gistfile1.m

self.startButton.rac_command = [RACCommand command];
self.stopButton.rac_command = [RACCommand command];

__unsafe_unretained id weakSelf = self;
id<RACSignal> tick = [[[[self.startButton.rac_command
    map:^(id _) {
        RTAFirstView *strongSelf = weakSelf;
        // Map each start button click to a new signal that fires every second
        // and stops when the stop button is clicked.
        return [[RACSignal interval:1] takeUntil:strongSelf.stopButton.rac_command];
    }]
    // At this point we have a signal of signals. We want to just use the signal
    // from the latest click.
    switch]
    // Increment a counter each time the signal fires.
    scanWithStart:@0 combine:^(NSNumber *previous, id _) {
        return @(previous.unsignedIntegerValue + 1);
    }]
    // Map the count to a string version of the count.
    map:^(NSNumber *tick) {
        return [NSString stringWithFormat:@"%lu", (unsigned long)tick.unsignedIntegerValue];
    }];

// The timer label's string value should always be the latest value yield by the
// tick signal.
RAC(self.timerLabel.stringValue) = tick;