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常用轮子之EventBus基本使用及原理

不忘初心 砥砺前行, Tomorrow Is Another Day !

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本文概要:

  1. EventBus的介绍
  2. 简单使用
  3. EventBus源码分析

引言

Android中消息传递有多种方式.

  • Handler : 线程间的通信.
  • BroadcastReceiver : 进程间的通信.接收系统广播.
  • 接口回调 : 事件触发通知.

当我们需要在多个地方接收事件通知,此时接口回调过于繁琐;广播又显得资源浪费.这时就需要用到EventBus了. EventBus是一种基于观察者模式,降低组件之间耦合,简化通信的方式.
官方介绍地址: github.com/greenrobot/…

一. EventBus的介绍

EventBus的三剑客 :

  • Publisher : 事件发布者
  • Event : 事件
  • Subscriber : 事件订阅者

事件由发布者通过EvenentBus传递给订阅者.

image

EventBus的四模型 :

  • ThreadMode.POSTING : 在哪个线程发布,就在哪个线程处理
  • ThreadMode.MAIN : 在主线程处理.
  • ThreadMode.BACKGROUND : 子~子 和 主~线程池
  • ThreadMode.ASYNC : 线程池处理

在3.0之前只能固定的方法来指定线程模型:

  • OnEvent
  • OnEventMainThread
  • OnEventBackgroundThread
  • OnEventAsync

二. 简单使用

  1. 定义一个事件类
public class DataEvent{
    String data;
    public DataEvent(String data){
        this.data = data;
    }
}
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  1. 注册订阅
EventBus.getDefault().register(this);
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  1. 发布事件
//发布普通事件
EventBus.getDefault().post(new DataEvent());

//发布黏性事件
EventBus.getDefault().postSticky(new DataEvent());
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  1. 定义订阅方法
//指定线程模型和优先级
@Subscriber (threadMode = ThreadMode.MAIN,priority = 0)
public void onDataEvent(DataEvent dataEvent){
}
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  1. 反注册订阅
EventBus.getDefault().unregister(this); 
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上述示例中,使用了黏性事件和优先级. 接下来看两者概念.

  • 黏性事件 : 事件发送之后再进行注册依然能收到该事件.
  • 优先级 : 默认为0,数值越大优先级越高.

三. 基本原理

EventBus的原理主要理解注册-发布事件-反注册流程三个方面.

3.1 定义注解

同样EventBus也定义了我们需要用到的注解.

@Documented
@Retention(RetentionPolicy.RUNTIME)//运行时注解
@Target({ElementType.METHOD})//修饰方法
public @interface Subscribe {
//定义了三个参数
    ThreadMode threadMode() default ThreadMode.POSTING;

    boolean sticky() default false;

    int priority() default 0;
}
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3.2 EvenBus的注册

先初始化:使用单例模式双重检查DCL方式创建EventBus对象.

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

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初始化完成后,就需要调用register方法进行注册.

public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        //获取当前订阅者所有订阅方法(findSubscriberMethods)
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
             //遍历所有方法,将他们保存起来(通过subscribe保存).
                subscribe(subscriber, subscriberMethod);
            }
        }
    }
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接着看如何查到所有订阅方法的

 List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
        //如果有缓存,则直接返回
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
        //无缓存,则通过反射活编译时生成的代码找到订阅方法集合(具体实现稍后在分析,先了解到这就行)
        if (ignoreGeneratedIndex) {
            //反射获取
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
            //3.0开启索引加速
            subscriberMethods = findUsingInfo(subscriberClass);
        }
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass
                    + " and its super classes have no public methods with the @Subscribe annotation");
        } else {
            //然后缓存起来保存
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }
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当找到所有订阅方法后,会保存订阅方法.我们看保存的具体过程

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class<?> eventType = subscriberMethod.eventType;
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);

        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
             //核心map,事件类型对应的Subscription订阅信息.
            //保存在subscriptionsByEventType(Map,key:eventType(事件类型的class对象);
            // value:CopyOnWriteArrayList<Subscription> (Subscription集合))
           
            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++) {
        //优先级排序
            if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
            //添加单个subscription
                subscriptions.add(i, newSubscription);
                break;
            }
        }

        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();

            //次核心map,订阅者对应的所有事件类型
            // 保存在typesBySubscriber(Map, key:subscriber(订阅者); value:List<Class<?>>(事件类型的class对象集合))
            
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);

        if (subscriberMethod.sticky) {
            if (eventInheritance) {//处理继承
                // 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);
            }
        }
    }
         
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3.3 EventBus的反注册

注册的时候核心是对核心Map和次核心Map,进行添加操作.那么反注册则是删除操作.

public synchronized void unregister(Object subscriber) {
        //从次核心Map取出所有事件类型
        List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
        if (subscribedTypes != null) {
            for (Class<?> eventType : subscribedTypes) {
                //从核心Map,移除订阅信息
                unsubscribeByEventType(subscriber, eventType);
            }
            //从次核心Map,移除订阅者
            typesBySubscriber.remove(subscriber);
        } else {
            logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
        }
    }
    
    //从核心Map,移除订阅信息的具体过程
    private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
        List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions != null) {
            int size = subscriptions.size();
            for (int i = 0; i < size; i++) {
                Subscription subscription = subscriptions.get(i);
                if (subscription.subscriber == subscriber) {
                    subscription.active = false;
                    subscriptions.remove(i);
                    i--;
                    size--;
                }
            }
        }
    }
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3.4 EventBus发布事件

发布事件,还是跟核心Map有很大关联,接着看发布事件流程.

由于Post方法内部调用了postSingleEvent来,其内部最终调用了postSingleEventForEventType发送事件,这里直接看该方法.

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            //从核心Map,根据事件类型找到所有订阅信息
            subscriptions = subscriptionsByEventType.get(eventClass);
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
            for (Subscription subscription : subscriptions) {
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted = false;
                try {
                    //遍历调用订阅信息里面的订阅方法
                    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;
    }
    
  //postToSubscription的具体实现
  private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        switch (subscription.subscriberMethod.threadMode) {
            case POSTING:
                //当前线程调用
                invokeSubscriber(subscription, event);
                break;
            case MAIN:
               //主线程调用
                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) {
                //线程池中调用 backgroundPoster.enqueue(subscription, event);
                } else {
                //当前子线程调用
                    invokeSubscriber(subscription, event);
                }
                break;
            case ASYNC:
                //线程池中调用
                asyncPoster.enqueue(subscription, event);
                break;
            default:
                throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
        }
    }   
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最后我们对整个EventBus流程做一个总结.

准备工作:
先初始化,使用单例模式双重检查DCL方式创建EventBus对象.

  1. 注册:

    1. 获取当前订阅者所有订阅方法(findSubscriberMethods)

      • 如果有缓存(METHOD_CACHE),则直接返回; 无缓存,则通过++反射或者编译时生成的代码++来找到订阅方法集合,然后缓存起来返回.
    2. 遍历所有方法,将他们保存起来(通过subscribe保存).

      1. 组合订阅信息,Subscription(订阅信息) = Subscriber(订阅者) + SubscriberMethods(订阅方法)
      2. 保存在事件类型对应的订阅信息集合的Map中.(核心Map)
      3. ++保存在订阅者对应的所有事件类型(次核心Map)++
  2. 反注册:

    1. 根据当前订阅者,从次核心Map找到所有订阅事件类型.
    2. 遍历根据订阅事件,从核心Map找到对应所有订阅信息.
    3. 最后一次移除订阅信息,订阅者.
  3. 发布事件

    • 根据当前事件类型,从核心Map找到所有订阅信息,遍历调用.

3.4 EventBus的3.0索引加速

之前在讲解EventBus的注册过程时,只是简单得讲到获取所有的订阅方法,如果没有缓存时,则从反射或者编译时注解生成的代码中获取.接下来继续看反射相关代码

a. 通过反射获取所有订阅方法
private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // This is faster than getMethods, especially when subscribers are fat classes like Activities
            //获取所有方法
            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;
        }
        //遍历
        for (Method method : methods) {
            int modifiers = method.getModifiers();
            //检查是否满足订阅方法的条件.
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class<?>[] parameterTypes = method.getParameterTypes();
                //获取参数长度
                if (parameterTypes.length == 1) {
                    //获取方法注解
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        Class<?> eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            //条件都满足
                            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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b. 通过编译时生成的代码获取所有订阅方法
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            //获取订阅信息(getSubscriberInfo方法在编译时索引类里)
            findState.subscriberInfo = getSubscriberInfo(findState);
            if (findState.subscriberInfo != null) {
                //获取所有订阅方法
                SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
                for (SubscriberMethod subscriberMethod : array) {
                    if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                        findState.subscriberMethods.add(subscriberMethod);
                    }
                }
            } else {
                //继续回去调用反射
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

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到此为止,我们知道3.0之前通过反射获取订阅的方法,对于性能上是有所欠缺.所以在3.0后提供了(Subscribe Index)索引加速,其实本质就是注解处理器的应用,这样就不用通过反射了,性能上得到了很大的提高.
接下来看如何开启索引. greenrobot.org/eventbus/do…

1. 添加注解处理器:
android {
    defaultConfig {
        javaCompileOptions {
            annotationProcessorOptions {
                //指定生成的索引文件名以及日志打印
                arguments = [ eventBusIndex : 'com.example.myapp.MyEventBusIndex', verbose : 'true' ]
            }
        }
    }
}
 
dependencies {
    implementation 'org.greenrobot:eventbus:3.1.1'
    annotationProcessor 'org.greenrobot:eventbus-annotation-processor:3.1.1'
}
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最后我们在MainActivity准备一些订阅方法方法

    @Subscribe(threadMode = ThreadMode.MAIN, priority = 0)

    public void onDataEventOne(Integer dataEvent) {
    }

    @Subscribe(threadMode = ThreadMode.MAIN, priority = 1)
    public void onDataEventTwo(String dataEvent) {
    }

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接着编译下项目,此时在build-generated-source-apt的包下,通过注解处理器生成了我们自己定义的索引文件MyEventBusIndex.java

public class MyEventBusIndex implements SubscriberInfoIndex {
    private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;

    static {
        //索引Map,key为订阅者,value为订阅者信息
        SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();

        //添加订阅者信息到Map中
        putIndex(
                //
                new SimpleSubscriberInfo(org.jasonhww.eventbusdemo.MainActivity.class, true,
                new SubscriberMethodInfo[] {
            //创建订阅方法对象
            new SubscriberMethodInfo("onDataEventOne", Integer.class, ThreadMode.MAIN),
            new SubscriberMethodInfo("onDataEventTwo", String.class, ThreadMode.MAIN, 1, false),
        }));

    }

    //将订阅信息添加到索引Map中
    private static void putIndex(SubscriberInfo info) {
        SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);
    }

    //获取订阅者对应的订阅信息
    //此方法就在注册查找所有订阅方法的具体实现.
    @Override
    public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {
        SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);
        if (info != null) {
            return info;
        } else {
            return null;
        }
    }
}

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接着在初始化EventBus时添加我们配置好的索引.

//方法一,使用自定义的EventBus
EventBus eventBus = EventBus.builder().addIndex(new MyEventBusIndex()).build();
//方法二,使用默认的EventBus,大多数情况下采取这种就行.
EventBus.builder().addIndex(new MyEventBusIndex()).installDefaultEventBus();
EventBus eventBus = EventBus.getDefault();
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2. 注解处理器-创建索引类

在之前分析编译时注解和ButterKnife,都有讲到注解处理器的运用.同样我们依葫芦画瓢来看EventBus是如何利用注解处理器去生成我们的索引类的.

相比之前分析的,这里注解处理器类上多了一个@SupportedOptions(value = {"eventBusIndex", "verbose"})配置,这里对应了最开始gradle的配置.

@SupportedAnnotationTypes("org.greenrobot.eventbus.Subscribe")
@SupportedOptions(value = {"eventBusIndex", "verbose"})
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这里直接看注解处理器process方法里,最重要的两步.

  • 收集信息

待生成的java文件信息存储在methodsByClass中.

private void collectSubscribers(Set<? extends TypeElement> annotations, RoundEnvironment env, Messager messager) {
        for (TypeElement annotation : annotations) {//遍历所有注解类对应的TypeElement
            //获取被注解的元素(如MainActivity的onDataEventOne方法对应的元素对象)
            Set<? extends Element> elements = env.getElementsAnnotatedWith(annotation);
            for (Element element : elements) {
                if (element instanceof ExecutableElement) {
                    ExecutableElement method = (ExecutableElement) element;
                    if (checkHasNoErrors(method, messager)) {
                        //获取被注解所在的类
                        TypeElement classElement = (TypeElement) method.getEnclosingElement();
                        //存入Map中,生成文件时调用.  
                        methodsByClass.putElement(classElement, method);
                    }
                } else {
                    messager.printMessage(Diagnostic.Kind.ERROR, "@Subscribe is only valid for methods", element);
                }
            }
        }
    }
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  • 生成java文件
    与ButterKnife不同,这里使用了JDK的JavaFileObject创建索引类.可以对照之前生成的索引类.
private void createInfoIndexFile(String index) {
        BufferedWriter writer = null;
        try {
            JavaFileObject sourceFile = processingEnv.getFiler().createSourceFile(index);
            int period = index.lastIndexOf('.');
            String myPackage = period > 0 ? index.substring(0, period) : null;
            String clazz = index.substring(period + 1);
            writer = new BufferedWriter(sourceFile.openWriter());
            if (myPackage != null) {
                writer.write("package " + myPackage + ";\n\n");
            }
            writer.write("import org.greenrobot.eventbus.meta.SimpleSubscriberInfo;\n");
            writer.write("import org.greenrobot.eventbus.meta.SubscriberMethodInfo;\n");
            writer.write("import org.greenrobot.eventbus.meta.SubscriberInfo;\n");
            writer.write("import org.greenrobot.eventbus.meta.SubscriberInfoIndex;\n\n");
            writer.write("import org.greenrobot.eventbus.ThreadMode;\n\n");
            writer.write("import java.util.HashMap;\n");
            writer.write("import java.util.Map;\n\n");
            writer.write("/** This class is generated by EventBus, do not edit. */\n");
            writer.write("public class " + clazz + " implements SubscriberInfoIndex {\n");
            writer.write("    private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;\n\n");
            writer.write("    static {\n");
            writer.write("        SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();\n\n");
            //此里面调用收集信息时存入的Map(methodsByClass)
            writeIndexLines(writer, myPackage);
            writer.write("    }\n\n");
            writer.write("    private static void putIndex(SubscriberInfo info) {\n");
            writer.write("        SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);\n");
            writer.write("    }\n\n");
            writer.write("    @Override\n");
            writer.write("    public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {\n");
            writer.write("        SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);\n");
            writer.write("        if (info != null) {\n");
            writer.write("            return info;\n");
            writer.write("        } else {\n");
            writer.write("            return null;\n");
            writer.write("        }\n");
            writer.write("    }\n");
            writer.write("}\n");
        } catch (IOException e) {
            throw new RuntimeException("Could not write source for " + index, e);
        } finally {
            if (writer != null) {
                try {
                    writer.close();
                } catch (IOException e) {
                    //Silent
                }
            }
        }
    }

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这样就成功的将订阅者的所有订阅方法都保存在了索引文件了,这样最终注册的时候就不需要再利用反射区查找订阅方法了.

最后还是老规矩做一个小的总结.

所谓索引加速,就是利用注解处理器通过解析注解信息,生成java文件的索引类,将所有订阅的方法保存在索引类而已.

由于本人技术有限,如有错误的地方,麻烦大家给我提出来,本人不胜感激,大家一起学习进步.

参考链接:

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