EventBus 原始碼分析（上篇）

EventBus 原始碼分析 · 發表 2018-09-29 18:34:22

摘要：可用於應用內的訊息事件傳遞，方便快捷，耦合性低 1.基本用法 public class EventBusMain extends AppCompatActivity { @Override protected void onCreate(@Nullable Bundl...

可用於應用內的訊息事件傳遞，方便快捷，耦合性低

1.基本用法

public class EventBusMain extends AppCompatActivity {

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.content_main);


EventBus.getDefault().register(this);

}

- 訂閱的事件 onEvent1
@Subscribe
public void onEvent1(RemindBean bean){

}
- 訂閱的事件 onEvent2
@Subscribe
public void onEvent2(UserInfo bean){

}

@Override
protected void onDestroy() {
super.onDestroy();
EventBus.getDefault().unregister(this);
}
}

需要傳送訊息傳遞的時候：

EventBus.getDefault().post(new RemindBean())

2.原始碼解讀

放上官網的一張原理圖，感覺挺清晰的：

image.png

釋出訊息的一方( Publisher )，只需要 post 一個 event 之後就不用管了， EventBus 內部會將 event 逐一分發給訂閱此 event 的訂閱者（ Subscriber ）. 不錯就是這樣一個東西。

還記得以往我要實現兩個不同的 activity 之間要傳遞一些資料的時候，我都是通過定義一個 interface 的形式完成，時間一長，定義的介面一堆，在回顧檢視程式碼也確實不夠美觀。好了話不多說，看下大家都在用的 Eventbus .

3.首先

EventBus.getDefault().register(this);

getDefault() :

EventBus 是一個單例模式，懶漢式，雙重判斷
 /** 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;
}

register 是什麼意思呢，就是就跟你訂閱報紙一樣，報社需要確定幾個重要的問題：

Subscriber
Event

就是我認為比較重要的，那麼 register 這一步就是 Subscriber 告訴報社，訂閱的 event

public void register(Object subscriber) {
- 1.先拿到這個訂閱者(subscriber)類的位元組碼
Class<?> subscriberClass = subscriber.getClass();

- 2. 通過這個類的位元組碼，拿到所有的訂閱的 event,存放在List中
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);

synchronized (this) {
- 3. 迴圈遍歷所有的訂閱的方法，完成subscriber 和 subscriberMethod 的關聯
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}

我們看下這個如何根據 subscriberClass 找到這個訂閱的 method 的， findSubscriberMethods :

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
- 1.先從快取中取
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);

- 2. 第一次肯定 null
if (subscriberMethods != null) {
return subscriberMethods;
}

- 3. 查詢預設也是false，感興趣的可以看下
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);
} else {

- 4. 所以是走這裡
subscriberMethods = findUsingInfo(subscriberClass);
}
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
- 5. 找到之後新增到快取中，key是 subscriber ;value 是：methods
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}

看下： findUsingInfo(subscriberClass)

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
- 1. 我認為就是準備一個查詢結果得儲存物件
FindState findState = prepareFindState();

- 2. 將訂閱者的subscriberClass 儲存起來，儲存在一個FindState 類中的subscriberClass 
同時賦值給clazz變數中,以下程式碼能夠看出
//void initForSubscriber(Class<?> subscriberClass) {
 //this.subscriberClass = clazz = subscriberClass;
//}
findState.initForSubscriber(subscriberClass);

while (findState.clazz != null) {進入迴圈中
//獲取subscriberInfo 資訊，返回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 {
- 3. 進入到這裡了
findUsingReflectionInSingleClass(findState);
}
- 4. 查詢父類中的方法
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}

findUsingReflectionInSingleClass 如下：

private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
// This is faster than getMethods, especially when subscribers are fat classes like Activities
- 1. 通過訂閱者的位元組碼查詢當前類中所有生命的方法
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;
}
- 2. 迴圈遍歷所有的方法
for (Method method : methods) {
- 3. 獲取方法的修飾符
int modifiers = method.getModifiers();

- 4.判斷修飾符，訂閱方法的修飾符不能是private，static
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
- 5. 獲取方法的所有的引數
Class<?>[] parameterTypes = method.getParameterTypes();

- 6.判斷引數的個數，只能有1個引數，訂閱方法中
if (parameterTypes.length == 1) {
- 7.獲取方法上具有subscribe 註解
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);

- 8.含有subscribe註解的方法，就是該類訂閱的方法，其它不符合的可能就是普通的方法
if (subscribeAnnotation != null) {

- 9. 獲取第一個引數eventType
Class<?> eventType = parameterTypes[0];

if (findState.checkAdd(method, eventType)) {
- 10. 獲取註解的mode，就是我們在註解上標識的，
有mainThread,Posting,background,async
ThreadMode threadMode = subscribeAnnotation.threadMode();

- 11. 將訂閱方法的一系列資訊（方法名稱，threadMode,優先順序，是否是粘性等）新增到集合subscriberMethods中去
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
}
}
} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
- 12. 引數是多個的時候丟擲異常
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)) {
- 13. 方法的修飾符不是public的，丟擲異常

String methodName = method.getDeclaringClass().getName() + "." + method.getName();
throw new EventBusException(methodName +
" is a illegal @Subscribe method: must be public, non-static, and non-abstract");
}
}
}

這樣我們將所有信息都儲存到 findState 類中去了。再回頭看我們原先那個方法,到第三步了：

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
- 1. 我認為就是準備一個查詢結果得儲存物件
FindState findState = prepareFindState();

- 2. 將訂閱者的subscriberClass 儲存起來，儲存在一個FindState 類中的subscriberClass 
同時賦值給clazz變數中,以下程式碼能夠看出
//void initForSubscriber(Class<?> subscriberClass) {
 //this.subscriberClass = clazz = subscriberClass;
//}
findState.initForSubscriber(subscriberClass);

while (findState.clazz != null) {進入迴圈中
//獲取subscriberInfo 資訊，返回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 {
- 3. 進入到這裡了，上面已經分析所有資訊儲存到findState中
findUsingReflectionInSingleClass(findState);
}
- 4. 查詢父類中的方法
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}

在這個 getMethodsAndRelease(findState) :

private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
- 1. 取出裡面的subscriberMethods
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;
}
}
}
- 2. 返回集合
return subscriberMethods;
}

至此，我們知道了根據訂閱者（ subscriber ）的 clazz 找到了所有訂閱的方法事件

methods

回到最初的第一步 register :

public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
- 2. 完成
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
- 3.迴圈遍歷所有的訂閱方法和訂閱者之間建立關聯
subscribe(subscriber, subscriberMethod);
}
}
}

subscribe(subscriber, subscriberMethod) 方法：

// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
- 1. 訂閱方法的eventType的位元組碼
Class<?> eventType = subscriberMethod.eventType;

- 2. 訂閱者和訂閱方法封裝成一個Subscription 物件
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);

- 3. subscriptionsByEventType 第一次也是null ,根據eventType
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);

- 4. 第一次肯定為null
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();

- 5. key 為 eventType, value 是subscriptions物件
subscriptionsByEventType.put(eventType, subscriptions);
} else {
- 丟擲異常
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}

- 6. 獲取所有新增的subscriptions
int size = subscriptions.size();
for (int i = 0; i <= size; i++) {
- 7. 會判斷每個訂閱方法的優先順序，新增到這個 subscriptions中，按照優先順序
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}

- 8.獲取訂閱的方法集合
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
- 9. 為空新增到 typesBySubscriber
typesBySubscriber.put(subscriber, subscribedEvents);
}
- 10. 訂閱事件新增到subscribedEvents集合中去
subscribedEvents.add(eventType);

- 11. 判斷是否是粘性事件的關聯
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);
}
}
}

到此，如果你跟著我一步步看到這裡，應該大概明白一些了，還有一部分沒完，就是 register 前半部分完成訂閱，儲存等工作；剩下 post(event) 方法就是將 event 分發給相應訂閱過此事件的訂閱者了。

EventBus 原始碼分析（上篇）

1.基本用法

2.原始碼解讀

3.首先

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