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")

Makes sense. Instance methods in swift are just curried class functions that take the instance as the first argument and return an optional function that then gets resolved... somehow. It's wild.

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
}