RxJava操作符之Share, Publish, Refcount

原文链接：http://nerds.weddingpartyapp.com/tech/2015/01/21/rxjava-share-publish-refcount-and-all-that-jazz/

看源码知道.share()操作符是.publish().refcount()调用链的包装。

先来看ConnectedObservable

“ConnectedObservable” – This is a kind of observable which doesn’t emit items even if subscribed to.
It only starts emitting items after its .connect() method is called.

因为这个原因，在ConnectedObservable的connect这个方法被调用之前，connected obesrvable也被认为是“冷”和不活跃。

再看publish方法

.publish()– This method allows us to change an ordinary observable into a “ConnectedObservable”.
Simply call this method on an ordinary observable and it becomes a connected one.

现在我们知道了share操作符的1/2,那么为什么需要运用Connected Observable这个操作符呢？文档上是这么写的：

In this way you can wait for all intended Subscribers to subscribe to the Observable before the Observable begins emitting items.

这就意味着publish可以调用多个subscriber。当你有超过一个订阅者的时候，处理每个订阅和正确的销毁他们变得棘手。 为了使这个更方便，Rx发明了这个魔法操作符refcount():

refcount() – This operator keeps track of how many subscribers are subscribed to the resulting Observable and
refrains from disconnecting from the source ConnectedObservable until all such Observables are unsubscribed.

refcount本质上在后台维护着一个引用计数器，当一个subscription需要取消订阅或者销毁的时候，发出一个正确的动作。

我们再次看一下debouncedBuffer的例子，看一下在哪，share是怎么用的。

Observable<Object> tapEventEmitter = _rxBus.toObserverable().share();
// which is really the same as:
Observable<Object> tapEventEmitter = _rxBus.toObserverable().publish().refcount();

我们现在有了一个"shareable"的名字叫"tapEventEmitter"的observable。 因为他是可以分享的，而且还不是“live”(share操作符中的publish调用使其变成一个ConnectedObservable)， 我们可以用他构成我们的Observables，而且要确保我们有了一个原始的observable的引用 （这个例子中原始的observable是_rxBus.toObserverable()）.

Observable<Object> tapEventEmitter = _rxBus.toObserverable().share();
Observable<Object> debouncedEventEmitter = tapEventEmitter.debounce(1, TimeUnit.SECONDS);
tapEventEmitter.buffer(debouncedEventEmitter)
//...

所有的这一切看起来都很好。然而这个实现会有一个可能的竞争条件。因为这有两个subscribers(debounce and buffer)而且会在不同的时间点发生,所以竞争条件就会发生。 记住RxBus是由hot/live Subject支持的不断的发射数据。通过使用share操作符，我们保证引用的是同一个资源。 而不是subscribers在不同的时间点订阅，他们会收到准确的相同的数据。

The race condition is when the two consumers subscribe. Often on a hot stream it doesn’t matter when subscribers come and go,and refCount is perfect for that.
The race condition refCount protects against is having only 1 active subscription upstream. However,if 2 Subscribers subscribe to a refcounted stream that emits 1, 2, 3, 4, 5, the first may get 1, 2, 3, 4, 5 and the second may get 2, 3, 4, 5.

To ensure all subscribers start at exactly the same time and get the exact same values, refCount can not be used.
Either ConnectableObservable with a manual, imperative invocation of connect needs to be done, or the variant of publish(function)which connects everything within the function before connecting the upstream.

在我们的用法中几乎立即执行所以没有什么关系。但是我们原始的意图是把debouncedBuffer方法作为一个单独的操作符。 如果相同的事件没有被发射出去，从概念上看起来是不正确的。

通过Bean后来的建议，我添加了一个更好的第三方的实现，用来处理这种竞争条件。

// don't start emitting items just yet by turning the observable to a connected one
ConnectableObservable<Object> tapEventEmitter = _rxBus.toObserverable().publish();

tapEventEmitter.publish((Func1) (stream) -> {

// inside `publish`, "stream" is truly multicasted

// applying the same technique for getting a debounced buffer sequence
    return stream.buffer(stream.debounce(1, TimeUnit.SECONDS));

}).subscribe((Action1) (taps) {
_showTapCount(taps.size());
});

// start listening to events now
tapEventEmitter.connect();