A hidden feature or SwiftUI API related bug?

Using Swift
1

Command click in Xcode 11 (on Catalina) on some types like Button or SwiftUI module in general reveals seemingly broken type aliases such as:

public struct Button<Label> where Label : View {
    public typealias Body
}

extension View {
    /// The type resulting from applying a view modifier `T`.
    public typealias Modified<T>
}

Is this a bug or a hidden feature that the framework required?

Same goes for the online docs:

https://developer.apple.com/documentation/swiftui/button/modified

Why can't we see what the type alias is aliasing here?

2

I think what you're seeing is the same interface printing behavior that you get for things like PlaygroundQuickLook or AnyObject.

3

Hmm so they are hiding implementation detail types?

4

You can't see the types because they are prefixed with _ and Xcode hides such types.

You can find the types in this file:

/Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks/SwiftUI.framework/Modules/SwiftUI.swiftmodule/arm64.swiftinterface

Here is struct Button:

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
public struct Button<Label> : __LegacyView where Label : SwiftUI.View {
  public init(action: @escaping () -> Void, @SwiftUI.ViewBuilder label: () -> Label)
  public var body: _View {
    get
  }
  public typealias Body = SwiftUI._AutoViewWrapper
}

And here is the extension View that defines Modified:

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
extension View {
  public typealias Modified<T> = _ModifiedContent<Self, T> where T : SwiftUI.ViewModifier
  @inlinable public func modifier<T>(_ modifier: T) -> Modified<T> where T : SwiftUI.ViewModifier {
        return .init(content: self, modifier: modifier)
    }
}
6 Likes
5

Cool thank you for sharing where I can check the interface files, that's helpful.

But for some reason this only applies to Apple frameworks and stdlib, I don't recall that this ever worked for custom types that begin with a single underscore.

1 Like