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Extending static member lookup in generic contexts

Available from Swift 5.5

Paul Hudson      @twostraws

SE-0299 allows Swift to perform static member lookup for members of protocols in generic functions, which sounds obscure but actually fixes a small but important legibility problem that hit SwiftUI particularly hard.

At this time SwiftUI hasn’t been updated to support this change, but if everything goes to plan we can stop writing this:

import SwiftUI

Toggle("Example", isOn: .constant(true))
    .toggleStyle(SwitchToggleStyle())

And instead write something like this:

Toggle("Example", isOn: .constant(true))
    .toggleStyle(.switch)

This was possible in early SwiftUI betas because Apple had put extensive workarounds in place, but these were withdrawn before release.

To see what’s actually changing here, imagine a Theme protocol with several structs conforming to it:

protocol Theme { }
struct LightTheme: Theme { }
struct DarkTheme: Theme { }
struct RainbowTheme: Theme { }

We could also define a Screen protocol that is able to have a theme() method called on it with some sort of theme:

protocol Screen { }

extension Screen {
    func theme<T: Theme>(_ style: T) -> Screen {
        print("Activating new theme!")
        return self
    }
}

And now we could create an instance of a screen:

struct HomeScreen: Screen { }

Following older SwiftUI code, we could enable a light theme on that screen by specifying LightTheme():

let lightScreen = HomeScreen().theme(LightTheme())

If we wanted to make that easier to access, we could try adding a static light property to our Theme protocol like this:

extension Theme where Self == LightTheme {
    static var light: LightTheme { .init() }
}

However, using that with the theme() method of our generic protocol was what caused the problem: before Swift 5.5 it was not possible and you had to use LightTheme() every time. However, in Swift 5.5 or later this is now possible:

let lightTheme = HomeScreen().theme(.light)
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