Understanding Cold and Hot Signals in ReactiveCocoa (RAC)

ReactiveCocoa (RAC) is a Functional Reactive Programming (FRP) framework originally created by the GitHub team for Cocoa. FRP expresses user input as time‑varying functions, eliminating mutable state. This article is the first in a series that explains the concepts of cold and hot signals in RAC.

The notions of cold and hot signals come from the .NET Reactive Extensions (Rx). A Hot Observable is active: it pushes events continuously even if no one has subscribed, similar to mouse movement. A Cold Observable is passive: it only starts emitting when a subscriber appears. Hot observables can have multiple subscribers sharing the same data (one‑to‑many), whereas cold observables deliver a separate sequence to each subscriber (one‑to‑one).

Below is a simple RAC example that creates a signal, sends the values @1, @2, @3, and then completes. Two subscribers subscribe at different times.

RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
    [subscriber sendNext:@1];
    [subscriber sendNext:@2];
    [subscriber sendNext:@3];
    [subscriber sendCompleted];
    return nil;
}];
NSLog(@"Signal was created.");
[[RACScheduler mainThreadScheduler] afterDelay:0.1 schedule:^{
    [signal subscribeNext:^(id x) {
        NSLog(@"Subscriber 1 recveive: %@", x);
    }];
}];
[[RACScheduler mainThreadScheduler] afterDelay:1 schedule:^{
    [signal subscribeNext:^(id x) {
        NSLog(@"Subscriber 2 recveive: %@", x);
    }];
}];

Running the program produces:

2015-08-11 18:33:21.681 RACDemos[6505:1125196] Signal was created.
2015-08-11 18:33:21.793 RACDemos[6505:1125196] Subscriber 1 recveive: 1
2015-08-11 18:33:21.793 RACDemos[6505:1125196] Subscriber 1 recveive: 2
2015-08-11 18:33:21.793 RACDemos[6505:1125196] Subscriber 1 recveive: 3
2015-08-11 18:33:22.683 RACDemos[6505:1125196] Subscriber 2 recveive: 1
2015-08-11 18:33:22.683 RACDemos[6505:1125196] Subscriber 2 recveive: 2
2015-08-11 18:33:22.683 RACDemos[6505:1125196] Subscriber 2 recveive: 3

Because the signal is cold, each subscriber receives the full sequence regardless of when it subscribes.

To turn the signal into a hot one, we use publish to obtain a RACMulticastConnection and then connect it:

RACMulticastConnection *connection = [[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
    [[RACScheduler mainThreadScheduler] afterDelay:1 schedule:^{
        [subscriber sendNext:@1];
    }];
    [[RACScheduler mainThreadScheduler] afterDelay:2 schedule:^{
        [subscriber sendNext:@2];
    }];
    [[RACScheduler mainThreadScheduler] afterDelay:3 schedule:^{
        [subscriber sendNext:@3];
    }];
    [[RACScheduler mainThreadScheduler] afterDelay:4 schedule:^{
        [subscriber sendCompleted];
    }];
    return nil;
}] publish];
[connection connect];
RACSignal *signal = connection.signal;
NSLog(@"Signal was created.");
[[RACScheduler mainThreadScheduler] afterDelay:1.1 schedule:^{
    [signal subscribeNext:^(id x) {
        NSLog(@"Subscriber 1 recveive: %@", x);
    }];
}];
[[RACScheduler mainThreadScheduler] afterDelay:2.1 schedule:^{
    [signal subscribeNext:^(id x) {
        NSLog(@"Subscriber 2 recveive: %@", x);
    }];
}];

The output is:

2015-08-12 11:07:49.943 RACDemos[9418:1186344] Signal was created.
2015-08-12 11:07:52.088 RACDemos[9418:1186344] Subscriber 1 recveive: 2
2015-08-12 11:07:53.044 RACDemos[9418:1186344] Subscriber 1 recveive: 3
2015-08-12 11:07:53.044 RACDemos[9418:1186344] Subscriber 2 recveive: 3

This demonstrates the hot‑signal behavior: the signal pushes values as they occur, and subscribers that join later miss earlier values. Summarizing the characteristics:

Hot signals are active ; they emit regardless of subscription, and multiple subscribers share the same stream.

Cold signals are passive ; they start emitting only after a subscription, and each subscriber gets its own independent sequence.

Understanding these differences is essential when choosing the appropriate signal type in RAC. The next article will discuss why the distinction matters.