类
注册接受消息的类并在在该类中定义接收消息的方法
发送消息
合适的地方取消注册

示例：

实体类：

package com.example.kj_eventbus;

public class EventMessage {
    public String name;

    public EventMessage(String name) {
        this.name = name;
    }
}

接收消息的类：

public class MainActivity extends AppCompatActivity {

 	...
 	//接收消息的方法
    @Subscribe(threadMode = ThreadMode.MAIN)
    public void eventBus(EventMessage msg) {
        Toast.makeText(this, msg.name, Toast.LENGTH_SHORT).show();
    }

    public void jump(View view) {
        //注册接收消息的类
        EventBus.getDefault().register(this);
        startActivity(new Intent(this, SendMsgAct.class));
    }

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

发送消息：

public class SendMsgAct extends AppCompatActivity {
	...
    public void send(View view) {
       new Thread(new Runnable() {
            @Override
            public void run() {
                //发送消息
                EventBus.getDefault().post(new EventMessage("你好朋羽毛！！"));
            }
        }).start();
    }
}

源码解析

注解的使用

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface Subscribe {
    //接收消息的线程
    ThreadMode threadMode() default ThreadMode.POSTING;
    //是否是粘性消息
    boolean sticky() default false;
	//优先级
    int priority() default 0;
}


public enum ThreadMode {
    POSTING,
    MAIN,
    MAIN_ORDERED,
    BACKGROUND,
    ASYNC
}

该注解运行时生效，用于注解方法，标示接收消息的线程，消息的优先级，以及是否是粘性的消息
利用该注解可以在运行时通过反射获取到（类—方法—MessageType）对应的关系；
具体代码后面会给出分析

初始化

这里使用了单例跟建造者模式构建evenbus实例。但是为什么构造函数是public的呢？有知道的同学可以告我一哈～

//单例 
static volatile EventBus defaultInstance;

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

public EventBus() {
    this(DEFAULT_BUILDER);
}
//建造者（将构造参数分离了出来）
EventBus(EventBusBuilder builder) {
    ...
        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
    sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
    throwSubscriberException = builder.throwSubscriberException;
    eventInheritance = builder.eventInheritance;
    executorService = builder.executorService;
}
public static EventBusBuilder builder() {
    return new EventBusBuilder();
}

注册类

public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();
    //1
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    //2   
    synchronized (this) {
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);
        }
    }
}

//接收消息的方法的封装实体类
public class SubscriberMethod {
    final Method method;//方法
    final ThreadMode threadMode;
    final Class<?> eventType;//MessagedType
    final int priority;
    final boolean sticky;
    /** Used for efficient comparison */
    String methodString;
}

 //类跟对应方法的实体类
 class Subscription {
    final Object subscriber;//注册类
    final SubscriberMethod subscriberMethod;
 }

该方法是注册入口，主要两步：

首先通过findSubscriberMethods找到该类的SubscirberMethod（接收消息的方法的封装）集合；
之后调用subscribe初始化subscriptionsByEventType（通过messageType找到subsciption）typesBySubscriber（通过注册类找到MessageType）

下面我们先跟踪下findSubscriberMethods：

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    //从cache中获取
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
    //ignoreGeneratedIndex如果为true，那么使用apt获取subscriberClass中接收消息的方法集合
    //为false，那么通过反射获取
    if (ignoreGeneratedIndex) {
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        subscriberMethods = findUsingInfo(subscriberClass);
    }
    if (subscriberMethods.isEmpty()) {
       ...
    } else {
        //放入cache
        METHOD_CACHE.put(subscriberClass, subscriberMethods);
        return subscriberMethods;
    }
}

该方法先从cache中获取，之后通过apt或者反射获取。我们先分析反射（默认），也就是findUsingInfo()

private List<SubscriberMethod> 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 {
            //通过反射调用
            findUsingReflectionInSingleClass(findState);
        }
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}

之后进入findUsingReflectionInSingleClass

private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
       	...
        methods = findState.clazz.getDeclaredMethods();
      	...
        for (Method method : methods) {
            ////获取方法修饰符（public private static,,
            //方法必须是publit not-static  not-abstract
            int modifiers = method.getModifiers();
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class<?>[] parameterTypes = method.getParameterTypes();
                if (parameterTypes.length == 1) {
                    //获取方法注解
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        //获取MesageType
                        Class<?> eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
                            //获取注解的值，将method，MessageType，ThreadMode，Prority，Sticky关联起来
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                        }
                    }
                } 
                ...
                }
            } 
            ...
        }
    }

之后会调用getMethodsAndRelease；

private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
    List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
    findState.recycle();
    synchronized (FIND_STATE_POOL) {
        for (int i = 0; i < POOL_SIZE; i++) {
            if (FIND_STATE_POOL[i] == null) {
                FIND_STATE_POOL[i] = findState;
                break;
            }
        }
    }
    return subscriberMethods;
}

最终返回的是该注册类中接收消息的方法集合。

接着我们进入第二步，subscribe：

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    Class<?> eventType = subscriberMethod.eventType;
    //1:初始化subscriptionsByEventType
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    //异步可以同时读写的list：CopyOnWriteArrayList
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
       ...
    }

    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;
        }
    }
	//2:初始化typesBySubscriber
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);

    if (subscriberMethod.sticky) {
        ...
    }
}

以上，regist分析完毕。

发送消息

public void post(Object event) {
    //将消息放入postingState的queue中
    PostingThreadState postingState = currentPostingThreadState.get();
    List<Object> eventQueue = postingState.eventQueue;
    eventQueue.add(event);

    if (!postingState.isPosting) {
        postingState.isMainThread = isMainThread();
        postingState.isPosting = true;
      	...
        try {
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}
//其中的currentPostingThreadState是ThreadLocal（以线程为单位，存放变量，也就是同一个变量在不同线程中是不一样的）
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
    @Override
    protected PostingThreadState initialValue() {
        return new PostingThreadState();
    }
};

//PostingThreadState
final static class PostingThreadState {
    final List<Object> eventQueue = new ArrayList<>();
    boolean isPosting;
    boolean isMainThread;
    Subscription subscription;
    Object event;
    boolean canceled;
}

其中的isMainThread()要从mainThreadSupport的初始化说起：

点击进入isMainThread()

1
2
3

private boolean isMainThread() {
    return mainThreadSupport != null ? mainThreadSupport.isMainThread() : true;
}

可见是根据mainThreadSupport来判断的，接着我们找到mainThreadSupport初始化的地方

EventBus(EventBusBuilder builder) {
	...
       mainThreadSupport = builder.getMainThreadSupport();
       mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
       backgroundPoster = new BackgroundPoster(this);
       asyncPoster = new AsyncPoster(this);
    	...
   }

进入getMainThreadSupport

MainThreadSupport getMainThreadSupport() {
    if (mainThreadSupport != null) {
        return mainThreadSupport;
    } else if (Logger.AndroidLogger.isAndroidLogAvailable()) {
        Object looperOrNull = getAndroidMainLooperOrNull();
        return looperOrNull == null ? null :
        new MainThreadSupport.AndroidHandlerMainThreadSupport((Looper) looperOrNull);
    } else {
        return null;
    }
}

我们看下Logger.AndroidLogger.isAndroidLogAvailable()

public static class AndroidLogger implements Logger {
      static final boolean ANDROID_LOG_AVAILABLE;

      static {
          boolean android = false;
          try {
              android = Class.forName("android.util.Log") != null;
          } catch (ClassNotFoundException e) {
              // OK
          }
          ANDROID_LOG_AVAILABLE = android;
      }

      public static boolean isAndroidLogAvailable() {
          return ANDROID_LOG_AVAILABLE;
      }
}

返回的是ANDROID_LOG_AVAILABLE，这个值如果存在“android.util.Log”，那么为true，否则为false，意思就是如果是运行在android中，那么为true，否则为false；

接着我们看下getAndroidMainLooperOrNull();

Object getAndroidMainLooperOrNull() {
    ...
    return Looper.getMainLooper();
    ...
}

返回的是主线程的looper，一定不为空，

那么getMainThreadSupport的返回值就是 MainThreadSupport.AndroidHandlerMainThreadSupport((Looper) looperOrNull)

接着我们看下AndroidHandlerMainThreadSupport

class AndroidHandlerMainThreadSupport implements MainThreadSupport {

    private final Looper looper;

    public AndroidHandlerMainThreadSupport(Looper looper) {
        this.looper = looper;
    }

    @Override
    public boolean isMainThread() {
        return looper == Looper.myLooper();
    }
  ...
}

其实就是把AndroidHandlerMainThreadSupport中的looper初始化为主线程的looper。

此时我们在看isMainThread()。如果Looper.myLooper返回的跟主线程的looper一致，那么自然就是主线程了。因为列子中是在子线程中发的消息，所以此时返回false。我们回到post方法中:

postingState的成员变量eventQueue中有我们发送的消息，isMainThread=false；

接着只要eventQueue不为空，就执行postSingleEvent(eventQueue.remove(0), postingState);

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    //是否算父类，默认是true
    if (eventInheritance) {
        List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
        int countTypes = eventTypes.size();
        for (int h = 0; h < countTypes; h++) {
            Class<?> clazz = eventTypes.get(h);
            subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
        }
    } else {
        subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
    }
    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));
        }
    }
}

那么接着我们进入lookupAllEventTypes

private static List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {
    synchronized (eventTypesCache) {
        List<Class<?>> eventTypes = eventTypesCache.get(eventClass);
        if (eventTypes == null) {
            eventTypes = new ArrayList<>();
            Class<?> clazz = eventClass;
            while (clazz != null) {
                eventTypes.add(clazz);
                addInterfaces(eventTypes, clazz.getInterfaces());
                clazz = clazz.getSuperclass();
            }
            eventTypesCache.put(eventClass, eventTypes);
        }
        return eventTypes;
    }
}

就是返回MessageType以及它的父类的List

返回之后接着执行postSingleEventForEventType(event, postingState, clazz)

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 {
                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;
}

这里通过subscriptionsByEventType找到该MessageType对应的subscriptions，将它赋给postingState.subscription，将该MessageType赋给postingState.event，之后调用postToSubscription(subscription, event, postingState.isMainThread);

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    switch (subscription.subscriberMethod.threadMode) {
        case POSTING:
            invokeSubscriber(subscription, event);
            break;
        case MAIN:
            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);
    }
}

例子中threadMode是Main，发送消息是在子线程中，接下来会执行mainThreadPoster.enqueue(subscription, event)

这里就要从mainThreadPoster的初始化说起(Eventbus构造方法中)：

1	mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;

mainThreadSupport.createPoster(this)

class AndroidHandlerMainThreadSupport implements MainThreadSupport {

	...
    @Override
    public Poster createPoster(EventBus eventBus) {
        return new HandlerPoster(eventBus, looper, 10);
    }
}

我们上面提到过这里的looper是主线程的looper。我们接着进入new HandlerPoster(eventBus, looper, 10)

public class HandlerPoster extends Handler implements Poster {

    private final PendingPostQueue queue;
    private final int maxMillisInsideHandleMessage;
    private final EventBus eventBus;
    private boolean handlerActive;

    protected HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {
        super(looper);
        this.eventBus = eventBus;
        this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage;
        queue = new PendingPostQueue();
    }

    public 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;
        }
    }
}

HandlerPoster是EventBus中比较重要的类，它继承自handler，因为我们传入的looper是主线程的，所以该handler是主线程的handler。

此时回到我们的初衷：mainThreadPoster.enqueue(subscription, event);

这里会将subscription跟event转化为PendingPost，pendingpost是用来做消息复用的，具体如下：

static PendingPost obtainPendingPost(Subscription subscription, Object event) {
    synchronized (pendingPostPool) {
        int size = pendingPostPool.size();
        if (size > 0) {
            PendingPost pendingPost = pendingPostPool.remove(size - 1);
            pendingPost.event = event;
            pendingPost.subscription = subscription;
            pendingPost.next = null;
            return pendingPost;
        }
    }
    return new PendingPost(event, subscription);
}

核心思路就是如果pendingPostPool这个池子里有，那么就从池子里拿，没有才会new 对象。拿到PendingPost之后，将它放入PendingPostQueue队列中。之后执行sendMessage(obtainMessage())发送一个空消息。

接着我们进入handleMessage。改方法会从PendingPostQueue队列中不断的poll数据，知道队列为空。每个数据都会执行eventBus.invokeSubscriber(pendingPost);

void invokeSubscriber(PendingPost pendingPost) {
    Object event = pendingPost.event;
    Subscription subscription = pendingPost.subscription;
    PendingPost.releasePendingPost(pendingPost);
    if (subscription.active) {
        invokeSubscriber(subscription, event);
    }
}

void invokeSubscriber(Subscription subscription, Object event) {
	...
    subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
	...
}

改方法就是通过反射调用接收消息的方法。

以上，完成了子线程给主线程传递消息的整个过程。

如果接收消息也是子线程（也就是threadMode设置的是backGround），如果发送消息在主线程，那么会调用backgroundPoster.enqueue(subscription, event);将线程切换到子线程中，否者直接反射调用即可。最后我们看下backgroundPoster的实现：

final class BackgroundPoster implements Runnable, Poster {

    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) {
                eventBus.getLogger().log(Level.WARNING, Thread.currentThread().getName() + " was interruppted", e);
            }
        } finally {
            executorRunning = false;
        }
    }

}

它本身是一个runnable，维护着一个队列，入队列之后执行eventBus.getExecutorService().execute(this)把自己放入线程池中（cache线程池），此时就会执行run方法，而run方法会不断的从队列中poll数据，之后反射执行。

反射优化：apt

运行时使用反射对会有一定的性能损耗，使用apt（注解工具）通过编译时生成对应的代码，从而规避了运行时反射。

总结

实质：当发送某个消息时执行特定类的特定方法

原理：其实就是利用注解跟反射通过注册类的方式将（类—方法–消息类型）对应的关系表维护在Eventbus中，当post某个消息时查询该关系表，通过反射执行指定类的指定方法，切换线程使用handler跟线程池。

最后：evenbus中相关的知识点有很多：设计模式，线程池，handler，ThreadLocal，反射，注解，多线程…源码还是推荐大家简单看一下。