Can you put functions in Dictionaries?

Question

logistes OP

Created Sep ’15

Replies 25

Boosts 0

Views 6.7k

Participants 7

Hi,

I have been trying to prove a concept in Swift that I implemented in another language. I thought it would be easy is Swift, but it doesn't seem to work.

My playground code is below. When I call the 2 btn.click() functions at the end it get

The caption is Say hello

Can't find proc for Say hello

The proc was just called

The caption is Greet Veronica

Can't find proc for Greet Veronica

The proc was just called

I was hoping that sayHello would be called and then the closure to greet Veronica.

Can anyone see what coud be going wrong here?

TIA

Mark

import UIKit
print("\nTesting assignment of functions\n")

typealias Proc = () -> ()
typealias Event = AnyObject -> ()

class Button {

    var caption: String
    var onClick: Event?

    init(caption: String, onClick: Event? = nil) {
        self.caption = caption
        self.onClick = onClick
    }

    func click() {
        onClick?(self)
    }

}

func sayHello() { print("Hello") }
func greet(name: String) { print("Hello", name) }

let sh = "Say hello"
let gv = "Greet Veronica"

struct Form {

    var commands = [String: Proc]()


    let btn1 = Button(caption: sh)
    let btn2 = Button(caption: gv)
    func stdBtnClick(sender: AnyObject) {
        guard let button = sender as? Button else { return }
        print("\nThe caption is", button.caption)
        if let proc = commands[button.caption] {
            proc()
        } else {
            print("Can't find proc for", button.caption)
        }
        print("The proc was just called\n")
    }


    init() {
        btn1.onClick = stdBtnClick
        btn2.onClick = stdBtnClick
     
        commands.updateValue(sayHello, forKey: sh)
        commands.updateValue( { greet("Veronica") }, forKey: gv)
     
        print("Commands =", commands)
    }

}

let form = Form()
form.btn1.click()
form.btn2.click()

Answered by jsslai in 59137022

Change Form to class instead of struct. I guess now the Form gets copied and commands property is empty.

Boost

Answer 1

Jens OP

Sep ’15

Why are you focusing so much on the fact that functions stored in a dictionary are called/executed after the code is compiled (ie runtime)?

I mean, of course they are. Just like any other function.

func foo() { print("foo") } // foo is this function.
foo()       // Calls foo.
let x = foo // The value of x is now a reference to the function foo.
x()         // Calls foo.
let myDictionary: [String : () -> ()] = ["foosKey" : foo] // This dictionary now has a value that is a reference to the function foo.
myDictionary["foosKey"]?() // Calls foo.

You certainly can put a (reference to a) function in a dictionary, just like you can in an array, in a variable, in a struct, in a class, whatever.

I can't see what the big deal/difference is.

Perhaps what you mean to point out is that you can't send a dictionary with function(reference)s in it over a network and expect to transmit the actual (code for those) functions? (And the same goes for eg archiving/unarchiving.)

I don't think anyone here have such expectations.

(Functions are reference types, and all that gets stored in the dictionary are references to them.)

0

Answer 2

RLKingSoftware OP

Sep ’15

You're missing the point entirely. Nothing stored in a dictionary is executable code. It's strings or numbers or possibly

even image data but not code.

Your example, adds fooskey to the dictionary, not the function foo().

0

Answer 3

Jens OP

Sep ’15

I'm not sure who is missing what point really.

Of course it is not code, who has said that it should be code? By that logic you can not store instances of classes in Dictionaries either. No one would say that you can not put instances of some class in a dictionary. You can do that, most people would say, assuming that we all know that it is only references to the class instances that is stored in the dictionary, since classes are reference types.

Read my previous post again. It clearly says that it is references to functions that gets stored in the values of the key-value pairs of the dictionary. I even wrote that part starting with "Perhaps what you mean to point out ..." which it doesn't seem like you have read. Have you copy pasted and run my code example? It prints three "foo", each one of them is the result of a call to the function foo. Also, the OP is happy with the accepted answer (which clearly works, even though you say it won't work).

Here's my dictionary again:

let myDictionary: [String : () -> ()] = ["foosKey" : foo]

That is exactly the same as eg:

let myDictionary: Dictionary<String, (() -> Void)> = ["foosKey" : foo]

All Dictionaries has an Element type which is (Key, Value), and in this particular case:

Key is the type String

Value is the type () -> Void

So this Dictionary contains Strings (for keys) and references to functions of the type (() -> Void) as values.

I don't understand what you mean when you say that my dictionary only has a key and no value in it.

A Dictionary can't have just a key in it ...