EventBus通過訂閱者/發佈者機制使得整個系統的耦合度更加的鬆散，在不使用Handler和AsyncTask的情況下也能實現對界面的更新，在這篇文章中主要對EventBus源碼進行一個簡要分析，如果還沒有接觸過這個開源庫可以參考一下官方給出的實例代碼~
還是從我們平時使用的地方開始說起。在使用時我們的入口都是EventBus類的靜態方法，那麼最常接觸的就是getDefault方法了，來看一下：
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}構造方法採用了單例模式的雙重檢查來保證單例的唯一性，再看一下defaultInstance：
static volatile EventBus defaultInstance;將實例聲明爲volatile來保證了及時可見性。
在這裏調用了EventBus的構造方法，我們就不再跟進構造方法中了，由於其構造涉及到的參數數量較多，所以在這裏採取了構造者模式，有興趣可以去看一下實現（鏈式）。
看過了getDefault方法，如果看過例子都應該瞭解，我們還需要在初始化時註冊當前的類，並且在銷燬時反註冊掉當前類，涉及到註冊的方法有如下幾個：
public void register(Object subscriber) {
register(subscriber, false, 0);
}
public void register(Object subscriber, int priority) {
register(subscriber, false, priority);
}
public void registerSticky(Object subscriber) {
register(subscriber, true, 0);
}
public void registerSticky(Object subscriber, int priority) {
register(subscriber, true, priority);
}這裏的subscriber顯然就是訂閱者類啦，priority是訂閱者是優先級，至於Sticky我們暫時先不去考慮它。這四個方法都調用了同一個方法：
private synchronized void register(Object subscriber, boolean sticky, int priority) {
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriber.getClass());
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod, sticky, priority);
}
}在這裏調用了subscriberMethodFinder對象的findSubscriberMethod方法，傳入的是當前的訂閱者Class對象。可以看到subscriberMethodFinder是在構造方法中創建出來的，我們直接來看它的findSubscriberMethod方法：
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
String key = subscriberClass.getName();
List<SubscriberMethod> subscriberMethods;
synchronized (methodCache) {
subscriberMethods = methodCache.get(key);
}
if (subscriberMethods != null) {
return subscriberMethods;
}
subscriberMethods = new ArrayList<SubscriberMethod>();
Class<?> clazz = subscriberClass;
HashSet<String> eventTypesFound = new HashSet<String>();
StringBuilder methodKeyBuilder = new StringBuilder();
while (clazz != null) {
String name = clazz.getName();
if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {
// Skip system classes, this just degrades performance
break;
}
// Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
Method[] methods = clazz.getDeclaredMethods();
for (Method method : methods) {
String methodName = method.getName();
if (methodName.startsWith(ON_EVENT_METHOD_NAME)) {
int modifiers = method.getModifiers();
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
Class<?>[] parameterTypes = method.getParameterTypes();
if (parameterTypes.length == 1) {
String modifierString = methodName.substring(ON_EVENT_METHOD_NAME.length());
ThreadMode threadMode;
if (modifierString.length() == 0) {
threadMode = ThreadMode.PostThread;
} else if (modifierString.equals("MainThread")) {
threadMode = ThreadMode.MainThread;
} else if (modifierString.equals("BackgroundThread")) {
threadMode = ThreadMode.BackgroundThread;
} else if (modifierString.equals("Async")) {
threadMode = ThreadMode.Async;
} else {
if (skipMethodVerificationForClasses.containsKey(clazz)) {
continue;
} else {
throw new EventBusException("Illegal onEvent method, check for typos: " + method);
}
}
Class<?> eventType = parameterTypes[0];
methodKeyBuilder.setLength(0);
methodKeyBuilder.append(methodName);
methodKeyBuilder.append('>').append(eventType.getName());
String methodKey = methodKeyBuilder.toString();
if (eventTypesFound.add(methodKey)) {
// Only add if not already found in a sub class
subscriberMethods.add(new SubscriberMethod(method, threadMode, eventType));
}
}
} else if (!skipMethodVerificationForClasses.containsKey(clazz)) {
Log.d(EventBus.TAG, "Skipping method (not public, static or abstract): " + clazz + "."
+ methodName);
}
}
}
clazz = clazz.getSuperclass();
}
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass + " has no public methods called "
+ ON_EVENT_METHOD_NAME);
} else {
synchronized (methodCache) {
methodCache.put(key, subscriberMethods);
}
return subscriberMethods;
}
}方法很長但是並不複雜，一點一點來看一下：
首先得到了訂閱者類的全類名，而後以這個全類名爲key查詢methodCach，下面是這個對象的定義：
private static final Map<String, List<SubscriberMethod>> methodCache = new HashMap<String, List<SubscriberMethod>>();很顯然第一次在緩存中是一定找不到的，我們繼續向下看。
創建一個ArrayList，從對象名字可以看出這是一個訂閱者方法的集合，而後進入while循環，判斷一下當前的訂閱者類是不是一個系統中的提供類，若是的話直接就終止掉循環。
然後獲取到當前類的所有方法，包括私有方法，但是在這裏作者註釋聲明瞭在這個版本中只允許訂閱者onEvent開頭的方法爲public，這點在後文中將會有所體現。
得到方法的參數，當方法的參數不是一個的時候拋出異常。
截取該類中以onEvent開頭的方法名，並且使用ThreadMode來儲存方法的名字。
後面進行了一個判定的操作然後將方法method，保存信息的threadMode和方法參數Class對象封裝成一個SubscriberMethod類的對象並且存放在subscriberMethods集合中。
而後將clazz對象置爲父類重新進行循環。
最後可以看到以當前訂閱者類全名爲key存入到了緩存中，方便下次使用，而後返回subscriberMethods集合也就是訂閱者方法集合。
到這裏findSubscriberMethods就結束了，我們回到主線上繼續看：
循環出了subscriberMethods集合中的每一個方法然後調用subscribe方法，我們進入看一下：
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod, boolean sticky, int priority) {
Class<?> eventType = subscriberMethod.eventType;
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
Subscription newSubscription = new Subscription(subscriber, subscriberMethod, priority);
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<Subscription>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
// Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
// subscriberMethod.method.setAccessible(true);
int size = subscriptions.size();
for (int i = 0; i <= size; i++) {
if (i == size || newSubscription.priority > subscriptions.get(i).priority) {
subscriptions.add(i, newSubscription);
break;
}
}
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<Class<?>>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
if (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);
}
}
}首先得到該方法參數的Class對象，然後以這個對象作爲key從subscriptionsByEventType集合中來找出value，還是看一下定義：
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;先簡單的從名字理解一下，以onEvent開頭的方法參數類型爲key的map，保存了所有包含這種參數類型的Subscription集合（Subscription是由當前訂閱者，訂閱者的單獨一個方法和訂閱者的優先級共同確定的）。
在第一次取出來的時候也是一定會取出一個null，這時候將創建這個集合並且將其放入map。
這時候將之前創建出的Subscription類的對象添加到集合當中。在這裏作者將Subscription類對象中封裝的方法設置權限的代碼註釋掉了，也是應了剛纔說的，只允許onEvent開頭的方法是public的。
而後又出現了一個typesBySubscriber，看一下它的定義：
private final Map<Object, List<Class<?>>> typesBySubscriber;從名字也可以很簡單的看出來，它保存了當前訂閱者對象所有可能接受的訂閱事件。
以當前訂閱者對象爲key來從這個map中找出集合，如果不存在就創建一個，然後把eventType訂閱事件放到裏面。剩下的一些代碼也暫時不去考慮，先只看一下主線。
到這裏註冊的邏輯就已經完成了，看起來還是挺簡單的，只是將訂閱者啊，Subscription向集合里加一加。看完了註冊我們再來看一下發布事件的post方法：
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;
}
}
}首先在currentPostingThreadState調用get方法，看一下這個對象的定義：
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
@Override
protected PostingThreadState initialValue() {
return new PostingThreadState();
}
};是一個ThreadLocal，那麼很顯然每個線程都將維護它自己的postingThreadState對象。
從這個對象中得到當前的事件隊列然後將發佈的事件放到隊列中。
看一下當前如果沒有處於正在發佈的狀態，下行。
保存一下當前是否是在主線程中進行的事件發佈，並且將狀態置爲正在發佈。
而後不斷的從隊列中取出事件調用postSingleEvent方法，進入方法看一下：
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
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) {
Log.d(TAG, "No subscribers registered for event " + eventClass);
}
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}在這裏我們只看主線的狀態，當最簡單的情況下應該走else中的方法，也就是postSingleEventForEventType，再進入看一下：
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;
}以當前發佈的事件爲key，從subscriptionsByEventType中找出Subscription的集合，如果把subscriptionsByEventType忘記了可以回到前面再看一下。
然後循環出這個集合中的每個Subscription類的對象調用postToSubscription方法，我們來看一下：
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
switch (subscription.subscriberMethod.threadMode) {
case PostThread:
invokeSubscriber(subscription, event);
break;
case MainThread:
if (isMainThread) {
invokeSubscriber(subscription, event);
} else {
mainThreadPoster.enqueue(subscription, event);
}
break;
case BackgroundThread:
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);
}
}判斷一下Subscription類對象中封裝的方法中ThreadMode，如果是PostThread那麼直接調用invokeSubscriber，看一下：
void invokeSubscriber(Subscription subscription, Object event) {
try {
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}簡單的通過反射來調用了onEventPostThread方法，並且將event事件傳入進去。值得注意的是這裏的postToSubscription方法是在事件發佈的線程中調用的。
如果switch選擇出來的是MainThread，那麼判斷當前發佈事件的線程是否是主線程，如果是直接執行，不是將subscription和event事件入隊mainThreadPoster。
同理BackgroundThread判斷當前是否是主線程，是的話入隊backgroundPoster，不是的話直接調用。
如果是Async則直接入隊asyncPoster。
如果有興趣可以查看一下mainThreadPoster、backgroundPoster和asyncPoster的內容，裏面的實現也比較簡單，這裏就不再贅述。