一名優秀的Android開發，需要一份完備的 知識體系，在這裏，讓我們一起成長爲自己所想的那樣~。
基本使用步驟
1）定義事件Event
public class MyEvent {
public MyEvent(String id, String message) {
this.id = id;
this.message = message;
}
public String id;
public String message;
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getMessage() {
return message;
}
public void setMessage(String message) {
this.message = message;
}
}
2）註冊訂閱者，並實現回調方法
public class MainActivity extends Activity {
private TextView message, next;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initView();
initData();
}
/**
* 初始化佈局組件
*/
private void initView() {
message = findViewById(R.id.message);
next = findViewById(R.id.next);
next.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Intent intent = new Intent(MainActivity.this, SecondActivity.class);
startActivity(intent);
}
});
}
@Subscribe(threadMode = ThreadMode.MAIN)
public void onMyEvent(MyEvent event) {
message.setText(event.getMessage());
}
/**
* 初始化數據
*/
private void initData() {
EventBus.getDefault().register(this);
}
@Override
protected void onDestroy() {
super.onDestroy();
EventBus.getDefault().unregister(this);
}
}
通過EventBus.getDefault().register(this);方法註冊訂閱者。
3）發送消息
public class SecondActivity extends Activity {
private TextView message;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
initView();
}
/**
* 初始化佈局組件
*/
private void initView() {
message = findViewById(R.id.message);
message.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
EventBus.getDefault().post(new MyEvent("1","這是從SecondActivity發送過來的消息"));
Toast.makeText(SecondActivity.this, "消息發送成功", Toast.LENGTH_SHORT).show();
}
});
}
}
通過EventBus.getDefault().post(new MyEvent(“1”,“這是從SecondActivity發送過來的消息”));方法發送消息
源碼分析
EventBus的使用先註冊訂閱者，並實現方法，然後再發送post消息，所以我們分析源碼也按照這個順序來。
1）實例化EventBus
EventBus.getDefault().register(this);
該方法首先獲取EventBus實例，然後再註冊，源碼如下所示：
/** 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;
}
可以看到，實例化EventBus是個雙重鎖的單例模式。
接下來看EventBus的構造函數，源碼如下：
EventBus(EventBusBuilder builder) {
logger = builder.getLogger();
subscriptionsByEventType = new HashMap<>();
typesBySubscriber = new HashMap<>();
stickyEvents = new ConcurrentHashMap<>();
mainThreadSupport = builder.getMainThreadSupport();
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
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;
}
可以看到，在EventBus的構造函數中，初始化了一大堆變量，這邊主要關注前面兩個。
subscriptionsByEventType = new HashMap<>();：key:事件類型（如：MyEvent ），value:新的訂閱對象的集合，包括訂閱者和訂閱者包含的方法。具體賦值邏輯在後面會講，這邊先了解即可。
typesBySubscriber = new HashMap<>();：key:事件的訂閱者（MainActivity ），value：事件類型的集合。訂閱者跟事件類型是一對多的關係，所以一個界面可以支持多個事件類型。
2）註冊
/**
* 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) {
//1.拿到訂閱對象的類型；
Class<?> subscriberClass = subscriber.getClass();
//2.通過findSubscriberMethods方法獲取該訂閱者中的所有訂閱方法，因爲可能包含多個訂閱方法，所以返回集合。
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
//3.通過subscribe方法爲每個訂閱方法進行訂閱。
subscribe(subscriber, subscriberMethod);
}
}
}
如何獲取該訂閱者中的所有訂閱方法，這邊主要看第二件事情的處理過程，及findSubscriberMethods方法的源碼
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//先拿訂閱對象在本地緩存中查找，提高性能。
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//默認情況下ignoreGeneratedIndex值是false的
if (ignoreGeneratedIndex) {
//使用反射方法拿到訂閱者中的訂閱方法
subscriberMethods = findUsingReflection(subscriberClass);
} else {
//使用編譯期間生成的SubscriberInfo
subscriberMethods = findUsingInfo(subscriberClass);
}
//如果沒有訂閱方法，則拋出異常
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
//本地存儲訂閱方法，方便下次獲取，提高了性能
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
可以看到上面方法做了三件事：
1.先拿訂閱對象在本地緩存中查找訂閱方法
2.如果本地獲取不到，則根據ignoreGeneratedIndex的值決定獲取訂閱方法的方式
3.本地保存訂閱方法。
可以看到獲取訂閱的關鍵方法爲：findUsingInfo()，那麼趕緊看下他的源碼，如下：
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
// 獲取訂閱者信息，沒有配置MyEventBusIndex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 {
// 通過反射來查找訂閱方法，所以爲了提高性能，我們還是要用索引的形式使用EventBus
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
獲取到訂閱方法後，我們看真正的註冊方法subscribe(subscriber, subscriberMethod);
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//獲取到事件類型（如：MyEvent ）
Class<?> eventType = subscriberMethod.eventType;
//把訂閱者和訂閱者方法重新封裝成新的對象
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//事件類型爲key，新的訂閱者對象列表爲value，存儲進subscriptionsByEventType
//該集合很重要，當post消息的時候，就是從該集合中查找訂閱者對象列表
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
//如果訂閱者對象列表爲空，則初始化出來，並加到subscriptionsByEventType中
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++) {
// 根據訂閱方法的優先級，添加到訂閱列表
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
//訂閱者爲key，事件類型列表爲value，存儲進typesBySubscriber
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
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);
}
}
}
可以看到上面方法做了兩件事：
1.根據訂閱方法的優先級，添加到訂閱列表，事件類型爲key，訂閱列表爲value，存儲進subscriptionsByEventType
2.訂閱者爲key，事件類型列表爲value，存儲進typesBySubscriber
3）發送消息
/** Posts the given event to the event bus. */
public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
//將當前要發送的事件加入到隊列中
eventQueue.add(event);
if (!postingState.isPosting) {
postingState.isMainThread = isMainThread();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
//循環從列表中拿event
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
循環從eventQueue取event，調用postSingleEvent方法：
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance) {
//查找event事件和event子類事件
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) {
logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
}
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}
遍歷每個event事件或者子類，會再調用postSingleEventForEventType：
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//subscriptionsByEventType有沒有很眼熟，在subscribe方法中，事件類型爲key，新的訂閱者對象列表爲value，存儲進subscriptionsByEventType
//獲取到新的訂閱對象列表
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
//循環訂閱對象列表
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;
}
可以看到，該方法其實就是從subscriptionsByEventType集合中，查找該event對應的訂閱者對象列表，然後遍歷訂閱者對象，調用postToSubscription方法。
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
switch (subscription.subscriberMethod.threadMode) {
case POSTING:
invokeSubscriber(subscription, event);
break;
case MAIN:
if (isMainThread) {
//通過反射，調用訂閱者的方法
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) {
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);
}
}
最後通過反射調用訂閱者的方法。
通過上面的分析可以看到發送消息其實是三重循環：
1.循環從eventQueue取event，調用postSingleEvent方法
2.遍歷每個event事件或者子類，調用postSingleEventForEventType方法
3.遍歷訂閱者對象，調用postToSubscription方法。
這種方式的算法複雜度還是很高的，event事件不做繼承可以提高發送流程的性能。
4）解綁
/** Unregisters the given subscriber from all event classes. */
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 {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
可以看到，先從typesBySubscriber集合中根據訂閱者獲取到事件類型列表subscribedTypes，然後循環事件類型，找出該事件類型對應的Subscriptions訂閱列表，循環遍歷，當Subscription對象中包含該訂閱者subscriber對象時，把該Subscription對象從訂閱列表中刪除，再從從typesBySubscriber中刪除對應的訂閱者。