gistfile1.m

    __block __typeof(self) weakSelf = self;
    __block int tick = 0;
    id<RACSignal> timer = [[[[RACSignal interval:1.0] startWithTrigger:self.startButton.rac_command] takeUntil:self.stopButton.rac_command] repeat];
    [timer subscribeNext:^(id x) {
        weakSelf.timerLabel.stringValue = [NSString stringWithFormat:@"%d", tick++];
    }];

RACSignalProtocol.m

- (id<RACSignal>)startWithTrigger:(id<RACSignal>)signalTrigger
{
    return [RACSignal createSignal:^(id<RACSubscriber> subscriber) {
        __block RACDisposable *selfDisposable = nil;
        __block RACDisposable *triggerDisposable = [signalTrigger subscribe:[RACSubscriber subscriberWithNext:^(id x) {
            selfDisposable = [self subscribeNext:^(id x2) {
                [subscriber sendNext:x2];
            } error:^(NSError *error) {
                [subscriber sendError:error];
            } completed:^{
                [subscriber sendCompleted];
            }];
        } error:^(NSError *error) {

        } completed:^{

        }]];

        return [RACDisposable disposableWithBlock:^{
            [triggerDisposable dispose];
            [selfDisposable dispose];
        }];
    }];
}