CODEWITHSUNDEEP
swift
thoxey
thoxey

Reputation: 322

How to pass a function as an optional parameter Swift

New to Swift and I am currently getting around passing a function as an optional parameter in the following way:

import Foundation 

func foo()
{
    print("Foo")
}

func bar()
{
    print("Bar")
}

func dummy()
{
    return
}

func myFunc()
{
    myFunc(callBack: dummy())
}

func myFunc(callBack: (Void))
{
    foo()
    callBack
}

myFunc()
myFunc(callBack: bar())

i.e. exploiting polymorphism

This seems like an inelegant way to do this, is there a better way,

Thanks, Tom

edit:

I am aware I could shorten this by doing:

import Foundation 

func foo()
{
    print("Foo")
}

func bar()
{
    print("Bar")
}

func dummy()
{
    return
}

func myFunc(callBack: (Void) = dummy())
{
    foo()
    callBack
}

myFunc()
myFunc(callBack: bar())

But I would still say this is an inelegant solution

Upvotes: 3

Views: 8058

Answers (4)

thoxey
thoxey

Reputation: 322

Full code with answer applied:

import Foundation 

func foo()
{
    print("Foo")
}

func bar()
{
    print("Bar")
}

func myFunc(completion: () -> Void = { })
{
    foo()
    completion()
}

myFunc()
myFunc(completion: bar)

Upvotes: 2

LinusG.
LinusG.

Reputation: 28982

You can easily define a default value for a function as parameter:

func foo(completion: () -> Void = { }) {
    completion()
}

You could also have it be nil by default:

func foo(completion: (() -> Void)? = nil) {

When it's an optional closure, you'll have to call it like this:

completion?()

Upvotes: 11

kabanus
kabanus

Reputation: 26005

Your are referring to default arguments:

You can define a default value for any parameter in a function by assigning a value to the parameter after that parameter’s type. If a default value is defined, you can omit that parameter when calling the function.

So you need something like:

func myfunc (callback: ()->void = dummy) {

LinusGeffarth notes that callbacks in Swift are called completion, or closure as avismara notes.

Upvotes: 2

avismara
avismara

Reputation: 5149

Umm, are you passing function as an optional parameter, though? It looks like you have written a method that accepts Void, you have a function that accepts Void and you are just calling that method. I don't see any trace of polymorphism here except probably that the myFunc has multiple signatures. This method is an inelegant solution not because it is inelegant, but because it isn't a solution.

Here is a correct example of polymorphism in functional systems: 

func printA() {
    print("A")
}

func printB() {
    print("B")
}

//This method accepts a function as a parameter
func higherOrderPrinter(with print: () -> Void ) { 
    print() //Can be anything, depends on the method you send here. Polymorphism much? :)
}

higherOrderPrinter(with: printA) //Prints A. See carefully how I am NOT doing printA()
higherOrderPrinter(with: printB) //Prints B
//In fact...
higherOrderPrinter {
    print("C") 
} //Prints C

Upvotes: 1

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