ReactiveCocoa: release a RACSignal when no one subscribes it - ios

Question

In my class, there are some values that may be observed by other part of the app. These values is can be read and written to database.
I have a collection that retains some RACSubjects.
When an object need to observe a value V1, I'll create a RACSignal for it, name it S1, and later if any other object is also interested in V1, I'll give it V1 too, so that when S1 changed, I can call [RACSubject sendNext:] to notify objects that is interested in it.
But there is a problem, I don't know when to release S1, since I don't know how to get notified when there is no object subscribing to S1.
Is there any way to do this?

Answer 1

I'm mentally rewording part of your question to the following (my changes italicized), because I think the original phrasing had some typos:
When an object need to observe a value V1, I'll create a RACSubject for it, name it S1, and later if any other object is also interested in V1, I'll give it S1 too, so that when V1 changed, I can call [RACSubject sendNext:] to notify objects that is interested in it.
If this is an incorrect interpretation, ignore this answer.
If you aren't explicitly retaining a signal yourself, ReactiveCocoa will automatically reclaim it when it runs out of subscribers. The relevant excerpt:
A created signal is automatically added to a global set of active signals.
The signal will wait for a single pass of the main run loop, and then remove itself from the active set if it has no subscribers. Unless the signal was retained somehow, it would deallocate at this point.
If something did subscribe in that run loop iteration, the signal stays in the set.
Later, when all the subscribers are gone, step 2 is triggered again.
But there's one problem: this doesn't apply to RACSubjects. They aren't added to the global set of active signals.
However, there's a... workaround you can apply in order to get this nice auto-retain behavior.
RACSignal *autoretainedSignal = [subject map:^(id x) { return x; }]
As long as you only subscribe to the autoretainedSignal, not the underlying RACSubject, you can take advantage of the auto-retain behavior that normal signals get.
So how do you hold onto it without retaining it? If you only have one signal that you're interesting in caching/sharing, you can just store it in a weak property. If there are multiple properties that you're dynamically managing, an NSMapTable with weak storage is your friend.
You'll need to hold onto both the underlying subject (so you can send events on it) and the derived signal (so you can provide it to subscribers) weakly. As long as the derived signal has subscribers, it will keep its underlying subject alive, and as soon as it runs out of subscribers both it and its subject will be deallocated.

Answer 2

I found an idea for you :)
#property (nonatomic,assign) NSInteger countOfSubscribers = 0;
later in your code
RACSignal *s; // here is your target signal
[[s rac_signalForSelector:#selector(subscribeNext:)]
subscribeNext:^(id x) {
self.countOfSubscribers++;
}];
[[[s rac_signalForSelector:#selector(subscribeNext:)] rac_willDeallocSignal]
subscribeNext:^(id x) {
self.countOfSubscribers--;
}];
Main idea is to subscribe to 'subscribeNext:' method, and when original subscriber will be removed - rac_willDeallocSignal
will called.
It's not completely tested snippet, but i hope this can help you to find right direction.