Self type as argument in closure

Question

Is it possible for an instance of a UIView to call a method which executes a closure, and inside that closure referring to the same instance? This is the non-generic version:

import UIKit

public extension UIView {
    func layout(from: (UIView) -> ()) {
        from(self)
    }
}

When I call it with a UILabel for example, I do not have access to e.g. the text aligment. Is it possible that inside the closure I can refer to the UILabel? I would expect something like this would work:

func layout(from: (Self) -> ()) {
    from(self)
}

But it doesn't compile. Is there a workaround? This is what I want:

let label = UILabel(frame: .zero)

label.layout { $0.textAlignment = .natural } // Currenly not working, since $0 = UIView.

@MoAbdul-Hameed This is a simplified version of something bigger, so don't mind the actual code :) — J. Doe, Apr 06 '19 at 13:34
See https://stackoverflow.com/a/42356615/2976878 – you can currently use a protocol extension to work around this limitation until it's implemented ([SR-10121](https://bugs.swift.org/browse/SR-10121)). — Hamish, Apr 06 '19 at 14:29
@Hamish vadian gave an answer with protocol extensions, or is there another way? — J. Doe, Apr 06 '19 at 14:32

score 3 · Accepted Answer · answered Apr 06 '19 at 13:42

3

Different approach: Protocol Extension with associated type.

protocol Layout {
    associatedtype View : UIView = Self
    func layout(from: (View) -> ())
}

extension Layout where Self : UIView {
    func layout(from: (Self) -> ()) {
        from(self)
    }
}

extension UIView : Layout {}

let label = UILabel(frame: .zero)
label.layout { $0.textAlignment = .natural }

answered Apr 06 '19 at 13:42

vadian

274,689
30
353
361

2

You don't need to declare an `associatedtype` for this to work (or any protocol requirements for that matter). You could also declare the protocol as `protocol Layout : UIView` to restrict it to being conformed by `UIView` (and then remove the constraint from the extension). – Hamish Apr 06 '19 at 14:33

Pentracchiano · Answer 2 · 2019-04-06T14:18:49.947

1

There are different ways to do it.

Firstly, you could use the closures' variable capturing system in order to directly use the variable inside the closure, without passing it as an argument.

public extension UIView {
    func layout(from: () -> ()) {
        from()
    }
}

label.layout { label.textAlignment = .natural }

Otherwise, if you want to pass a generic UIView and change the behaviour accordingly to the specific one - since it looks like you know for sure what type you are working on - you can use a downcast:

public extension UIView {
    func layout(from: (UIView) -> ()) {
        from(self)
    }
}

let label = UILabel(frame: .zero)

label.layout { ($0 as! UILabel).textAlignment = .natural }

Anyway, why are you doing:

label.layout { $0.textAlignment = .natural }

instead of:

label.textAlignment = .natural

Is there any particular reason not to do it? I imagine there's something bigger behind the scenes, I'm just curious.

edited Apr 06 '19 at 14:18

answered Apr 06 '19 at 13:50

Pentracchiano

188
7

1

Hey thanks for your response. The code in the question isn't the real code, but a simplified version. When using the closure method as you described at the top of your answer, I do not get an argument of type `self` (which is the question). – J. Doe Apr 06 '19 at 14:22
Hello! I see, what about the second piece of the answer? I mean, the downcast. Is it too *dirty*? – Pentracchiano Apr 06 '19 at 14:29
2

Yes, I do not want to downcast everytime I use it, but still thank you for your explaination and answer :) – J. Doe Apr 08 '19 at 12:57

Self type as argument in closure

2 Answers2