源碼閱讀系列：EventBus
title: 源碼閱讀系列：EventBus
date: 2016-12-22 16:16:47
tags: 源碼閱讀
---java
EventBus 是人們在平常開發中常常會用到的開源庫，即便是不直接用的人，也多少借鑑過事件總線的用法。並且EventBus的代碼實際上是很是簡單的，能夠試着閱讀一下。android
源碼閱讀系列不採用對功能進行歸類的方法進行閱讀，而是採用一個剛開始閱讀源碼的視角，從咱們平時的API調用，一步步的去理解設計意圖和實現原理。git
從這裏開始
從這裏開始吧，咱們最經常使用的地方就是給一個函數添加上註解，咱們先拋開apt生成的table，只看這個運行時版本的訂閱設定。github
// eventbus/Subscribe
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
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;
}
這個設定仍是很是簡單的，並且都是咱們熟悉的東西，線程類型（默認的是拋出線程），是不是粘性事件，時間的優先級。通過這個類的出現，咱們就能夠在類裏面寫咱們常常寫的某個函數是訂閱函數了。緩存
@Subscribe (...)
public void getMessage(Event event) { ... }
下面的問題是咱們改怎麼讓EventBus找到這些方法呢？經過apt的版本咱們知道這裏面確定有一個map或者是table的東西記錄了Object和Method之間的訂閱關係，並且仍是一對多的。這個地方就是從每一個咱們進行register的地方進行的。多線程
register & unregister
// eventbus/EventBus
/**
* 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) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
咱們在Activity／Fragment中都有可能會調用這個方法，若是是Fragment裏面咱們還會在onDestoryView()中進行unregister(...)。在這段函數裏咱們發現使用反射從這個Class中找到了全部的訂閱者函數了，而後對每一個訂閱者函數進行註冊。併發
這裏咱們看看咱們的SubribeMethod被包裝成了什麼樣子：app
/** 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;
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;
}
@Override
public boolean equals(Object other) {
if (other == this) {
return true;
} else if (other instanceof SubscriberMethod) {
checkMethodString();
SubscriberMethod otherSubscriberMethod = (SubscriberMethod)other;
otherSubscriberMethod.checkMethodString();
// Don't use method.equals because of http://code.google.com/p/android/issues/detail?id=7811#c6
return methodString.equals(otherSubscriberMethod.methodString);
} else {
return false;
}
}
private synchronized void checkMethodString() {
if (methodString == null) {
// Method.toString has more overhead, just take relevant parts of the method
StringBuilder builder = new StringBuilder(64);
builder.append(method.getDeclaringClass().getName());
builder.append('#').append(method.getName());
builder.append('(').append(eventType.getName());
methodString = builder.toString();
}
}
@Override
public int hashCode() {
return method.hashCode();
}
}
SubscribeMethod 攜帶了Method函數原型，還有就是咱們在註解類裏面提供的全部信息。還有一個Class<?>類型的EventType是指咱們的事件類所對應的Class，其他的方法都是爲了比較和判斷是否相等來作的，equal/checkMethodString都是各類的拼字串來進行存儲和判斷。異步
下面咱們來看register裏面調用的這段subscribe，這段很是的重要涉及了EventBus運行時處理的絕大多數部分，還有就是粘性事件的分發。這段使用了大量的JDK的反射包的API，自己註釋也提醒咱們了這段代碼須要加鎖，畢竟裏面這一堆併發容器。因此咱們最好先明確這段裏面用的併發容器到底都是什麼，這段代碼纔好繼續看的下去。async
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
private final Map<Object, List<Class<?>>> typesBySubscriber;
private final Map<Class<?>, Object> stickyEvents;
主要的有這幾個：
- 第一個Map存儲的Key是Class類型，Value是一個併發的ArrayList裏面存的是對訂閱者和訂閱函數的一種綁定類
Subscription從名字上也能看出Key是Event的Class對象。 - 第二個存儲的是訂閱者（Activity什麼的啊）和Event類型的List。
- 第三個Map存儲的是粘性事件，Key是Event類型，Value是真實存在的StickyEvent對象。
知道這三個都是什麼以後，這段代碼就好看了。咱們來看前一部分。
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
// Map<Class<?>, CopyOnWriteArrayList<Subscription>>
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType) ;
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);
}
}
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;
}
}
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
這段寫的雖然有點亂套，但實際上寫的挺簡單的，並且一堆堆的O(n)遍歷，性能也就那樣(?)。
首先這裏面出現了Subscription：
final class Subscription {
final Object subscriber;
final SubscriberMethod subscriberMethod;
/**
* Becomes false as soon as {@link EventBus#unregister(Object)} is called, which is checked by queued event delivery
* {@link EventBus#invokeSubscriber(PendingPost)} to prevent race conditions.
*/
volatile boolean active;
Subscription(Object subscriber, SubscriberMethod subscriberMethod) {
this.subscriber = subscriber;
this.subscriberMethod = subscriberMethod;
active = true;
}
@Override
public boolean equals(Object other) {
if (other instanceof Subscription) {
Subscription otherSubscription = (Subscription) other;
return subscriber == otherSubscription.subscriber
&& subscriberMethod.equals(otherSubscription.subscriberMethod);
} else {
return false;
}
}
@Override
public int hashCode() {
return subscriber.hashCode() + subscriberMethod.methodString.hashCode();
}
}
咱們發現了這是訂閱者和訂閱方法類的一個契約關係類。
因此說上面subscribe函數主要作了，
- 建立了訂閱者和方法類的綁定，而後存進了
subscriptionsByEventType中 - 對每一個類型從新排列了一次優先級
- 對
typesBySubscriber添加了對應的類型
而後咱們能夠看一下這個函數的下一半，咱們會驚奇地發現，StickyEvent的發送時機竟然是在register的時候：
...
if (subscriberMethod.sticky) {
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 {
Object stickyEvent = stickyEvents.get(eventType);
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
這時候輪了一遍全部的粘性事件。isAssignableFrom相似於使用在Class之間的instance of 就是判斷兩個類是否有相同的接口關係，也就是說有繼承和實現關係的事件類，都會被判斷處理。
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());
}
}
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);
}
}
以後就是針對各類的ThreadMode進行了處理，同一線程的直接依賴Java的反射invoke執行了，各類不能夠的狀況，好比說發到主線程但還沒在主線程的時候，都是用隊列進行發送到對應線程。
接下來咱們看看這裏面在各線程之間的發送是怎麼實現的。
消息轉換線程
咱們發如今Subscription和event入隊的時候咱們把他們封裝成了一個PendingPost類：
// HandlePoster
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");
}
}
}
}
而後才進行的入隊和發送，這個PendingPost就是一個帶有回收池的掩飾傳送類：
final class PendingPost {
private final static List<PendingPost> pendingPostPool = new ArrayList<PendingPost>();
Object event;
Subscription subscription;
PendingPost next;
private PendingPost(Object event, Subscription subscription) {
this.event = event;
this.subscription = subscription;
}
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);
}
static void releasePendingPost(PendingPost pendingPost) {
pendingPost.event = null;
pendingPost.subscription = null;
pendingPost.next = null;
synchronized (pendingPostPool) {
// Don't let the pool grow indefinitely
if (pendingPostPool.size() < 10000) {
pendingPostPool.add(pendingPost);
}
}
}
}
這裏的設計其實挺不錯的，一個靜態的回收池，初始化靠一個靜態方法，優先使用被回收的對象，實現和Message其實很像。另外一個release方法就是把用完的對象回收起來。
PendingPostQueue 就是一個PendingPost的隊列，裏面的操做基本上就是入隊出隊之類的，有點特殊的是入隊和出隊都有一把鎖。
接着這個隊列被用在了好幾個Poster類中，實現了向各個線程的消息轉換，首先咱們來看向主線程發送數據的:
HandlePoster
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;
}
}
}
HandlePoster 繼承自 Handler 再加上初始化的時候傳進去的是Looper.getMainThread();因此能向主線程發送消息。每次入隊以後都會發送一條空消息去通知handleMessage函數處理隊列數據，使用handlerActive做爲控制標記位。handleMessage是個死循環兩段的if判斷用來處理多線程的狀況，invokeSubscriber的方式和以前相似。以後就是有一個閥值，當時間超過10ms的時候就會發一個消息重入，而且退出此次循環，這是防止時間太長阻塞主線程。
BackgroundPoster
final class BackgroundPoster implements Runnable {
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) {
Log.w("Event", Thread.currentThread().getName() + " was interruppted", e);
}
} finally {
executorRunning = false;
}
}
}
BackgroundPoster 自身是一個Runnable ，入隊以後就調用EventBus攜帶的一個線程池進行運行，一樣也是一個死循環，用了一個生產者 vs 消費者模式 進行了有限等待，這1000ms內入隊的消息都會被彈出處理。
synchronized PendingPost poll(int maxMillisToWait) throws InterruptedException {
if (head == null) {
wait(maxMillisToWait);
}
return poll();
}
PendingPostQueue的poll(int)方法對隊列爲空的狀況進行了等待，喚醒則出如今enqueue:
synchronized void enqueue(PendingPost pendingPost) {
if (pendingPost == null) {
throw new NullPointerException("null cannot be enqueued");
}
if (tail != null) {
tail.next = pendingPost;
tail = pendingPost;
} else if (head == null) {
head = tail = pendingPost;
} else {
throw new IllegalStateException("Head present, but no tail");
}
notifyAll(); // 在這進行了喚醒
}
AsyncPoster
若是說Background尚且能保證在同一個線程內完成，AsyncPoster就徹底進行了異步操做。
class AsyncPoster implements Runnable {
private final PendingPostQueue queue;
private final EventBus eventBus;
AsyncPoster(EventBus eventBus) {
this.eventBus = eventBus;
queue = new PendingPostQueue();
}
public void enqueue(Subscription subscription, Object event) {
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
queue.enqueue(pendingPost);
eventBus.getExecutorService().execute(this);
}
@Override
public void run() {
PendingPost pendingPost = queue.poll();
if(pendingPost == null) {
throw new IllegalStateException("No pending post available");
}
eventBus.invokeSubscriber(pendingPost);
}
}
這裏面基本上什麼都不控制，直接就來一個運行一次，也不會有什麼問題。。。
到這爲止咱們不但知道了方法是怎麼註冊和綁定的，咱們甚至還知道了粘性事件是怎麼發送的了，接着咱們來看方法查找和普通事件的發送是怎麼進行的。
方法查找
// package org.greenrobot.eventbus.meta;
/** Base class for generated index classes created by annotation processing. */
public interface SubscriberInfo {
// 獲取訂閱的類
Class<?> getSubscriberClass();
// 全部的method
SubscriberMethod[] getSubscriberMethods();
// 獲取父類的info
SubscriberInfo getSuperSubscriberInfo();
// 是否檢查父類
boolean shouldCheckSuperclass();
}
SubscriberInfo 描述了能經過註解類生成的Index的方法（具體功能我加了主食）。
/**
* Interface for generated indexes.
*/
public interface SubscriberInfoIndex {
SubscriberInfo getSubscriberInfo(Class<?> subscriberClass);
}
這個接口是查找info的。
另外能夠說這其中的SubscriberMethodInfo存儲着SubscriberMethod所需的元信息：
public class SubscriberMethodInfo {
final String methodName;
final ThreadMode threadMode;
final Class<?> eventType;
final int priority;
final boolean sticky;
...
AbstractSubscriberInfo是一個抽象類，主要負責從Info建立出Method，又是一個反射：
protected SubscriberMethod createSubscriberMethod(String methodName, Class<?> eventType, ThreadMode threadMode,
int priority, boolean sticky) {
try {
Method method = subscriberClass.getDeclaredMethod(methodName, eventType);
return new SubscriberMethod(method, eventType, threadMode, priority, sticky);
} catch (NoSuchMethodException e) {
throw new EventBusException("Could not find subscriber method in " + subscriberClass +
". Maybe a missing ProGuard rule?", e);
}
}
另外還有一個SimpleSubscriberInfo做爲他的子類。
接下來的SubscriberMethodFinder也很是重要運行時的方法查找都來自這裏:
剛纔咱們在EventBus.register(...)中調用了這個函數：
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return 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;
}
}
其中的METHOD_CACHE是對每一個類方法進行緩存，防止屢次查找，畢竟運行時查找仍是個複雜的操做，根據是否忽略生成Index。
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findUsingReflectionInSingleClass(findState);
findState.moveToSuperclass();
}
return getMethodsAndRelease(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");
}
}
}
findUsingReflectionInSingleClass對反射類進行了處理，這裏面經過掩模運算檢查了訪問權限， 檢查了參數個數。
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.
Object existing = anyMethodByEventType.put(eventType, method);
if (existing == null) {
return true;
} else {
if (existing instanceof Method) {
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);
}
}
其中的checkAdd檢查了類型和方法簽名，每次輪轉完成以後都會進行一次findState.moveToSuperclass();對父類進行處理。
使用索引
由於反射所使用的運行時查找速度緩慢，因此咱們也常常會經過apt使用已經建立好的Index。
剛纔另外一個分支的findUsingInfo就是使用已有的Index:
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 {
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
這段很是簡單，幾乎就是剛纔的驗證而已，若是沒拿到數據的話，還會進行正常的反射查找。
// EventBusAnnotationProcessor 負責生成註解路由表
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");
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
}
}
}
}
private void writeIndexLines(BufferedWriter writer, String myPackage) throws IOException {
for (TypeElement subscriberTypeElement : methodsByClass.keySet()) {
if (classesToSkip.contains(subscriberTypeElement)) {
continue;
}
String subscriberClass = getClassString(subscriberTypeElement, myPackage);
if (isVisible(myPackage, subscriberTypeElement)) {
writeLine(writer, 2,
"putIndex(new SimpleSubscriberInfo(" + subscriberClass + ".class,",
"true,", "new SubscriberMethodInfo[] {");
List<ExecutableElement> methods = methodsByClass.get(subscriberTypeElement);
writeCreateSubscriberMethods(writer, methods, "new SubscriberMethodInfo", myPackage);
writer.write(" }));\n\n");
} else {
writer.write(" // Subscriber not visible to index: " + subscriberClass + "\n");
}
}
}
有了這兩個方法以後咱們就知道，日常的index就是經過這種方式拼接出來的。
Post消息
/** Posts the given event to the event bus. */
public void post(Object event) {
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;
}
}
}
PostingThreadState是一個存儲在ThreadLocal中的對象，包含有如下各類內容，線程信息，是不是主線程，是否取消，還有一個相應的事件隊列。
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance) {
/** Looks up all Class objects including super classes and interfaces. Should also work for interfaces. */
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
// 對全部的訂閱函數，都調用發送數據
for (int h = 0; h < countTypes; h++) {
// 全部的訂閱類
Class<?> clazz = eventTypes.get(h);
subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
}
} else {
// 只發送一次
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
}
if (!subscriptionFound) {
if (logNoSubscriberMessages) {
Log.d(TAG, "No subscribers registered for event " + eventClass);
}
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
// 無訂閱者的處理
post(new NoSubscriberEvent(this, event));
}
}
}
以後：
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;
}
以後對全部的訂閱類的全部訂閱者都發送一次數據，發送數據方法和上文相同。
發送粘性數據就是拿鎖而後保存到隊列中去,這樣就能夠在從新發送：
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);
}
由於咱們沒法肯定何時粘性事件應該中止繼續傳播，這取決於咱們應用的須要，因此咱們應當手動remove掉Sticky Event ：
// 系統提供了以下方法
public <T> T removeStickyEvent(Class<T> eventType) {
synchronized (stickyEvents) {
return eventType.cast(stickyEvents.remove(eventType));
}
}
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;
}
}
}
public void removeAllStickyEvents() {
synchronized (stickyEvents) {
stickyEvents.clear();
}
}
至此咱們就分析完了EventBus的基本上全部的代碼（處理util包下的錯誤日誌），EventBus自己的實現並不複雜，使用運行時的反射技巧也很簡單，單純的使用註解類可能會拖慢速度，可是經過apt生成的靜態表把速降提高到了一個新的高度，apt的生成你們也看到了並非很複雜，幾乎就是類型檢查和拼接字串，不過想法決定了EventBus仍然是一個優秀的開源庫，但願咱們在使用的同時，仍能對實現原理有所瞭解。
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