Android技术前沿：RxBus的实践

RxBus的简单和强大是基于RxJava技术的，RxJava天生就是类似sub/pub的观察者模式，而且很容易处理线程切换。这就使得RxBus天然的支持事件总线。本文基于对RxJava的理解，自定义RxBus，并在目前一个相对轻量级的应用中得到了验证。

自定义RxBus

RxJava中有一种实体对象叫做Subject，官方的解释是“A Subject is a sort of bridge or proxy that is available in some implementations of ReactiveX that acts both as an observer and as an Observable. Because it is an observer, it can subscribe to one or more Observables, and because it is an Observable, it can pass through the items it observes by reemitting them, and it can also emit new items.” 这句话的意思就是Subject既是observer（观察者） 也是 Observable（订阅者），作为observer，它可以订阅多个Observable并观察之；作为Observable，它可以被订阅，并抛出事件。

按照不同的需求，Subject被设计有四种，分别为：AsyncSubject、BehaviorSubject、PublishSubject和ReplaySubject。这四种Subject的说明可查阅 http://reactivex.io/documentation/subject.html。
这里主要讲述PublishSubject。

PublishSubject可以实现从哪里订阅就从哪里开始发送数据，这就保证了事件的订阅和接收的有序。示意图如下：

错误发生，事件传递中断

public class RxBus {
  private static final String TAG = "RxBus";

  public RxBus() {
  }

  private ConcurrentHashMap> subjectMapper = new ConcurrentHashMap<>();

  public  Observable register(@NonNull Object tag) {
    List subjectList = subjectMapper.get(tag);
    if (null == subjectList) {
      subjectList = new ArrayList<>();
      subjectMapper.put(tag, subjectList);
    }

    Subject subject = PublishSubject.create();
    subjectList.add(subject);
    Logger.t(TAG).i("[register]subjectMapper: %s", subjectMapper);
    return subject;
  }

  @SuppressWarnings("unchecked")
  public void unregister(@NonNull Object tag, @NonNull Observable observable) {
    List subjects = subjectMapper.get(tag);
    if (null != subjects) {
      subjects.remove(observable);
      if (subjects.size() == 0) {
        subjectMapper.remove(tag);
      }
    }
    Logger.t(TAG).i("[unregister]subjectMapper: %s", subjectMapper);
  }

  @SuppressWarnings("unchecked")
  public void post(@NonNull Object tag, @NonNull Object content) {
    List subjectList = subjectMapper.get(tag);

    if (subjectList != null) {
      for (Subject subject : subjectList) {
        subject.onNext(content);
      }
    }
    Logger.t(TAG).i("[send]subjectMapper: %s", subjectMapper);
  }
}

代码解读：ConcurrentHashMap> subjectMapper 保存了所有的regist的 observer，每次regist就会PublishSubject.create()并存到该Map中；post事件之后，check subjectMapper是否有对应的observer，有则取出执行onNext()方法。

使用：

public class TestFragment extends BaseFragment {
  @Inject
  RxBus rxBus;
  private Observable