How to trigger Timer.publish() right away?

Question

I created a timer via combine which emits Date and ignores errors using this code:

let timer: AnyPublisher<Date, Never> = Timer.publish(every: 5, on: .main, in: RunLoop.Mode.common)
  .autoconnect()
  .map { _ in Date() }
  .replaceError(with: Date())
  .eraseToAnyPublisher()

(I'm sure there are better ways than mapping and replacing the error, but for this example, I wanted to keep the type simple, AnyPublisher<Date, Never>.)

The timer fires correctly, but there is a delay between when the timer is created to when it fires the first time (i.e. it waits 5 seconds). With NSTimer, we can invoke timer.fire() to force it to fire immediately.

Is there an equivalent way to force a timer to post immediately when using Timer.publish()?

Alternatively, is there a way to merge Just(Date()) with the above Timer.publish so that it fires immediately and every 5 seconds, while still keeping the AnyPublisher<Date, Never> type?

score 12 · Accepted Answer · answered Jul 07 '20 at 08:29

I'm sure there are better ways than mapping and replacing the error

Timer.TimerPublisher.Failure is Never, so you don't need any mapping because it can't fail.

Furthermore, Timer.TimerPublisher.Output is already the current Date(), so you don't need to map the output either.

To emit the current date immediately on subscription, you want to combine a Deferred { Just(Date()) } with the timer publisher. The Deferred means the current date won't be computed until the subscription happens.

Here is one way to put it all together:

let timer = Deferred { Just(Date()) }
    .append(Timer.publish(every: 5, on: .main, in: .common).autoconnect())
    .eraseToAnyPublisher()

Great point about `Deferred`. Also for other scenarios, it seems like `Publishers.Merge` is another solution, but `append` sounds more accurate for this situation. — Senseful, Jul 07 '20 at 14:38

score 10 · Answer 2 · answered May 31 '22 at 09:12

10

Also, there is an option using prepend

let delayPublisher = Timer.publish(every: 5, on: .main, in: .common)
  .autoconnect()
  .receive(on: RunLoop.main)
  .prepend(Date())

answered May 31 '22 at 09:12

Đỗ Long Thành

369
2
7

3

I think simplest solution so far, not need to merge with another publisher when you can just add an initial value – Daniel Fernandez Y May 31 '22 at 09:15

score 6 · Answer 3 · answered Jan 11 '21 at 16:03

6

Alternative to @rob mayoff answer and @Senseful comment, it's possible to merge the "first value" of the timer doing:

let delayPublisher = Timer.publish(every: 5, on: .main, in: .common)
  .autoconnect()
  .receive(on: RunLoop.main)
  .merge(with: Just(Date()))

answered Jan 11 '21 at 16:03

Martin Romañuk

1,039
10
14

Far more simpler alternative. +1. – Ruiz Jun 08 '21 at 21:04

How to trigger Timer.publish() right away?

3 Answers3