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swiftswift3closures

What does the expression () -> () = {} mean in Swift?

I'm going over some Swift code and I've encounter this function signature:

func foo(withCompletion completion: @escaping () -> () = {}) { ... }

I'm not sure what does the part () -> () = {} mean? And if it's a default value, how should it be used?

Any idea?

The code is in Swift 3

Solution

  • The completion argument has a type of () -> (). That's a closure that has no parameters and has an empty (void) return type.

    The = {} is the default value for the parameter meaning that you don't actually need to pass in a closure if you don't need one.

    So you can call this as:

    foo(withCompletion: {
    // your code here
})

    or (using trailing closure syntax):

    foo() {
    // your code here
}

    or (if you don't want to use the completion closure):

    foo()