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Timers in Swift

In Swift, timers are used to create repeating tasks to schedule a work with a delay. It is a class which is formerly known as NSTimer. Swift's timer class provides a flexible way to schedule work to happen in future either once or repeatedly.

Let's see how to create repeating and non-repeating timers, using run loops, keeping track of timers, and how you can reduce their energy and power impact.

Creating a Repeating Timer

We can create and start a repeating counter by using the following syntax:


Let's see an example to demonstrate how to create a repeating counter:


In the above example,

  • A timer is created using the Timer.scheduledTimer(...) class method. The return value of this method is assigned to the constant timer. This constant now contains a reference to the timer, which will be used later.
  • The parameters of scheduledTimer() are the timer interval of 1 second. It uses a mechanism known as target-action, some userInfo that's set to nil, and the parameter repeats set to true.
  • We've also coded a function fire(). This is the function that's called when the timer fires, i.e. roughly every second. By setting target to self and selector to #selector(fire) you're indicating that whenever the timer fires, the function fire() of self needs to be called.

Parameter explanation

In this example, 5 parameters are used to create a timer.

  • timeInterval: It specifies the interval between timer fires in seconds, type is Double.
  • target: It specifies a class instance that the function for selector should be called on
  • selector: It specifies the function to call when the timer fires, with #selector(...)
  • userInfo: It specifies a dictionary with data that's provided to the selector, or nil.
  • repeats: It specifies whether this timer is repeating or non-repeating.

Creating a Nonrepeating Timer

To create a nonrepeating timer, you have to just set the repeats parameter to false. The timer will only fire once and immediately invalidate itself after.


Note: The above code must run in a class context, for example in a view controller class. The fire() function is part of the class, and self refers to the current class instance.

Create a timer using Closure

In the above code, the last parameter block takes a closure. The closure has one parameter timer itself.

Here, the @objc attribute is used because it makes the fire() function available in Objective-C. The Timer class is part of the Objective-C runtime that is the reason we use that @objc attribute.

Difference between Repeating and Nonrepeating Timers

You have to specify whether a timer is repeating or nonrepeating at the time of creation. The main difference between repeating and nonrepeating timer is:

A nonrepeating timer fires once and then invalidates itself automatically, so, it prevents the timer from firing again.

A repeating timer fires and then reschedules itself on the same run loop. A repeating timer always schedules itself based on the scheduled firing time, as opposed to the actual firing time.

For example, if a timer is scheduled to fire at a specific time and every 10 seconds after that, the scheduled firing time will always fall on the original 10-second time intervals, even if the actual firing time gets delayed.

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