How to delay execution of code using the defer keyword

Swift version: 5.6

Paul Hudson @twostraws May 28th 2019

The defer keyword is new in Swift 2 and lets you schedule some code to be run at a later date. That later date is when your code exits its current scope, which might be when a function returns or at the end of a loop, for example.

If you've used other programming languages, defer will seem similar to try/finally. Any code you defer will run no matter what, even if you throw an exception.

In the example code below, the closeFile() function will get called no matter how the writeLog() function ends:

func writeLog() {
    let file = openFile()
    defer { closeFile(file) }

    let hardwareStatus = fetchHardwareStatus()
    guard hardwareStatus != "disaster" else { return }
    file.write(hardwareStatus)

    let softwareStatus = fetchSoftwareStatus()
    guard softwareStatus != "disaster" else { return }
    file.write(softwareStatus)

    let networkStatus = fetchNetworkStatus()
    guard networkStatus != "disaster" else { return }
    file.write(networkStatus)
}