ReactiveCocoa Design Patterns Tweak a single line into justifying

Auto-updating relative timestamp

April 14, 2014

This first very practical example in this blog on how to user ReactiveCocoa in real world. The idea is simple: we have a label somewhere in UI showing the date timestamp. First and simplest coming to mind is simply format the date. We saw this example in first post related to ViewModel. Like this:

RAC(self, dateString) = [RACObserve(self, map:^id (NSDate *date) {
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd"];

    return [dateFormatter stringFromDate:date];

Auto-updating label

Now imagine that we want to represent timestamp in relative manner like '5d ago' or '5m ago'. Or '5s ago'. Obviously, there's nothing wrong with days or minutes, but showing seconds is tricky. Once next second ticks, you value '5s ago' becomes invalid, as it's actually '6s ago' now and so on. You need to put this update on timer.

With ReactiveCocoa and FRP you recall, that we deal with signals and we can construct signal that sends new value every time current becomes invalid. Here's how I do this:

RAC(self, timestamp) = [[RACObserve(self, model.timestamp) map:^id(NSDate *value) {
            return [[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
                NSUInteger days, hours, minutes, seconds;
                absoluteTimeUnitsSinceDate(value, &days, &hours, &minutes, &seconds);
                NSUInteger updateAfter = NSNotFound;
                NSString *currentValue = nil;
                if (days) {
                    currentValue = [NSString stringWithFormat:@"%ud", (unsigned int)days];
                else if (hours) {
                    currentValue = [NSString stringWithFormat:@"%uh", (unsigned int)hours];
                else if (minutes) {
                    currentValue = [NSString stringWithFormat:@"%um", (unsigned int)minutes];
                    updateAfter = 60;
                else {
                    if (seconds < 5.0) {
                        currentValue = @"just now";
                    else {
                        currentValue = [NSString stringWithFormat:@"%us", (unsigned int)seconds];
                    updateAfter = 1;

                [subscriber sendNext:currentValue];
                RACSerialDisposable *disposable = [RACSerialDisposable serialDisposableWithDisposable:nil];
                if (updateAfter != NSNotFound) {
                    disposable.disposable = [[RACScheduler currentScheduler] afterDelay:updateAfter schedule:^{
                        [subscriber sendCompleted];
                return disposable;
            }] repeat];

        }] switchToLatest];

This may me hard to get from the first sight. Let me explain what's going on there. The function absoluteTimeUnitsSinceDate returns absolute number of seconds, minutes and hours and days from passed from the given day and current time. The main idea of this stucture is that we create signal that sends current value and when it's time to update it completes by scheduling that code in current scheduler. Once signal completes, we reconnect to it immediately — that's was repeat does. Upon reconnection, or resubscribing, signal will calculate and send current value to that new subscription, and depending on the updateAfter will schedule another delayed complete-and-update.

At the end of the chain there is -switchToLatest call. We will be using this often. Basically, it's one of the ways to flatten signal. In fact, combination -map: + -switchToLatest and flattenMap: are very similar and can be confused. We'll learn what the difference between them is later, in the meantime I'll explain what switchToLatest does: it applies to the signal of signals (e.g. to the signal, all the values passed through which are other signals) and once getting new signal, it subscribes it's subscriber to that signal, up until new signals arrives. In other words, it switches it's own subscriber between inner signals. In our case, for each incoming timestamp we generate signal and switchToLatest connects to that signal. When new timestamps arrives, switchToLatest will abandon existing internal signal, generate new one for new timestamp and subscribe to it.

I assume, relation we coded is happened in ViewModel, and thus, ViewModel has property timestamp and you can then bind UI's label text property ViewModel's timestamp property:

RAC(self, label.text) = RACObserve(self, viewModel.timestamp);

That's it. It will look like this:

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