Checking for the presence of an optional method in a protocol

optional_protocol.swift

import Foundation

@objc protocol Foo {
    optional func say() -> String
}

class Doesnt: NSObject, Foo {
}

class Does: NSObject, Foo {
    func say() -> String {
        return "hi"
    }
}

let doesnt: Foo = Doesnt()
let does: Foo = Does()


// You can easily check to see if an object implements an optional protocol method:

if let f = doesnt.say {
    f() // Never called
} else {
    // Fallback to something
}

if let f = does.say {
    f() // Returns "hi"
}


// You can also just do the following if you don't care if it's implemented or not.
doesnt.say?() // .None
does.say?() // Optional("hi")

You can also do this:

doesnt?.say()  // no-op
does?.say()  // returns "hi"

@subdigital typo?

doesnt.say?()  // .None
does.say?()  // returns Optional("hi")

@subdigitial that is great if you want to just call it if it doesn't exists or not. This is more for:

if let f = does.say {
    f()
} else {
   // Some fallback
}