EventBus3.0 源码解析
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基本使用
EventBus是专门为Android设计的用于订阅,发布总线的库,用到这个库的app很多,因为它有很多的优点。比如,它可以简单android组件之间的通信;它可以避免了android四大组件复杂的生命周期处理;它可以让你的代码更为简洁。先一起了解下如何使用,然后在分析它的源码,知道它的工作原理。我们直接来使用EventBus 3.0,3.x主要的一个新的特性就是使用了注解,我们的Subscribe可以在代码中就指定我们的EventBus使用什么ThreadMode,是否粘性事件,优先级。
public @interface Subscribe { ThreadMode threadMode() default ThreadMode.POSTING; /** * If true, delivers the most recent sticky event (posted with * {@link EventBus#postSticky(Object)}) to this subscriber (if event available). */ boolean sticky() default false; /** Subscriber priority to influence the order of event delivery. * Within the same delivery thread ({@link ThreadMode}), higher priority subscribers will receive events before * others with a lower priority. The default priority is 0. Note: the priority does *NOT* affect the order of * delivery among subscribers with different {@link ThreadMode}s! */ int priority() default 0;}
这些,等会在源码分析的时候会讲,先看看如何使用:
public class MainActivity extends AppCompatActivity { private TextView tv; private Button btn; private NetTask netTask; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); EventBus.getDefault().register(this); netTask = new NetTask(); tv = (TextView)findViewById(R.id.first_tv); btn = (Button)findViewById(R.id.first_btn); btn.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { netTask.runTask(); } }); } @Subscribe(threadMode = ThreadMode.MAIN) public void onEventMainThread(Object object) { if(object instanceof NetTask){ String result= ((NetTask) object).getResult(); tv.setText(result); } } @Override protected void onDestroy() { try{ EventBus.getDefault().unregister(this); }catch (Exception e){ } super.onDestroy(); }}
视图上显示一个TextView和一个Button,点击按钮的时候,就执行NetWork任务请求,请求执行结束后,在onEventMainThread方法更新UI,下面是NetWork :
public class NetTask { private String result = ""; private static final int SUCCESS = 0x23; private static final int ERROR = 0x24; public String getResult(){ return result; } public void runTask(){ new AsyncTask<Void,Void,Integer>(){ @Override protected Integer doInBackground(Void... params) { //模拟网络请求 try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); } result = "Event Bus"; return SUCCESS; } @Override protected void onPostExecute(Integer integer) { if(integer == SUCCESS){ EventBus.getDefault().post(NetTask.this); } } }.execute(); }}
代码看起来要比广播和回调方便多了,下面是UI显示:
源码分析
初始化
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<>(); mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10); backgroundPoster = new BackgroundPoster(this); asyncPoster = new AsyncPoster(this); indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0; subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes, builder.strictMethodVerification, builder.ignoreGeneratedIndex); logSubscriberExceptions = builder.logSubscriberExceptions; logNoSubscriberMessages = builder.logNoSubscriberMessages; sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent; sendNoSubscriberEvent = builder.sendNoSubscriberEvent; throwSubscriberException = builder.throwSubscriberException; eventInheritance = builder.eventInheritance; executorService = builder.executorService; }
上面是EventBus初始化的三个步骤,直观上看用到单例模式和Builder模式,将构造参数给分离了出来,实际上还用到了策略模式,其中Builder中有些参数用于代码执行的策略,就说,你传的参数不一样,我执行的方式不一样,像ignoreGeneratedIndex 作用就是让EventBus如何查找出订阅方法的策略。这些布尔类型的参数,在分析代码中可以逐步的了解到,先了解一些缓存对象,以更容易的了解源码:
- subscriptionsByEventType : 内部是一个Map集合,可以根据EventType查找订阅事件
- typesBySubscriber : 根据我们的订阅对象找到EventType
- stickyEvents : 粘性事件的缓存
- 事件投递者 : mainThreadPoster,backgroundPoster,asyncPoster 根据订阅注解ThreadMode去选择不同的投递者,不同投递者投递事件,接收函数会执行在不同的线程中
- subscriberMethodFinder :查找方法用的,内部维护了一个订阅方法的集合
注册
public void register(Object subscriber) { Class<?> subscriberClass = subscriber.getClass(); List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass); synchronized (this) { for (SubscriberMethod subscriberMethod : subscriberMethods) { subscribe(subscriber, subscriberMethod); } } }
当我们调用register(this)的时候就把订阅者给传了进来,代码量很少,主要就两个步骤,第一个findSubscriberMethods找出一个SubscriberMethod的集合,然后就遍历SubscriberMethod去订阅事件,我们先看看findSubscriberMethods()里面到底做了什么,返回的是什么。
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) { List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass); if (subscriberMethods != null) { return subscriberMethods; } /** * ignoreGeneratedIndex 为true则表明用反射生成 subscriberMethods * false则使用apt获取 subscriberMethods */ if (ignoreGeneratedIndex) { 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; } } }
首先从缓存中查找,如果找到了就立马返回。如果缓存中没有的话,则根据 ignoreGeneratedIndex 选择如何查找订阅方法,最后,找到订阅方法后,放入缓存,以免下次继续查找。ignoreGeneratedIndex 默认就是false,那我们会执行findUsingInfo()方法,但是这里先分析findUsingReflection(),因为在我们没有做任何配置的情况下还是会执行上面的findUsingReflection(),就是通过反射来解析注解。
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) { FindState findState = prepareFindState(); findState.initForSubscriber(subscriberClass); while (findState.clazz != null) { findUsingReflectionInSingleClass(findState); findState.moveToSuperclass(); } return getMethodsAndRelease(findState); }
从上面的代码块,我们可以看到,第一步准备FindState,我们点进去看看:
private FindState prepareFindState() { synchronized (FIND_STATE_POOL) { for (int i = 0; i < POOL_SIZE; i++) { FindState state = FIND_STATE_POOL[i]; if (state != null) { FIND_STATE_POOL[i] = null; return state; } } } return new FindState(); }
这里我们先了解从池中拿出一个FindState对象,FindState中维护的就是我们对订阅方法查找结果的封装。其实,往后面,我们就知道作者这里设计的非常精妙。第二步,initForSubscriber()就是将我们的订阅者传给FindState对象。第三步做的就是不断从订阅者和订阅者的父类去查找订阅方法,一起看findUsingReflectionInSingleClass()。
/** * 用发射对注解的处理 * @param 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"); } } }
代码很长,其实就是对订阅者方法的遍历,看有没有注解,有的话就解析注解,最后将找到的订阅方法的集合封装到FindState对象中的subscriberMethods集合中。我们解析完了之后,在看findUsingReflection()方法的最后,返回了getMethodsAndRelease(FindState),将我们的FindState穿给了getMethodsAndRelease()方法,好,我们点进去:
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; }
从这里,我们就知道作者设计FindState池的初心了,解析完了之后,将订阅方法赋给List集合,再回收FindState,继续接收解析,内存没有半点浪费。最后,我们返回的是一个订阅方法的集合。这样,我们通过反射解析注解,找到订阅方法的方式,我们已经分析完了。再看看通过apt处理器来找,我们知道apt处理,是针对源码的处理,是执行在编译过程中的。所以性能要比反射好的多,所以推荐大家使用这种方式。好,那最开始,我说使用findUsingInfo()方法如果没有配置还是使用反射呢,我们一起看看:
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) { /** * prepareFindState从池中获取FindState */ FindState findState = prepareFindState(); /** * 把 订阅类交给 FindState */ findState.initForSubscriber(subscriberClass); //什么时候为false,就是当当前类和父类的订阅方法全部检查完 while (findState.clazz != null) { //从EventBusIndex中查找订阅信息 /** * SubscriberInfo中维护的对象 * private final Class subscriberClass; * private final Class<? extends SubscriberInfo> superSubscriberInfoClass; * private final boolean shouldCheckSuperclass; * private final SubscriberMethodInfo[] methodInfos; */ findState.subscriberInfo = getSubscriberInfo(findState); if (findState.subscriberInfo != null) { SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods(); //根据订阅信息遍历他的订阅方法 /** * SubscriberMethod *final String methodName; *final ThreadMode threadMode; *final Class<?> eventType; *final int priority; *final boolean sticky; */ for (SubscriberMethod subscriberMethod : array) { /* * 根据eventType对SubscriberMethod进行检查 * 如果有这种类型的方法,或者有这个方法的类型的子类就返回false了 */ if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) { findState.subscriberMethods.add(subscriberMethod); } } } else { //如果没有EventBusIndex的信息,则用反射处理注解 findUsingReflectionInSingleClass(findState); } //移动到父类 findState.moveToSuperclass(); } //置空FindState池 return getMethodsAndRelease(findState); }
上面,我注释解析的很清楚了,前几步的操作基本一致,主要看getSubscriberInfo(findState)这个方法:
private SubscriberInfo getSubscriberInfo(FindState findState) { //查找是否存在订阅信息,有则直接返回 if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) { SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo(); if (findState.clazz == superclassInfo.getSubscriberClass()) { return superclassInfo; } } //从我们传入的MyEventBusIndex中查找是否存在订阅信息,订阅信息就是订阅方法,包括父类的订阅方法 if (subscriberInfoIndexes != null) { for (SubscriberInfoIndex index : subscriberInfoIndexes) { SubscriberInfo info = index.getSubscriberInfo(findState.clazz); if (info != null) { return info; } } } return null; }
上面一段代码是针对缓存的,如果缓存中有就直接返回,底下,是从subscriberInfoIndexes中查找订阅信息的。那么,我们的subscriberInfoIndexes从哪来的呢,是EventBus Builder对象,
public class EventBusBuilder { private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool(); ...... List<SubscriberInfoIndex> subscriberInfoIndexes;
默认是空的,所以结合上面的代码,还是执行了findUsingReflection()方法。那么,我们应该如何操作呢?我们需要配置下。第一步引入EventBusAnnotationProcessor库
这个库是在预编译过程,对我们的代码进行处理,找到我们有订阅方法的类的。第二步,配置项目的build.gradle文件。
将我们的库和andorid-apt引入,其中参数arguments是指定,我们处理完我们的源码文件,生成的文件名的。好,我们rebuild一下我们的项目。然后在build文件夹下面发现了:
我们点进去看看,
把我们的订阅类和订阅方法全找出来了,当,我们这个类初始化的时候,我们的SUBSCRIBER_INDEX这个集合就已经有我们的订阅信息了。所以,你知道为什么,叫ignoreGeneratedIndex了把。好,最后一步,我们就通过EventBus初始化,将这个类给传进去。
EventBus.builder().addIndex(new MyEventBusIndex()). installDefaultEventBus().register(this);
这样,它就会通过apt方式来处理我们的注解了,需要强调的是,我们是在编译过程中就已经找到我们的订阅信息了,不会在运行期做任何事情,所以这个效率是非常高的。好了,我们查找订阅的代码已经全部分析完了,这个部分是3.0最核心的代码,作者很精心的设计以提高EventBus的性能,我们看完之后也受益良多啊。好,继续分析,继续看findUsingInfo()方法,之前,我注释的也很清楚,但是还有个地方需要注意:
for (SubscriberMethod subscriberMethod : array) { /* * 根据eventType对SubscriberMethod进行检查 * 如果有这种类型的方法,或者有这个方法的类型的子类就返回false了 */ if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) { findState.subscriberMethods.add(subscriberMethod); } }
我们添加订阅方法的时候还有个检查,我们分析下:
boolean checkAdd(Method method, Class<?> eventType) { // 2 level check: 1st level with event type only (fast), 2nd level with complete signature when required. // Usually a subscriber doesn't have methods listening to the same event type. //两层检查:一层是只根据EventType,第二层根据签名 Object existing = anyMethodByEventType.put(eventType, method); if (existing == null) { return true; } else { if (existing instanceof Method) { //只有子类中没有发现这种类型的方法类型才返回true if (!checkAddWithMethodSignature((Method) existing, eventType)) { // Paranoia check throw new IllegalStateException(); } // Put any non-Method object to "consume" the existing Method anyMethodByEventType.put(eventType, this); } return checkAddWithMethodSignature(method, eventType); } }
这里,有两步检查,第一步检查,很好懂,第二步,根据什么签名来检查,我们看看:
private boolean checkAddWithMethodSignature(Method method, Class<?> eventType) { methodKeyBuilder.setLength(0); methodKeyBuilder.append(method.getName()); methodKeyBuilder.append('>').append(eventType.getName()); //将方法名和事件类型当作key,保存方法 String methodKey = methodKeyBuilder.toString(); Class<?> methodClass = method.getDeclaringClass(); Class<?> methodClassOld = subscriberClassByMethodKey.put(methodKey, methodClass); //如果我们传递过来的methodClass是父类,则直接返回 if (methodClassOld == null || methodClassOld.isAssignableFrom(methodClass)) { // Only add if not already found in a sub class return true; } else { //否则,保存我们的签名的MethodClass // Revert the put, old class is further down the class hierarchy subscriberClassByMethodKey.put(methodKey, methodClassOld); return false; } }
其实作者,这里又做了一个优化,将方法名和事件类型当作key,来保存方法,将传来的方法类型和我们签名的保存的比较,如果我们保存的是父类,就返回true,如果是子类,就将传来的方法保存起来,返回false。这样做的意图是,如果有父类的方法了,就没有必要添加子类的方法了,因为继承会执行到的。至此,我们对查找订阅方法的过程已经完全分析完了。看懂了之后,非常的过瘾。无论哪种方式查找,都返回了SubscriberMethod对象,我们看看它维护了什么属性:
/** Used internally by EventBus and generated subscriber indexes. */public class SubscriberMethod { final Method method; final ThreadMode threadMode; final Class<?> eventType; final int priority; final boolean sticky; /** Used for efficient comparison */ String methodString;
可以看到,基本都是我们注解标识的内容。好,我们继续分析订阅过程:
2.订阅
// Must be called in synchronized block private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) { Class<?> eventType = subscriberMethod.eventType; /** * Subsciption * final Object subscriber; * final SubscriberMethod subscriberMethod; * volatile boolean active; 如果为false,那么当unregister(this)调用的时候,那么在队列中检查,以防止竞争条件 */ //根据订阅者和订阅方法构造一个订阅事件 Subscription newSubscription = new Subscription(subscriber, subscriberMethod); //subscriptionsByEventType 是根据eventType去查找Subsciption的集合 CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType); //看看缓存中有没有,没有则放进缓存 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); } } /** * 遍历订阅事件,找到比subscriptions中订阅事件小的位置,然后插进去 * 这样就可以根据优先级依次投递事件了 */ 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; } } //typesBySubscriber是根据订阅者去查找EventType的缓存 //为了是unregister(this),根据this,去解绑事件的。 List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber); if (subscribedEvents == null) { subscribedEvents = new ArrayList<>(); typesBySubscriber.put(subscriber, subscribedEvents); } subscribedEvents.add(eventType); //如果是粘性事件,就立马处理 if (subscriberMethod.sticky) { if (eventInheritance) { //EventType的子类也应该考虑 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); } } }
订阅的代码比较长,但是理解起来并不难,而且,我也做了很详细的注释,其实里面就做了两件事,将我们的订阅方法和订阅者,封装到subscriptionsByEventType和typesBySubscriber,至于这两个对象是干什么的呢?第一个是我们投递订阅事件的时候,就是根据我们的EventType找到我们的订阅事件,从而去分发事件,处理事件的;第二个是在调用unregister(this)的时候,根据订阅者找到我们的EventType,又根据我们的EventType找到订阅事件,从而解绑用的,由于篇幅问题就不详细分析了。第二件事,就是如果是粘性事件的话,就立马投递、执行。
3.发布
/** Posts the given event to the event bus. */ public void post(Object event) { /** * PostingThreadState: * final List<Object> eventQueue = new ArrayList<Object>(); * boolean isPosting; * boolean isMainThread; * Subscription subscription; * Object event; * boolean canceled; */ //每个线程中都维护了一个投递的状态 PostingThreadState postingState = currentPostingThreadState.get(); List<Object> eventQueue = postingState.eventQueue; eventQueue.add(event); if (!postingState.isPosting) { postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper(); postingState.isPosting = true; if (postingState.canceled) { throw new EventBusException("Internal error. Abort state was not reset"); } try { while (!eventQueue.isEmpty()) { postSingleEvent(eventQueue.remove(0), postingState); } } finally { postingState.isPosting = false; postingState.isMainThread = false; } } }
当我们调用post(Object object)的方法的时候就执行了上面的代码,PostingThreadState是维护了投递的状态,最后循环投递,直到PostingThreadState中的EventQueue为空。那么代码最终执行到:
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); } }
那么,我们就会根据ThreadMode去处理事件了。也是由于篇幅的问题,就分析一种了,当线程模式是主线程的时候,意味着,我们需要执行的代码在主线程中操作。如果是主线程,就是通过反射,直接运行订阅的方法,如果不是主线程,我们需要mainThreadPoster将我们的订阅事件入队列,一起看看mainThreadPoster的工作原理:
final class HandlerPoster extends Handler { private final PendingPostQueue queue; private final int maxMillisInsideHandleMessage; private final EventBus eventBus; private boolean handlerActive; HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) { super(looper); this.eventBus = eventBus; this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage; queue = new PendingPostQueue(); } 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; } }}
其实,在EventBus初始化的时候,mainThreadPoster就已经获取主线程的Looper了,就是用到了我们Android的消息处理机制,Looper,Handler。至于消息队列是自己维护的一个单向的链表。每次向Andorid的主线程Looper投递一个空消息,然后在HandlerMessage()方法里面从自己维护的队列中取出PendingPost 进行处理。
final class PendingPost { private final static List<PendingPost> pendingPostPool = new ArrayList<PendingPost>(); Object event; Subscription subscription; PendingPost next;
而PendingPost中维护的是订阅事件,EventType和下一个PendingPost的地址。
4.反注册
public synchronized void unregister(Object subscriber) { List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber); if (subscribedTypes != null) { for (Class<?> eventType : subscribedTypes) { unsubscribeByEventType(subscriber, eventType); } typesBySubscriber.remove(subscriber); } else { Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass()); } }
反注册就是通过,我们EventBus中typesBySubscriber这个属性,通过订阅者去查找订阅事件,然后去一一解绑的。当然,反注册主要是为了提高效率的,不然订阅的事件太多,非常影响性能。
5.新特性-粘性事件
是否还记得,我们看订阅中的最后一段代码呢?不记得,也没事,继续看:
//如果是粘性事件,就立马处理 if (subscriberMethod.sticky) { if (eventInheritance) { //EventType的子类也应该考虑 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); } }
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()); } }
看到没有,如果我们在注册的时候,指定我们要发布粘性事件,那么我们在订阅的时候,就立马调用postToSubscription,去发布了,至于它从缓存中stickyEvents中获取订阅事件,你可能要问,我们什么时候把EventType放进去的呢?
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); }
没错,就是我们在调用postSticky()方法的时候,放入了缓存。当然,你也可以移除粘性事件。
public boolean removeStickyEvent(Object event) { synchronized (stickyEvents) { Class<?> eventType = event.getClass(); Object existingEvent = stickyEvents.get(eventType); if (event.equals(existingEvent)) { stickyEvents.remove(eventType); return true; } else { return false; } } }
根据源码的分析,我们把上面的代码改成粘性事件。首先,在NetWork里面:
@Override protected void onPostExecute(Integer integer) { if(integer == SUCCESS){ EventBus.getDefault().postSticky(NetTask.this); } }
然后,在MainActivity跳转到SecondActivity,让SecondActivity粘性注册:
public class SecondActivity extends AppCompatActivity { private TextView tv2; private Handler handler = new Handler(); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_second); EventBus.getDefault().registerSticky(this); tv2 = (TextView) findViewById(R.id.second_tv); } @Subscribe(threadMode = ThreadMode.MAIN,sticky = true) public void onEventMainThread(final NetTask netTask) { handler.post(new Runnable() { @Override public void run() { String result= netTask.getResult(); tv2.setText(result); } }); }}
这样,我们只要一打开SecondActivity,TextView上面就会显示Event Bus,下面是UI显示:
思考
分析了好久的源码,我们收获很多,除了对框架的理解得到了升华,其次,我们也体会到了作者的设计思想,这在我们平常工作中都可以去尝试,体验 。最后,我们来思考下使用它的优点。首先和Android自带的组件广播相比,除了代码简洁之外,更多的是,我们能获取到更多的信息,众所周知,观察者有两种模式,一种被观察者推,一种是观察者自己拉取。
@Override protected void onPostExecute(Integer integer) { if(integer == SUCCESS){ EventBus.getDefault().postSticky(NetTask.this); } } @Subscribe(threadMode = ThreadMode.MAIN,sticky = true) public void onEventMainThread(Object object) { if(object instanceof NetTask){ String result= ((NetTask) object).getResult(); tv.setText(result); } }
我们可以在投递的过程把被观察着本身投递出去,然后从被观察者身(NetTask)上拉取数据。既然提到观察者模式,我们和jdk自带的观察者模式对比,相信很多人都在android代码中使用jdk封装好的类把,那么我就随便举个例子。
public abstract class BaseTask extends Observable{ public static final int NET_ERROR = -1; public static final int PARSER_ERROR = -2; public static final int PARAMS_ERROR = -3; public static final int SUCCESSFUL = 0; public static final int NOT_VALUE = -4; protected int resultCode = NET_ERROR; public int getResultCode(){ return resultCode; } public void runTask(final Bundle bundle){ new AsyncTask<Void, Void, Integer>() { @Override protected Integer doInBackground(Void... params) { // TODO Auto-generated method stub WorkInBackground(bundle); return resultCode; } @Override protected void onPostExecute(Integer result) { WorkInUI(result); BaseTask.this.setChanged(); BaseTask.this.notifyObservers(); } }.execute(); } protected abstract void WorkInBackground(Bundle bundle); protected abstract void WorkInUI(Integer result);}
我们写个任务的基类来继承Observable,当任务执行完了之后,通知观察者们去取,我们只要在Activity中实现Observer,实现方法update就可以完成数据的更新。
public class SimpleActivity extends Activity implements Observer{ @Override protected void onCreate(Bundle savedInstanceState) { ... } @Override public void update(Observable observable, Object data) { //完成数据更新 }}
这种实现方式虽然也能达到和EventBus相同的作用,但是和我们的EventBus相比,就更能突出我们EventBus最大的优势就是解耦和,将业务和视图分离。只要在BaseAcitity中注册,解绑。通过注解我们就能很随意的获取到任何回调的结果,更为任性的是我们想在哪个线程中运行就再哪个线程中运行,并且接收事件的先后顺序也可以自定义。代码写出来更加的优雅,灵活。也许,你可能会说,性能相比呢?确实,我们的EventBus用用到了大量的缓存和反射,但是3.0后,已经做了很大改变了,我们可以通过apt预编译找到订阅着,避免了运行期间的反射处理解析。更为重要的是,牺牲一点内存达到代码的解耦,以后维护起来更加的方便,我觉得还是有所需要的。最后和Otto框架来比较,你就会知道EventBus框架是多么的优秀了!
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