Functions With Closures

Using functions that take in and execute closures can be extremely useful for sending a block of code to be executed elsewhere. We can start by allowing our function to take in an optional closure that will (in this case) return Void.

func closedFunc(block: (()->Void)? = nil) {
    print("Just beginning")

    if let block = block {
        block()
    }
}

Now that our function has been defined, let’s call it and pass in some code:

closedFunc() { Void in
    print("Over already")
}

By using a trailing closure with our function call, we can pass in code (in this case, print) to be executed at some point within our closedFunc() function.

The log should print:

Just beginning

Over already

A more specific use case of this could include the execution of code between two classes:

class ViewController: UIViewController {

    override func viewDidLoad() {
        let _  = A.init(){Void in self.action(2)}
    }

    func action(i: Int) {
        print(i)
    }
}

class A: NSObject {
    var closure : ()?

    init(closure: (()->Void)? = nil) {
        // Notice how this is executed before the  closure
        print("1")
        // Make sure closure isn't nil
        self.closure = closure?()
    }
}

The log should print:

1

2