ReactiveCocoa stopwatch with reset

gistfile1.m

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

static const CGFloat interval = 0.01;
__unsafe_unretained id weakSelf = self;
// Signal id -> Signal Signal Number
// Map each click of the start button to a signal that fires at our interval
// and stops when the stop button's clicked.
id<RACSignal> start = [self.startButton.rac_command map:^(id _) {
    RTAFirstView *strongSelf = weakSelf;
    return [[[RACSignal
        interval:interval]
        mapReplace:@1]
        takeUntil:strongSelf.stopButton.rac_command];
}];

// Signal id -> Signal Signal Number
// Map each click of the reset button to a signal that simply sends a 0.
id<RACSignal> reset = [self.resetButton.rac_command map:^(id _) {
    return [RACSignal return:@0];
}];

// [Signal Signal Number] -> Signal String
id<RACSignal> tick = [[[[RACSignal
    // By merging start and reset, we'll get the most recent signal to yield
    // a value. Then we'll switch to use the latest signal.
    merge:@[ start, reset ]]
    switch]
    // Increment the tick count and multiply it by the multiplier our
    // current signal has sent us.
    scanWithStart:@0 combine:^(NSNumber *previous, NSNumber *multiplier) {
        return @((previous.doubleValue + interval) * multiplier.doubleValue);
    }]
    // Map the ticket to a string representation.
    map:^(NSNumber *tick) {
        return [NSString stringWithFormat:@"%.2f", (double)tick.doubleValue];
    }];

RAC(self.timerLabel.stringValue) = tick;