2019独角兽企业重金招聘Python工程师标准>>> 
##写在前面
###1.前言 曾经，一层又一层的业务逻辑让我不知所措，一个又一个的回调让你头晕眼花，一个又一个的参数让你混乱不堪。EventBus，，一个耦合度低到令你害怕的框架。 ###2.什么是EventBus EventBus是一个消息总线，以观察者模式实现，用于简化程序的组件、线程通信，可以轻易切换线程、开辟线程。EventBus3.0跟先前版本的区别在于加入了annotation @Subscribe，取代了以前约定命名的方式。 ###3.相似产品比较
| 产品名 | 开发者 | 备注 |
|---|---|---|
| EventBus | greenrobot | 用户最多，简洁，方便，小巧，文档简洁明了 |
| Guava | 一个庞大的工具类库，EventBus只是一个小功能 | |
| otto | square | fork guava ，用的人不少 |
| AndroidEventBus | 何红辉 | 模仿EventBus开发的 |
##使用EventBus3.0三部曲
###1.定义事件
public class MessageEvent {
public final String message;
public MessageEvent(String message) {
this.message = message;
}
}
###2.准备订阅者
// This method will be called when a MessageEvent is posted
@Subscribe
public void onMessageEvent(MessageEvent event){
Toast.makeText(getActivity(), event.message, Toast.LENGTH_SHORT).show();
}
// This method will be called when a SomeOtherEvent is posted
@Subscribe
public void handleSomethingElse(SomeOtherEvent event){
doSomethingWith(event);
}
@Override
public void onStart() {
super.onStart();
EventBus.getDefault().register(this);
}
@Override
public void onStop() {
EventBus.getDefault().unregister(this);
super.onStop();
}
###3.发送事件
EventBus.getDefault().post(new MessageEvent("Hello everyone!"));
##深入了解 ###1.ThreadMode线程通信 EventBus可以很简单的实现线程间的切换，包括后台线程、UI线程、异步线程 ####ThreadMode.POSTING
//默认调用方式，在调用post方法的线程执行，避免了线程切换，性能开销最少
// Called in the same thread (default)
@Subscribe(threadMode = ThreadMode.POSTING) // ThreadMode is optional here
public void onMessage(MessageEvent event) {
log(event.message);
}
####ThreadMode.MAIN
// Called in Android UI's main thread
@Subscribe(threadMode = ThreadMode.MAIN)
public void onMessage(MessageEvent event) {
textField.setText(event.message);
}
####ThreadMode.BACKGROUND
// 如果调用post方法的线程不是主线程，则直接在该线程执行
// 如果是主线程，则切换到后台单例线程，多个方法公用同个后台线程，按顺序执行，避免耗时操作
// Called in the background thread
@Subscribe(threadMode = ThreadMode.BACKGROUND)
public void onMessage(MessageEvent event){
saveToDisk(event.message);
}
####ThreadMode.ASYNC
//开辟新独立线程，用来执行耗时操作，例如网络访问
//EventBus内部使用了线程池，但是要尽量避免大量长时间运行的异步线程，限制并发线程数量
//可以通过EventBusBuilder修改，默认使用Executors.newCachedThreadPool()
// Called in a separate thread
@Subscribe(threadMode = ThreadMode.ASYNC)
public void onMessage(MessageEvent event){
backend.send(event.message);
}
###2.配置EventBusBuilder EventBus提供了很多配置，一般的情况下我们可以不用配置.但是，如果你有一些其他要求,比如控制日志在开发的时候输出,发布的时候不输出，在开发的时候错误崩溃，而发布的时候不崩溃...等情况。 EventBus提供了一个默认的实现，但不是单例。
EventBus eventBus = new EventBus();
//下面这一条的效果是完全一样的
EventBus eventBus = EventBus.builder().build();
//修改默认实现的配置，记住，必须在第一次EventBus.getDefault()之前配置，且只能设置一次。建议在application.onCreate()调用
EventBus.builder().throwSubscriberException(BuildConfig.DEBUG).installDefaultEventBus();
###3.StickyEvent StickyEvent在内存中保存最新的消息，取消原有消息，执行最新消息，只有在注册后才会执行，如果没有注册，消息会一直保留来内存中
//在注册之前发送消息
EventBus.getDefault().postSticky(new MessageEvent("Hello everyone!"));
//限制，新界面启动了
@Override
public void onStart() {
super.onStart();
EventBus.getDefault().register(this);
}
//在onStart调用register后，执行消息
@Subscribe(sticky = true, threadMode = ThreadMode.MAIN)
public void onEvent(MessageEvent event) {
// UI updates must run on MainThread
textField.setText(event.message);
}
@Override
public void onStop() {
EventBus.getDefault().unregister(this);
super.onStop();
}
你也可以手动管理StickyEvent
MessageEvent stickyEvent = EventBus.getDefault().getStickyEvent(MessageEvent.class);
// Better check that an event was actually posted before
if(stickyEvent != null) {
// "Consume" the sticky event
EventBus.getDefault().removeStickyEvent(stickyEvent);
//or
EventBus.getDefault().removeAllStickyEvents();
// Now do something with it
}
在这里，或许你会有个疑问， StickyEvent=true的订阅者能否接收post的事件？ StickyEvent=false的订阅者能否接收postSticky的事件？ 查看源码发现
/**
* Posts the given event to the event bus and holds on to the event (because it is sticky). The most recent sticky
* event of an event's type is kept in memory for future access by subscribers using {@link Subscribe#sticky()}.
*/
public void postSticky(Object event) {
synchronized (stickyEvents) {
stickyEvents.put(event.getClass(), event);
}
// Should be posted after it is putted, in case the subscriber wants to remove immediately
post(event);
}
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
...省略部分代码
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).
Set, Object>> entries = stickyEvents.entrySet();
for (Map.Entry, 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);
}
}
}
boolean checkAdd(Method method, Class eventType) {
// 2 level check: 1st level with event type only (fast), 2nd level with complete signature when required.
// Usually a subscriber doesn't have methods listening to the same event type.
Object existing = anyMethodByEventType.put(eventType, method);
if (existing == null) {
return true;
} else {
if (existing instanceof Method) {
if (!checkAddWithMethodSignature((Method) existing, eventType)) {
// Paranoia check
throw new IllegalStateException();
}
// Put any non-Method object to "consume" the existing Method
anyMethodByEventType.put(eventType, this);
}
return checkAddWithMethodSignature(method, eventType);
}
}
发现，post方法没有过滤StickyEvent，而postSticky是调用post方法的，所以，无论post还是postSticky，StickyEvent是true或false，都会执行 ###4.priority事件优先级
//priority越大，级别越高
@Subscribe(priority = 1);
public void onEvent(MessageEvent event) {
…
}
//优先级实现方式，遍历当前列表，把当前
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;
}
}
###5.中止事件传递
// 中止事件传递，后续事件不在调用,注意，只能在传递事件的时候调用
@Subscribe
public void onEvent(MessageEvent event){
…
EventBus.getDefault().cancelEventDelivery(event) ;
}
###6.index索引加速 EventBus使用了annotation，默认在编译时生成代码，生成索引， 添加index后会在编译时运行，自动生成相应代码。 ps:由于apt的限制，匿名内部类中的annotation不会被识别，会自动降级在运行时反射，此时，效率会降低
buildscript {
dependencies {
classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'
}
}
apply plugin: 'com.neenbedankt.android-apt'
dependencies {
compile 'org.greenrobot:eventbus:3.0.0'
apt 'org.greenrobot:eventbus-annotation-processor:3.0.1'
}
apt {
arguments {
eventBusIndex "com.example.myapp.MyEventBusIndex"
}
}
EventBus eventBus = EventBus.builder().addIndex(new MyEventBusIndex()).build();
EventBus.builder().addIndex(new MyEventBusIndex()).installDefaultEventBus();
// Now the default instance uses the given index. Use it like this:
EventBus eventBus = EventBus.getDefault();
###7.NoSubscriberEvent 如果没找到订阅者事件，可以通过EventBusBuilder设置是否默认发送NoSubscriberEvent，默认是打开的
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
....
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));
}
}
}
###8.混淆
-keepattributes *Annotation*
-keepclassmembers class ** {
@org.greenrobot.eventbus.Subscribe ;
}
-keep enum org.greenrobot.eventbus.ThreadMode { *; }
# Only required if you use AsyncExecutor
-keepclassmembers class * extends org.greenrobot.eventbus.util.ThrowableFailureEvent {
(java.lang.Throwable);
}
###9.利弊 好处
简单，方便，小巧，文档清晰，性能消耗少，可定制行强，耦合度低
坏处
耦合度太低
这绝对不是个笑话，，EventBus的耦合太低了，如果不加以控制管理，你会不知道，你发的消息到跑哪里去了。也不知道你的这条消息，会在哪里发出。如果你没有很好的方法解决这个问题，建议不好用太多。
##使用建议
###1、EventBus管理 EventBus运行创建多个，那么，明确事件的生命周期，根据不同生命周期使用不同的EventBus？
/**
* 方法1
* 用annotation配合使用工厂
* EventBusFactory.getBus(EventBusFactory.START);
* EventBusFactory.getBus();
*/
public class EventBusFactory {
private static SparseArray mBusSparseArray = new SparseArray<>(2);
@IntDef({CREATE, START})
@Retention(RetentionPolicy.SOURCE)
public @interface BusType {
}
public static final int CREATE = 0;
public static final int START = 1;
static {
mBusSparseArray.put(CREATE, EventBus.builder().build());
mBusSparseArray.put(START, EventBus.getDefault());
}
public static EventBus getBus() {
return getBus(START);
}
public static EventBus getBus(@BusType int type) {
return mBusSparseArray.get(type);
}
}
/**
* 方法2
* 用枚举工厂
* EventBusFactory.START.getBus();
*/
public enum EventBusFactory {
CREATE(0),
START(1);
private int mType;
EventBusFactory(int type) {
mType = type;
}
public EventBus getBus() {
return mBusSparseArray.get(mType);
}
private static SparseArray mBusSparseArray = new SparseArray<>(2);
static {
mBusSparseArray.put(CREATE.mType, EventBus.builder().build());
mBusSparseArray.put(START.mType, EventBus.getDefault());
}
}
###2、以事件为对象 将数据封装到一个事件类。所有事件放到一个包下。如果事件太多，同个模块的事件可以考虑使用静态内部类，或者再分包。
/**
* This Event is posted by EventBus when no subscriber is found for a posted event.
*
* @author Markus
*/
public final class NoSubscriberEvent {
/** The {@link EventBus} instance to with the original event was posted to. */
public final EventBus eventBus;
/** The original event that could not be delivered to any subscriber. */
public final Object originalEvent;
public NoSubscriberEvent(EventBus eventBus, Object originalEvent) {
this.eventBus = eventBus;
this.originalEvent = originalEvent;
}
}
public class Event {
public static class UserListEvent {
public List users ;
}
public static class ItemListEvent {
public List- items;
}
}
注意，不是相同类型就一定要作为一个事件封装，具体需要考虑业务情景跟代码情况，比如事件行为不同、事件生命周期不同，如果有必要，写封装成两个Event可能是更好的选择。
public class Event {
public static class UserListUpdateEventOnCreate {
public List users;
}
public static class UserListUpdateEventOnStart {
public List users ;
}
public static class UserListRemoveEventOnStart {
public List users;
}
}
##参考文献
- EventBus官网地址
- EventBus github地址
- EventBus 3.0的用法详解
##说了这么多废话，，下面进入正题
