+ (RACSignal *)createSignal:(RACDisposable *( ^ ) ( id<RACSubscriber> subscriber ))didSubscribe
Events can be sent to new subscribers immediately in the
didSubscribe block, but the subscriber will not be able to dispose of the signal until a RACDisposable is returned from
didSubscribe. In the case of infinite signals, this won’t ever happen if events are sent immediately.
To ensure that the signal is disposable, events can be scheduled on the [RACScheduler currentScheduler] (so that they’re deferred, not sent immediately), or they can be sent in the background. The RACDisposable returned by the
didSubscribe block should cancel any such scheduling or asynchronous work.
didSubscribe - Called when the signal is subscribed to. The new subscriber is passed in. You can then manually control the RACSubscriber by sending it -sendNext:, -sendError:, and -sendCompleted, as defined by the operation you’re implementing. This block should return a RACDisposable which cancels any ongoing work triggered by the subscription, and cleans up any resources or disposables created as part of it. When the disposable is disposed of, the signal must not send any more events to the
subscriber. If no cleanup is necessary, return nil.
didSubscribe block is called every time a new subscriber subscribes. Any side effects within the block will thus execute once for each subscription, not necessarily on one thread, and possibly even simultaneously!