CODEWITHSUNDEEP
swiftclosures
Fred Faust
Fred Faust

Reputation: 6800

Method parameters in nested closure

I am trying to understand how parameters passed to a method are available to nested closures. I'm nervous that something I wrote won't always have the original parameters available.

(these are drastically simplified examples)

I have a method that I wrote that specifies a closure as a parameter:

func saveNameAndAgeToServer(serverParams: [String:String], completionHandler: (age: NSNumber) -> ()) {

    // Connect to a server
    // POST a name and dob from serverParams
    // Receives a current age in completion:

    completionHandler(age: 22)
}

Now somewhere else I create another method, that also specifies a closure, takes two parameters and calls the first function:

func AwesomeFunc(name: String, dob: NSDate, completionHandler: (isOverTwentyOne: Bool) -> ()) {

    let formattedDob = NSDateFormatter().stringFromDate(dob)

    saveNameAndAgeToServer([name:formattedDob]) { (age) -> () in

        if (age as Int) >= 21 {

            print("\(name) can have a beer.")
            completionHandler(isOverTwentyOne: true)

        } else {

            print("\(name) is too young to drink, he can have a water.")
            completionHandler(isOverTwentyOne: false)
        }
    }

}

Am I able to guarantee that the parameters (name and dob) passed into this latter function will always be available?

What I'm trying to ask is will the memory that the saveNameAndAgeToServer closure runs within always have the parameters of AwesomeFunc available to it? I'm pretty sure the function is all being held while whatever it calls is completed but would love a 2nd opinion.

Upvotes: 0

Views: 1502

Answers (1)

Kevin DiTraglia
Kevin DiTraglia

Reputation: 26078

You are correct, captured variables will last for the life of the closure. Here's an exert on capturing variables from apple's swift documentation:

A closure can capture constants and variables from the surrounding context in which it is defined. The closure can then refer to and modify the values of those constants and variables from within its body, even if the original scope that defined the constants and variables no longer exists.

In Swift, the simplest form of a closure that can capture values is a nested function, written within the body of another function. A nested function can capture any of its outer function’s arguments and can also capture any constants and variables defined within the outer function.

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html

Upvotes: 3

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