探索Android开源框架 - 5. EventBus使用及源码解析
相关概念
定义
Android事件发布/订阅框架，简化应用程序内组件间，组件与后台线程间的通信；
优点
代码简洁，使用简单，将事件发布和订阅充分解耦；
五种ThreadMode
POSTING（默认）
接收事件方法应执行在发射事件方法所在的线程
MAIN
接收事件方法应执行在主线程
MAIN_ORDERED
接收事件方法会被扔进 MessageQueue 中等待执行
BACKGROUND
发射事件方法在主线程中执行，则接收事件方法应执行在 EventBus 维护的单一子线程 执行
发射事件方法在子线程中执行，则接收事件方法应执行在发射事件方法所在的线程
ASYNC
接收方法应执行在不同于发射事件方法所在的另一个线程。常用于耗时操作
使用
添加依赖
导入依赖包
implementation 'org.greenrobot:eventbus:3.2.0'复制代码
Subscriber Index[可选]
作者在EventBus 3中引入了EventBusAnnotationProcessor（注解分析生成索引）技术，大大提高了EventBus的运行效率;
1. java
android {
defaultConfig {
javaCompileOptions {
annotationProcessorOptions {
arguments = [ eventBusIndex : 'com.example.myapp.MyEventBusIndex' ]
}
}
}
}
dependencies {
def eventbus_version = '3.2.0'
implementation "org.greenrobot:eventbus:$eventbus_version"
annotationProcessor "org.greenrobot:eventbus-annotation-processor:$eventbus_version"
}复制代码2. kotlin
apply plugin: 'kotlin-kapt' // ensure kapt plugin is applied
dependencies {
def eventbus_version = '3.2.0'
implementation "org.greenrobot:eventbus:$eventbus_version"
kapt "org.greenrobot:eventbus-annotation-processor:$eventbus_version"
}
kapt {
arguments {
arg('eventBusIndex', 'com.example.myapp.MyEventBusIndex')
}
}复制代码此时需要我们先编译一次，生成索引类。编译成功之后，就会发现在\ProjectName\app\build\generated\source\apt\PakageName\下看到通过注解分析生成的索引类，这样我们便可以在初始化EventBus时应用我们生成的索引了
使用
要应用我们生成好的索引时
val mEventBus = EventBus.builder().addIndex(MyEventBusIndex()).build()复制代码
如果想把自定义的设置应用到EventBus默认的单例中，则可以用installDefaultEventBus()方法
EventBus.builder().addIndex(MyEventBusIndex()).installDefaultEventBus()复制代码
基本使用
基本使用步骤分为注册，自定义Event，订阅，发布，注销几个步骤，其中自定义Event一般单独提取出来复用，发布一般在其他页面或组件中执行，示例代码如下
class EventBusActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_event_bus)
}
override fun onStart() {
super.onStart()
//1. 注册
EventBus.getDefault().register(this)
}
//2. 自定义Event
class MessageEvent(val what: Int)
//3. 订阅：通过设置不同的ThreadMode来指定订阅者的工作线程
@Subscribe(threadMode = ThreadMode.POSTING)
// @Subscribe(threadMode = ThreadMode.MAIN)
// @Subscribe(threadMode = ThreadMode.MAIN_ORDERED)
// @Subscribe(threadMode = ThreadMode.BACKGROUND)
// @Subscribe(threadMode = ThreadMode.ASYNC)
fun onMessageEvent(event: MessageEvent?) {
LjyLogUtil.d("${Thread.currentThread().name}_event.what=${event?.what}")
}
fun onBtnCLick(view: android.view.View) {
when (view.id) {
R.id.btn_postInMain -> {
//4.1 在主线程发布
LjyLogUtil.d("${Thread.currentThread().name}_post")
EventBus.getDefault().post(MessageEvent(1001))
}
R.id.btn_postInThread -> {
//4.2 在子线程发布
Thread {
LjyLogUtil.d("${Thread.currentThread().name}_post")
EventBus.getDefault().post(MessageEvent(1002))
}.start()
}
}
}
override fun onStop() {
super.onStop()
//5. 注销
EventBus.getDefault().unregister(this)
}
}复制代码粘性事件
订阅者注解开启sticky
@Subscribe(threadMode = ThreadMode.MAIN, sticky = true)
fun onMessageEventSticky(event: MessageEvent?) {
LjyLogUtil.d("${Thread.currentThread().name}_event.what=${event?.what}")
}复制代码发布时调用postSticky方法
EventBus.getDefault().postSticky(MessageEvent(1003))复制代码
源码解析
EventBus.getDefault
通过上面的使用我们发现都是调用EventBus.getDefault获取的EventBus实例,那么以此为入口，看一下其实现代码
public static EventBus getDefault() {
EventBus instance = defaultInstance;
//一个经典的懒汉式 double check 获取单例
if (instance == null) {
synchronized (EventBus.class) {
instance = EventBus.defaultInstance;
if (instance == null) {
instance = EventBus.defaultInstance = new EventBus();
}
}
}
return instance;
}复制代码EventBus 构造方法
通过上面的getDefault中调用的无参构造方法我们点进来看一下
public EventBus() {
this(DEFAULT_BUILDER);
}复制代码DEFAULT_BUILDER是默认的EventBusBuilder
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();复制代码
this调用了EventBus另一个构造函数, 主要做一些初始化工作
EventBus(EventBusBuilder builder) {
logger = builder.getLogger();
subscriptionsByEventType = new HashMap<>();
typesBySubscriber = new HashMap<>();
stickyEvents = new ConcurrentHashMap<>();
mainThreadSupport = builder.getMainThreadSupport();
//下面三个Poster很重要
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;
}复制代码register
下面我们看一下订阅者的注册方法register
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
//寻找 subscriber（我们实例中register(this)的this，当前Activity，Fragment等）中订阅的方法
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
//注册订阅
subscribe(subscriber, subscriberMethod);
}
}
}复制代码可以看到register中主要是调用findSubscriberMethods查找订阅方法，再调用subscribe进行订阅
findSubscriberMethods
查找传进来的订阅者的所有订阅方法，保存到SubscriberMethod集合中；
SubscriberMethod类主要是保存订阅方法的Method对象、线程模式、事件类型、优先级、是否是粘性事件等属性；
//存着注册类与其所有需要回调的 Event 方法列表的键值对
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//先从缓存中取
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//判断是否忽略注解器生成的MyEventBusIndex,也就是我们开头添加依赖时的Subscriber Index部分
if (ignoreGeneratedIndex) {
//通过反射获取
subscriberMethods = findUsingReflection(subscriberClass);
} else {
subscriberMethods = findUsingInfo(subscriberClass);
}
//如果订阅者中不存在被@Subscribe注解的public的方法，则抛出异常
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
//如果订阅者中存在订阅方法
//放入缓存中
METHOD_CACHE.put(subscriberClass, subscriberMethods);
//返回集合
return subscriberMethods;
}
}
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
//初始化 FindState 对象
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
// while 循环中，不停地去反射获取当前类和其父类的订阅方法并添入列表中，
//注意，在 Java 中，如果当前类实现了一个接口，即使该接口的方法被 @Subscribe 所修饰，
//当前类中的方法也是不包含该注解属性的，所以如果在接口中对某个方法使用了 @Subscribe 修饰然后让类去实现这个接口是没有任何作用的
while (findState.clazz != null) {
findUsingReflectionInSingleClass(findState);
findState.moveToSuperclass();
}
最终返回这个列表并重置 FindState 对象利于下一次重复使用
return getMethodsAndRelease(findState);
}复制代码从上面代码我们可以知道，调用register的类中，需要有被@Subscribe注解的方法，且必须为public方法；
上面代码中ignoreGeneratedIndex默认为false的，项目中经常通过EventBus单例模式来获取默认的EventBus对象，也就是ignoreGeneratedIndex为false的情况，这种情况调用了findUsingInfo方法：
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//获取订阅者信息,没有配置MyEventBusIndex返回null
findState.subscriberInfo = getSubscriberInfo(findState);
if (findState.subscriberInfo != null) {
//如果通过EventBusBuilder配置了MyEventBusIndex，便会获取到subscriberInfo，
//调用subscriberInfo的getSubscriberMethods方法可以得到订阅方法相关的信息，
//这个时候就不在需要通过注解进行获取订阅方法
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
for (SubscriberMethod subscriberMethod : array) {
//将订阅方法保存到findState
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
} else {
//通过反射查找订阅方法
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
//对findState做回收处理并反回订阅方法的List集合
return getMethodsAndRelease(findState);
}复制代码findUsingReflectionInSingleClass通过Java的反射和对注解的解析查找订阅方法，并保存到FindState中，代码如下
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
try {
methods = findState.clazz.getMethods();
} catch (LinkageError error) { // super class of NoClassDefFoundError to be a bit more broad...
String msg = "Could not inspect methods of " + findState.clazz.getName();
if (ignoreGeneratedIndex) {
msg += ". Please consider using EventBus annotation processor to avoid reflection.";
} else {
msg += ". Please make this class visible to EventBus annotation processor to avoid reflection.";
}
throw new EventBusException(msg, error);
}
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");
}
}
}复制代码subscribe
继续回到register，查找完订阅方法后便通过遍历调用subscribe，对所有的订阅方法进行注册
//
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
private final Map<Object, List<Class<?>>> typesBySubscriber;
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
//根据订阅者和订阅方法构造一个订阅事件
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//根据EventType找到订阅事件，从而去分发事件，处理事件
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//订阅者已经注册则抛出EventBusException
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
//遍历订阅事件，找到比subscriptions中订阅事件优先级小的位置，然后插进去
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;
}
}
//通过订阅者获取该订阅者所订阅事件的集合
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
//将当前的订阅事件添加到subscribedEvents中
subscribedEvents.add(eventType);
//如果是粘性事件的话，就立即投递、执行
if (subscriberMethod.sticky) {
//默认为true
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);
}
}
}复制代码post
下面来看一下post方法，代码如下
public void post(Object event) {
//PostingThreadState保存着事件队列和线程状态信息
PostingThreadState postingState = currentPostingThreadState.get();
//获取事件队列，并将当前事件插入到事件队列中
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
//确保不会被多次执行
if (!postingState.isPosting) {
postingState.isMainThread = isMainThread();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
//遍历处理队列中的事件
while (!eventQueue.isEmpty()) {
//post单个事件
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
//重置状态
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}复制代码post方法中调用了postSingleEvent处理单个事件，代码如下
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
//eventInheritance表示是否向上查找事件的父类,默认为true
if (eventInheritance) {
//取出 Event 及其父类和接口的 class 列表
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);
}
//找不到该事件时的异常处理
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));
}
}
}复制代码postSingleEvent中又调用了postSingleEventForEventType，代码如下
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//取出该事件对应的Subscription集合
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted;
try {
//对事件进行处理
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;
}复制代码postSingleEventForEventType中又调用了postToSubscription，代码如下,通过下面代码我们也可以知道之前介绍五种ThreadMode的不同之处
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
//取出订阅方法的线程模式，之后根据线程模式来分别处理
switch (subscription.subscriberMethod.threadMode) {
case POSTING:
//直接执行 invokeSubscriber() 方法，内部直接采用反射调用
invokeSubscriber(subscription, event);
break;
case MAIN:
//判断当前是否在 UI 线程
if (isMainThread) {
//直接采用反射调用
invokeSubscriber(subscription, event);
} else {
//把当前的方法加入到队列之中，然后通过 handler 去发送一个消息，在 handler 的 handleMessage 中去执行方法
mainThreadPoster.enqueue(subscription, event);
}
break;
case MAIN_ORDERED:
// 与MAIN类似，不过是确保是顺序执行的
if (mainThreadPoster != null) {
mainThreadPoster.enqueue(subscription, event);
} else {
invokeSubscriber(subscription, event);
}
break;
case BACKGROUND:
//判断当前是否在 UI 线程
if (isMainThread) {
backgroundPoster.enqueue(subscription, event);
} else {
//直接采用反射调用
invokeSubscriber(subscription, event);
}
break;
case ASYNC:
asyncPoster.enqueue(subscription, event);
break;
default:
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}复制代码postToSubscription处理完成后又会调用invokeSubscriber去回调订阅方法
void invokeSubscriber(PendingPost pendingPost) {
Object event = pendingPost.event;
Subscription subscription = pendingPost.subscription;
PendingPost.releasePendingPost(pendingPost);
if (subscription.active) {
invokeSubscriber(subscription, event);
}
}
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);
}
}复制代码postSticky
看完了post的流程，再来看一下postSticky，代码如下,先将该事件放入 stickyEvents 中，再正常调用post；
public void postSticky(Object event) {
//为避免多线程操作 postSticky(Object) 和 removeStickyEvent(Class<?>) 引发的冲突，所以对 stickyEvents 对象添加了 synchronized 关键字
synchronized (stickyEvents) {
stickyEvents.put(event.getClass(), event);
}
// Should be posted after it is putted, in case the subscriber wants to remove immediately
post(event);
}复制代码unregister
接下来看一下注销的方法unregister，代码如下
public synchronized void unregister(Object subscriber) {
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
//对 subscriptionsByEventType 移除了该 subscriber 的所有订阅信息
unsubscribeByEventType(subscriber, eventType);
}
//移除了注册对象和其对应的所有 Event 事件链表
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}复制代码番外
为什么使用事件总线机制来替代广播？
广播：耗时、容易被捕获（不安全）;
事件总线：更节省资源、更高效，能将信息传递给原生以外的各种对象
EventBus2.x的版本和3.x是有很大区别的
2.x使用的是运行时注解,采用了反射的方式对整个注册的类的所有方法进行扫描来完成注册，因而会对性能有一定影响;
3.x使用的是编译时注解，Java文件会编译成.class文件，再对class文件进行打包等一系列处理。在编译成.class文件时，EventBus会使用EventBusAnnotationProcessor注解处理器读取@Subscribe()注解并解析、处理其中的信息，然后生成Java类来保存所有订阅者的订阅信息。这样就创建出了对文件或类的索引关系，并将其编入到apk中；
从EventBus3.0开始使用了对象池缓存减少了创建对象的开销；
跨进程问题
目前EventBus只支持跨线程，而不支持跨进程。如果一个app的service起到了另一个进程中，那么注册监听的模块则会收不到另一个进程的EventBus发出的事件。这里可以考虑利用IPC做映射表，并在两个进程中各维护一个EventBus，不过这样就要自己去维护register和unregister的关系，比较繁琐，而且这种情况下通常用广播会更加方便;
RxBus
RxBus不是一个库，而是一个文件，实现只有短短30行代码。RxBus本身不需要过多分析，它的强大完全来自于它基于的RxJava技术;
基于RxJava2的RxBus实现代码如下
public final class RxBus2 {
private final Subject<Object> bus;
private RxBus2() {
// toSerialized method made bus thread safe
bus = PublishSubject.create().toSerialized();
}
public static RxBus2 getInstance() {
return Holder.BUS;
}
private static class Holder {
private static final RxBus2 BUS = new RxBus2();
}
public void post(Object obj) {
bus.onNext(obj);
}
public <T> Observable<T> toObservable(Class<T> tClass) {
return bus.ofType(tClass);
}
public Observable<Object> toObservable() {
return bus;
}
public boolean hasObservers() {
return bus.hasObservers();
}
}复制代码RxBus 与 EventBus 比较
其实也就是rxJava的优点:
RxJava的Observable有onError、onComplete等状态回调；
RxJava使用组合而非嵌套的方式，避免了回调地狱；
RxJava的线程调度设计的更加优秀，更简单易用；
RxJava可使用多种操作符来进行链式调用来实现复杂的逻辑；
RxJava的信息效率高于EventBus2.x，低于EventBus3.x；
那么技术选型时如何取舍呢？如果项目中使用了RxJava，则使用RxBus，否则使用EventBus3.x；
LiveDataBus
LiveDataBus是基于LiveData实现的类似EventBus的消息通信框架，它是基于LiveData实现的，完全可以代替EventBus，RxBus；
为什么会有LiveDataBus呢？
Handler : 容易导致内存泄漏，空指针，高耦合，不利于维护
EventBus ：原理实现复杂，无法混淆，需要手动绑定生命周期
RxBus：依赖于RxJava，包太大，影响apk大小，app启动时间
初代实现如下
public final class LiveDataBus {
private final Map<String, MutableLiveData<Object>> bus;
private LiveDataBus() {
bus = new HashMap<>();
}
private static class SingletonHolder {
private static final LiveDataBus DATA_BUS = new LiveDataBus();
}
public static LiveDataBus get() {
return SingletonHolder.DATA_BUS;
}
public <T> MutableLiveData<T> getChannel(String target, Class<T> type) {
if (!bus.containsKey(target)) {
bus.put(target, new MutableLiveData<>());
}
return (MutableLiveData<T>) bus.get(target);
}
public MutableLiveData<Object> getChannel(String target) {
return getChannel(target, Object.class);
}
}
//使用
注册订阅：
LiveDataBus.get().getChannel("key_test", Boolean.class)
.observe(this, new Observer<Boolean>() {
@Override
public void onChanged(@Nullable Boolean aBoolean) {
}
});
发送消息：
LiveDataBus.get().getChannel("key_test").setValue(true);复制代码LiveDataBus最终实现
public final class LiveDataBus {
private final Map<String, BusMutableLiveData<Object>> bus;
private LiveDataBus() {
bus = new HashMap<>();
}
private static class SingletonHolder {
private static final LiveDataBus DEFAULT_BUS = new LiveDataBus();
}
public static LiveDataBus get() {
return SingletonHolder.DEFAULT_BUS;
}
public <T> MutableLiveData<T> with(String key, Class<T> type) {
if (!bus.containsKey(key)) {
bus.put(key, new BusMutableLiveData<>());
}
return (MutableLiveData<T>) bus.get(key);
}
public MutableLiveData<Object> with(String key) {
return with(key, Object.class);
}
private static class ObserverWrapper<T> implements Observer<T> {
private Observer<T> observer;
public ObserverWrapper(Observer<T> observer) {
this.observer = observer;
}
@Override
public void onChanged(@Nullable T t) {
if (observer != null) {
if (isCallOnObserve()) {
return;
}
observer.onChanged(t);
}
}
private boolean isCallOnObserve() {
StackTraceElement[] stackTrace = Thread.currentThread().getStackTrace();
if (stackTrace != null && stackTrace.length > 0) {
for (StackTraceElement element : stackTrace) {
if ("android.arch.lifecycle.LiveData".equals(element.getClassName()) &&
"observeForever".equals(element.getMethodName())) {
return true;
}
}
}
return false;
}
}
private static class BusMutableLiveData<T> extends MutableLiveData<T> {
private Map<Observer, Observer> observerMap = new HashMap<>();
@Override
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<T> observer) {
super.observe(owner, observer);
try {
hook(observer);
} catch (Exception e) {
e.printStackTrace();
}
}
@Override
public void observeForever(@NonNull Observer<T> observer) {
if (!observerMap.containsKey(observer)) {
observerMap.put(observer, new ObserverWrapper(observer));
}
super.observeForever(observerMap.get(observer));
}
@Override
public void removeObserver(@NonNull Observer<T> observer) {
Observer realObserver = null;
if (observerMap.containsKey(observer)) {
realObserver = observerMap.remove(observer);
} else {
realObserver = observer;
}
super.removeObserver(realObserver);
}
private void hook(@NonNull Observer<T> observer) throws Exception {
//get wrapper's version
Class<LiveData> classLiveData = LiveData.class;
Field fieldObservers = classLiveData.getDeclaredField("mObservers");
fieldObservers.setAccessible(true);
Object objectObservers = fieldObservers.get(this);
Class<?> classObservers = objectObservers.getClass();
Method methodGet = classObservers.getDeclaredMethod("get", Object.class);
methodGet.setAccessible(true);
Object objectWrapperEntry = methodGet.invoke(objectObservers, observer);
Object objectWrapper = null;
if (objectWrapperEntry instanceof Map.Entry) {
objectWrapper = ((Map.Entry) objectWrapperEntry).getValue();
}
if (objectWrapper == null) {
throw new NullPointerException("Wrapper can not be bull!");
}
Class<?> classObserverWrapper = objectWrapper.getClass().getSuperclass();
Field fieldLastVersion = classObserverWrapper.getDeclaredField("mLastVersion");
fieldLastVersion.setAccessible(true);
//get livedata's version
Field fieldVersion = classLiveData.getDeclaredField("mVersion");
fieldVersion.setAccessible(true);
Object objectVersion = fieldVersion.get(this);
//set wrapper's version
fieldLastVersion.set(objectWrapper, objectVersion);
}
}
}
//注册订阅
LiveDataBus.get()
.with("key_test", String.class)
.observe(this, new Observer<String>() {
@Override
public void onChanged(@Nullable String s) {
}
});
//发送消息
LiveDataBus.get().with("key_test").setValue(s);复制代码想了解更多LiveDataBus可以参考下面的开源项目：
https://github.com/JeremyLiao/LiveEventBus复制代码
事件总线的考量
其实目前常用的各种事件总线xxBus原理都差不多，那么在项目中如何使用这些事件总线呢：
EventBus,RxBus: 将xxEvent消息容器和事件总线框架的依赖放到base module，其他模块组件依赖于base module; 但是这样每个模块改动都需要增删改baseModule中的消息容器, 组件化要求功能模块独立, 各组件应该尽量避免影响base module；
LiveDataBus: 无需建立消息模型，但无法想前两者一样拥有类名索引，无法引导正确的编写代码，也无法传递自定义实体到其他模块；
使用EventBus,RxBus，为了更大程度的解耦，可以独立出一个事件总线module，添加事件的实体都在这个module中，base module依赖 这个事件总线module对事件通信的解耦, 抽离事件到事件总线module中减少对base module的影响；
作者：今阳
链接：https://juejin.cn/post/7020671328193609765