Android 应用内常用通信方式

从过去一直到现在，常用到的几种通信方式：

• Handler；

• 自定义回调接口；

• 使用广播；

• EventBus进行应用内通信；
  EventBus in 3 steps:
  1.从它的注册开始：

  EventBus.getDefault().register(this);
  

EventBus.getDefault()在源码里的实现如下：

    /** 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;
    }

获取实例的时候使用的是单例模式。
register()方法：

 /**
     * Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they
     * are no longer interested in receiving events.
     * <p/>
     * 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) {
        //获取当前类的字节码
        Class<?> subscriberClass = subscriber.getClass();
        //它会查找在当前类里含有注解Subscriber的public的event方法，通俗的说就是查看这个订阅者订阅了哪些事件，因为可能有多个事件，所以是集合。
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            //循环遍历集合，将订阅者和这些事件subscribe起来，订阅事件
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                //遍历所有的方法，依次完成订阅者对方法的订阅
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

注释中说明：注册订阅者去接受事件，订阅者在它们一旦不想接受事件的时候必须调用unregister(object)方法。
订阅者处理事件的方法必须使用注解：Subscribe.
这个Subscribe同时也允许配置像 ThreadMode 一样还有优先级。

其中：subscriberMethodFinder 是用来查找订阅者订阅了哪些事件的类。
SubscriberMethod 对象是 订阅方法的对象，包括优先级啊，是否是黏性啊等，类部分源码如下：

  public class SubscriberMethod {
  //订阅的方法
  final Method method;
  //线程
  final ThreadMode threadMode;
  //事件类型
  final Class<?> eventType;
  //优先级
  final int priority;
  //是否黏性
  final boolean sticky;
  /** Used for efficient comparison */
  String methodString;

  public SubscriberMethod(Method method, Class<?> eventType, ThreadMode threadMode, int priority, boolean sticky) {
    this.method = method;
    this.threadMode = threadMode;
    this.eventType = eventType;
    this.priority = priority;
    this.sticky = sticky;

例如：我们订阅的一个类的方法如下：

public class MediaPlayer {

    public MediaPlayer(){
        EventBus.getDefault().register(this);
    }

    @Subscribe(sticky = true)
    public void OnMessageEvent(String message){

        Log.d("xx","message::"+message);


        EventBus.getDefault().removeStickyEvent(this);
    }

    @Subscribe(sticky = true)
    public void OnMessageEvent2(MessageBean1 message){

        Log.d("xx","message::"+message.getMessage());


        EventBus.getDefault().removeStickyEvent(this);
    }
}

那么我们对应SubscriberMethod的值就为：

past.png

然后我们看下这个findSubscriberMethods方法，是如何查找的：

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
        //如果缓存map里有，直接取出
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
        //是否忽略注解器生成的index类，默认为false
        if (ignoreGeneratedIndex) {
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
          //通过反射找到注解方法
            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;
        }
    }

其中这个 METHOD_CACHE是缓存订阅者的方法的，key是订阅者，value 是订阅者订阅的事件，还是拿我上面那个MediaPlayer类举例，如图所示：

1.png

这个无论是findUsingRefection还是findUsingInfo都会调用findUsingReflectionInSingleClass(findState);这个方法：

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");
            }
        }
    }

通过这个代码我们可以看出来，就是通过反射得到所有的方法，然后再通过是否标识注解，最终保存到

 findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));```
需要注意的是，如果注解方法中传的参数不是一个，就会抛出`must have exactly 1 parameter`异常；还有就是如果被注解的方法不是public，也会抛出`
is a illegal @Subscribe method: must be public...`的异常。



接下来，我们重点看下`subscribe(subscriber, subscriberMethod);`的实现：

// Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        //获取订阅方法的事件类型
        Class<?> eventType = subscriberMethod.eventType;
        //将这个订阅者还有订阅的事件封装到一个Subscription的对象中
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        //根据订阅者得到订阅者订阅方法一些信息,包括级别，线程等,可参考下图
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            //如果不为null，看是否定义相同的方法
            if (subscriptions.contains(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }
        //而后根据优先级添加到subscriptions
        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;
            }
        }
        //这个`typesBySubscriber` 是
        Map<Object, List<Class<?>>> typesBySubscriber，key 是订阅者，value 是订阅的方法。
        值得话还是看下图3更清晰直白一些；根据订阅者获取订阅的方法集合。
        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        //如果不为null 将订阅的方法添加到`subscribedEvents`集合中
        subscribedEvents.add(eventType);
        //判断订阅的事件是否是黏性的
        if (subscriberMethod.sticky) {
            //eventInheritance 是否分发事件
            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();
                        //最后都调用这个方法
                        checkPostStickyEventToSubscription(newSubscription, stickyEvent);
                    }
                }
            } else {
                ////最后都调用这个方法`checkPostStickyEventToSubscription`中就是立刻通知订阅者调用的订阅事件的方法。
                Object stickyEvent = stickyEvents.get(eventType);
                checkPostStickyEventToSubscription(newSubscription, stickyEvent);
            }
        }
    }

上述CopyOnWriteArrayList<Subscription> subscriptions的值

2.png

图3：

3.png
我们看下这个checkPostStickyEventToSubscription方法：

private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
        if (stickyEvent != null) {
            // If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
            // --> Strange corner case, which we don't take care of here.
            postToSubscription(newSubscription, stickyEvent, Looper.getMainLooper() == Looper.myLooper());
        }
    }

然后就直接invoke 订阅者订阅的事件：

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 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);
        }
    }

Eventbus大概就是这样的，花了一天的时间。