Swift Closures for Laymen

What are Closures?

In technical jargon, Closures are self-contained blocks of functionality that can be passed around and used in your code.

However, speaking in laymen's terms, Closures are bodies of function without syntax — func <func_name>().

Example:

func display(message: String) {
    print(message)
}

The above function can be re-written as a closure, like

{ (message) in 
    print(message)
}

How/When to Use Closure:

One of the most common uses of closure is callbacks. It can be used as an alternative to delegate when one-to-one communication is required. To understand its use better, consider the following scenario.

Let us start with two classes named class A and class B.

class A has:

• a stored property “name”
• a function whose only purpose is to call its argument (which is another function in this case)

class A {
    let name = “Nish”
    func callMyArgument(argument: (String) -> Void ) {
        argument(name)
    }
}

class B has:

• an object of class A
• a function that prints its own argument

class B {
    func callMeNow(name: String) {
        print(name)
    }
    var a = A()
    a.callMyArgument(callMeNow)
}

In simpler terms, this means that class B wants to call the function “callMeNow”. But B knows that this function should be called by class A because only class A knows the name argument to print.

Therefore, B cannot call the function “callMeNow(name: String)” directly because B does not have the value to its argument “name”.

But B knows that class A has a function:

func callMyArgument(argument: (String) -> Void )

that takes another function “(String) -> Void ” as its argument. So B pass its function name (without any parameter values) to class A’s function as an argument, like

a.callMyArgument(callMeNow)

Now the body of the function in A just calls its parameter with argument value, like

func callMyArgument(argument: (String) -> Void ) {
    argument(name)
}

This “argument(name)” calls “callMeNow(name)”, and thus, printing the output as:

Nish

Cheers!