swift - defer - lovelejess

Defer

Uncommonly used, but typically used to execute code withinin a function everytime it exits.

For example the function below will attempt to write “Hello World” to the file. Whether or not this function fails, will still print “Exiting.”

func writeHelloWorldToFile() {
    defer { print("Exiting." }

    let contents = "Hello World"
    let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
    let filename = path.appendingPathComponent("helloWorld.txt")

    do {
        try str.write(to: filename, atomically: true, encoding: String.Encoding.utf8)
    } catch {
        // Catch Error
    }
}

Ordering of Defer

If there are multiple usages of defer, the order is important. The defer statements are executed in reverse order of appearance, similar to how a stack works. (Last In First Out)

func printNumbers() {
    defer { print("1" }
    defer { print("2" }
    defer { print("3" }

    print("A")
}

printNumbers() // prints "A" "3" "2" "1"