EventBus github 地址
在Android Studio中添加如下依赖：
compile 'org.greenrobot:eventbus:3.1.1'
以前在用EventBus之前，一直认为其原理 实现是观察者模式，经过本次分析才发现其原理是不正常的观察者模式，是使用反射加注解的方式实现的。本文章只介绍源码，然后自己手动手写一个类似的 功能。
register源码解析
public void register(Object subscriber) {
//获取到注册对象，一般为MainActivity
Class<?> subscriberClass = subscriber.getClass();
//这里获取到解析此对象中有Subscribe注解信息列表封装成SubscriberMethod对象
List<SubscriberMethod> subscriberMethods =
//此方法比较重要
subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
其中subscriberMethodFinder为：
subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
builder.strictMethodVerification, builder.ignoreGeneratedIndex);
其中eventBusBuilder为
//默认的eventBusBuilder
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
eventBusBuilder中变量值为，这些值会在下面分析中用到。
boolean logSubscriberExceptions = true;
boolean logNoSubscriberMessages = true;
boolean sendSubscriberExceptionEvent = true;
boolean sendNoSubscriberEvent = true;
boolean throwSubscriberException;
boolean eventInheritance = true;
boolean ignoreGeneratedIndex;
boolean strictMethodVerification;
ExecutorService executorService = DEFAULT_EXECUTOR_SERVICE;
List<Class<?>> skipMethodVerificationForClasses;
List<SubscriberInfoIndex> subscriberInfoIndexes;
Logger logger;
MainThreadSupport mainThreadSupport;
言归正传，我们回到findSubscriberMethods方法
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//从缓存中获取SubscriberMethod对象，第一次应该为空
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//这里默认为false，可以查看builder中数据
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);
} else {
//方法走到这里面
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;
}
}
进入到findUsingInfo
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
//享元模式获取对象
FindState findState = prepareFindState();
//将返回的对象重新赋值
findState.initForSubscriber(subscriberClass);
//当findState.clazz不为空时
while (findState.clazz != null) {
findState.subscriberInfo = getSubscriberInfo(findState);
//因此不会走if
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 {
//程序会走到这里
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
程序会走到findUsingReflectionInSingleClass
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
//获取到类（activity）中所有方法，不包含继承的方法
methods = findState.clazz.getDeclaredMethods();
} catch (Throwable th) {
methods = findState.clazz.getMethods();
findState.skipSuperClasses = true;
}
//遍历类中的方法
for (Method method : methods) {
//获取方法的类型
int modifiers = method.getModifiers();
//当方法为public时，这里就知道previte方法接收不到的原因了
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
//获取到方法的参数数组，有可能会是多参
Class<?>[] parameterTypes = method.getParameterTypes();
//当方法参数为1时才会向下走，这里就知道为啥eventBus只能接收单个参数
if (parameterTypes.length == 1) {
//获取到方法上面的Subscribe注解
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
//如果注解不为空
if (subscribeAnnotation != null) {
//获取到参数的第一个类型（String.class）
Class<?> eventType = parameterTypes[0];
//检查数据中是否需要加入此方法和参数类型
if (findState.checkAdd(method, eventType)) {
//获取到方法上的threadMode
ThreadMode threadMode = subscribeAnnotation.threadMode();
//将method，eventType，threadMode，priority，sticky加入到集合中
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");
}
}
}
这时，我们便把当前类中需要的东西都封装成了SubscriberMethod对象。
其中SubscriberMethod中字段意义为：
public SubscriberMethod(Method method, Class<?> eventType, ThreadMode threadMode, int priority, boolean sticky) {
//方法对象
this.method = method;
//方法上的threadMode
this.threadMode = threadMode;
//方法中参数类(String)
this.eventType = eventType;
//方法上priority，优先级
this.priority = priority;
//方法上sticky，粘性
this.sticky = sticky;
}
进入getMethodsAndRelease方法，获取数据集合。
private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
//获取到当前subscriberMethods集合
List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
//清空findState对象中数据
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;
}
}
}
//返回subscriberMethods
return subscriberMethods;
}
至此findSubscriberMethods方法分析就完成了。可以理解为如下图：
findSubscriberMethods.png
注：图片引用自红橙Darren大神。懒得画了。哈哈哈!
继续分析subscribe方法
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//获取subscriberMethod中的参数类型
Class<?> eventType = subscriberMethod.eventType;
//根据主类(MAinActivity)和SubscriberMethod 创建Subscription 对象
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//从subscriptionsByEventType中获取Subscription list集合，第一次一般为空
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
//当数据为空时，创建list，并且加入到subscriptionsByEventType中
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);
}
}
//获取到Subscription集合大小
int size = subscriptions.size();
//这里进行优先级排序
for (int i = 0; i <= size; i++) {
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
//通过类对象(MainActivity)获取到 方法参数(String)集合，第一次为空
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
//创建新数组，并且把数据加入到typesBySubscriber
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
//将本方法参数对象加入到集合中
subscribedEvents.add(eventType);
//sticky分析省略
}
其中Subscription结构为：
Subscription(Object subscriber, SubscriberMethod subscriberMethod) {
//主类Mactivity
this.subscriber = subscriber;
//类中对应的对象
this.subscriberMethod = subscriberMethod;
active = true;
}
其中用到的变量为：
// subscriptionsByEventType 这个集合存放的是？
// key 是 Event 参数的类
// value 存放的是 Subscription 的集合列表
// Subscription 包含两个属性，一个是 subscriber 订阅者（反射执行对象）,
// 一个是 SubscriberMethod 注解方法的所有属性参数值
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
// typesBySubscriber 这个集合存放的是？
// key 是所有的订阅者
// value 是所有订阅者里面方法的参数的 class，eventType
private final Map<Object, List<Class<?>>> typesBySubscriber;
这一部分代码就是根据方法类型进行了分类，如图所示：
-
subscriptionsByEventType结构
subscriptionsByEventType.png
-
typesBySubscriber 结构
typesBySubscriber .png
看到这里大家想说了，把这些东西存放到这些结构里面有什么用呢，为什么要这样做呢，那请大家带着这些疑问来看看我们下面讲的unregister
unregister源码解析
当我们再使用EventBus时候在某个类中注册了事件，当不需要接收事件时，我们应该解除注册，这样可以停止接收事件，也为了防止内存泄漏，优化应用。
unregister方法
public synchronized void unregister(Object subscriber) {
//获取到本类的方法参数类型集合
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
//遍历集合解绑数据，防止内存泄漏
for (Class<?> eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType);
}
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not
registered before: " + subscriber.getClass());
}
}
这里就看到了，先从typesBySubscriber数据中获取参数列表
unsubscribeByEventType方法
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
//从subscriptionsByEventType中获取Subscription
List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions != null) {
int size = subscriptions.size();
for (int i = 0; i < size; i++) {
Subscription subscription = subscriptions.get(i);
//如果数组中的类和当前解绑类相等，则移除
if (subscription.subscriber == subscriber) {
subscription.active = false;
subscriptions.remove(i);
i--;
size--;
}
}
}
}
然后根据参数列表从subscriptionsByEventType中获取List<Subscription>数据集，然后移除对应对象。
至此EventBus的register和unregister就讲完了
post源码解析
public void post(Object event) {
//获取到当前线程的postingState
PostingThreadState postingState = currentPostingThreadState.get();
//获取到当前线程的event事件
List<Object> eventQueue = postingState.eventQueue;
//将本event加入数组中
eventQueue.add(event);
//当不是posting状态
if (!postingState.isPosting) {
//当前是不是主线程
postingState.isMainThread = isMainThread();
//设置为posting状态
postingState.isPosting = true;
//如果当前已被取消，需要抛出异常
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
//开始遍历数组，发送消息
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
post数据时候，会先检查当前发送线程，然后获取到eventQueue事件队列，把数据加入到队列中，开始发送数据。
postSingleEvent方法
private void postSingleEvent(Object event, PostingThreadState postingState){
//获取到传入参数的类型(String)
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 {
//程序会走到这里
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));
}
}
}
postSingleEventForEventType方法
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState,
Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//根据参数类型获取到Subscription集合
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
//遍历Subscription集合
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
//开始发送数据
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;
}
这个方法从subscriptionsByEventType数据结构中获取到当前需要通知的Subscription数组。
postToSubscription方法，终于发送数据了。。。
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
//根据mode选择执行方式
switch (subscription.subscriberMethod.threadMode) {
//在当前线程执行方法
case POSTING:
invokeSubscriber(subscription, event);
break;
//如果是主线程在当前线程执行，不是主线程，通过handler方式执行
case MAIN:
if (isMainThread) {
invokeSubscriber(subscription, event);
} else {
mainThreadPoster.enqueue(subscription, event);
}
break;
//如果主线程订阅了则在主线程中执行，否则在当前线程
case MAIN_ORDERED:
if (mainThreadPoster != null) {
mainThreadPoster.enqueue(subscription, event);
} else {
invokeSubscriber(subscription, event);
}
break;
//始终在子线程中执行
case BACKGROUND:
if (isMainThread) {
backgroundPoster.enqueue(subscription, event);
} else {
invokeSubscriber(subscription, event);
}
break;
//始终在另外一个新线程中执行
case ASYNC:
asyncPoster.enqueue(subscription, event);
break;
default:
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}
invokeSubscriber方法
void invokeSubscriber(Subscription subscription, Object event) {
try {
//通过反射，执行具体的方法，并把参数传进去
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}
通过反射，执行了方法。
到此为止post的分析也完成了。感兴趣的可以去看看如何发送到主线程的，其原理也是handler，有空再补上。
网友评论