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How to run code at a specific time

Swift version: 5.2

Paul Hudson    @twostraws   

You can use perform(_:with:afterDelay:) to run a method after a certain number of seconds have passed, but if you want to run code at a specific time – say at exactly 4pm – then you should use Timer instead. This class is great for executing code repeatedly at a specific time interval, but it's also great for running code at an exact time that you specify.

This is accomplished using a Timer constructor that accepts an Date for when the timer should fire. You can make this date however you want, which is what makes this approach so flexible.

As a simple example, this will create a timer that calls a runCode() method in five seconds:

let date = Date().addingTimeInterval(5)
let timer = Timer(fireAt: date, interval: 0, target: self, selector: #selector(runCode), userInfo: nil, repeats: false)
RunLoop.main.add(timer, forMode: .common)

Notice how you can specify a interval parameter? That's for when you set repeats to be true. If you have a Date 5 seconds from now and an interval of 1 (after setting repeat to be true!), it means "call runCode() after five seconds, then every one second after that."

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