Perform action on the hour, every hour, with ReactiveCocoa - ios

Question

Trying to follow the best practices of ReactiveCocoa to update my UI on the hour, every hour. This is what I've got:
NSDateComponents *components = [[[NSCalendar sharedCalendar] calendar] components:NSMinuteCalendarUnit fromDate:[NSDate date]];
// Generalization, I know (not every hour has 60 minutes, but bear with me).
NSInteger minutesToNextHour = 60 - components.minute;
RACSubject *updateEventSignal = [RACSubject subject];
[updateEventSignal subscribeNext:^(NSDate *now) {
// Update some UI
}];
[[[RACSignal interval:(60 * minutesToNextHour)] take:1] subscribeNext:^(id x) {
[updateEventSignal sendNext:x];
[[RACSignal interval:3600] subscribeNext:^(id x) {
[updateEventSignal sendNext:x];
}];
}];
This has some obvious flaws: manual subscription and sending, and it just "feels wrong." Any ideas on how to make this more "reactive"?

Answer 1

You can do this using completely vanilla operators. It's just a matter of chaining the two intervals together while still passing through both of their values, which is exactly what -concat: does.
I would rewrite the subject as follows:
RACSignal *updateEventSignal = [[[RACSignal
interval:(60 * minutesToNextHour)]
take:1]
concat:[RACSignal interval:3600]];
This may not give you super ultra exact precision (because there might be a minuscule hiccup between the two signals), but it should be Good Enough™ for any UI work.

Answer 2

Sounds like you need something like +interval:startingIn:.
With that thought, you could make your own version of +interval:startingIn: by slightly tweaking the implementation of +interval:.
+ (RACSignal *)interval:(NSTimeInterval)interval startingIn:(NSTimeInterval)delay {
return [[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
int64_t intervalInNanoSecs = (int64_t)(interval * NSEC_PER_SEC);
int64_t delayInNanoSecs = (int64_t)(delay * NSEC_PER_SEC);
dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0));
dispatch_source_set_timer(timer, dispatch_time(DISPATCH_TIME_NOW, delayInNanoSecs), (uint64_t)intervalInNanoSecs, (uint64_t)0);
dispatch_source_set_event_handler(timer, ^{
[subscriber sendNext:[NSDate date]];
});
dispatch_resume(timer);
return [RACDisposable disposableWithBlock:^{
dispatch_source_cancel(timer);
dispatch_release(timer);
}];
}] setNameWithFormat:#"+interval: %f startingIn: %f", (double)interval, (double)delay];
}
With this in place, your code could be refactored to:
NSDateComponents *components = [[[NSCalendar sharedCalendar] calendar] components:NSMinuteCalendarUnit fromDate:[NSDate date]];
// Generalization, I know (not every hour has 60 minutes, but bear with me).
NSInteger minutesToNextHour = 60 - components.minute;
RACSubject *updateEventSignal = [[RACSignal interval:3600 startingIn:(minutesToNextHour * 60)];
[updateEventSignal subscribeNext:^(NSDate *now) {
// Update some UI
}];