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Combine: Asynchronous Programming With Swift

Fourth Edition · iOS 16 · Swift 5.8 · Xcode 14

11. Timers
Written by Florent Pillet

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Repeating and non-repeating timers are always useful when coding. Besides executing code asynchronously, you often need to control when and how often a task should repeat.

Before the Dispatch framework was available, developers relied on RunLoop to asynchronously perform tasks and implement concurrency. You could use Timer to create repeating and non-repeating timers. Then, Apple released the Dispatch framework, including DispatchSourceTimer.

Although all of the above are capable of creating timers, not all timers are equal in Combine. Read on!

Using RunLoop

The main thread and any thread you create, preferably using the Thread class, can have its own RunLoop. Just invoke RunLoop.current from the current thread: Foundation would create one for you if needed. Beware, unless you understand how run loops operate — in particular, that you need a thread that runs the run loop — you’ll be better off simply using the main RunLoop that runs the main thread of your application.

Note: One important note and a red light warning in Apple’s documentation is that the RunLoop class is not thread-safe. You should only call RunLoop methods for the run loop of the current thread.

RunLoop implements the Scheduler protocol you’ll learn about in Chapter 17, “Schedulers.” It defines several methods which are relatively low-level, and the only one that lets you create cancellable timers:

let runLoop = RunLoop.main

let subscription = runLoop.schedule(
  interval: .seconds(1),
  tolerance: .milliseconds(100)
) {
  print("Timer fired")

This timer does not pass any value and does not create a publisher. It starts at the date specified in the after: parameter with the specified interval and tolerance, and that’s about it. Its only usefulness in relation to Combine is that the Cancellable it returns lets you stop the timer after a while.

An example of this could be:

runLoop.schedule(after: .init(Date(timeIntervalSinceNow: 3.0))) {

But all things considered, RunLoop is not the best way to create a timer. You’ll be better off using the Timer class!

Using the Timer Class

Timer is the oldest timer that was available in the original Mac OS X, long before Apple renamed it “macOS.” It has always been tricky to use because of its delegation pattern and tight relationship with RunLoop. Combine brings a modern variant you can directly use as a publisher without all the setup boilerplate.

let publisher = Timer.publish(every: 1.0, on: .main, in: .common)
let publisher = Timer.publish(every: 1.0, on: .current, in: .common)
let publisher = Timer
  .publish(every: 1.0, on: .main, in: .common)
let subscription = Timer
  .publish(every: 1.0, on: .main, in: .common)
  .scan(0) { counter, _ in counter + 1 }
  .sink { counter in
    print("Counter is \(counter)")

Using DispatchQueue

You can use a dispatch queue to generate timer events. While the Dispatch framework has a DispatchTimerSource event source, Combine surprisingly doesn’t provide a timer interface to it. Instead, you’re going to use an alternative method to generate timer events in your queue.

let queue = DispatchQueue.main

// 1
let source = PassthroughSubject<Int, Never>()

// 2
var counter = 0

// 3
let cancellable = queue.schedule(
  interval: .seconds(1)
) {
  counter += 1

// 4
let subscription = source.sink {
  print("Timer emitted \($0)")

Key Points

  • Create timers using good old RunLoop class if you have Objective-C code nostalgia.
  • Use Timer.publish to obtain a publisher which generates values at given intervals on the specified RunLoop.
  • Use DispatchQueue.schedule for modern timers emitting events on a dispatch queue.

Where to Go From Here?

In Chapter 18, “Custom Publishers & Handling Backpressure,” you’ll learn how to write your own publishers, and you’ll create an alternative timer publisher using DispatchSourceTimer.

Have a technical question? Want to report a bug? You can ask questions and report bugs to the book authors in our official book forum here.
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