swift/swiftui inheritance with opaque types

Question

I have a lot of different types of elements in an array, and I want them all to know how to understand how to render themselves... it's quite complex and I'm always adding new types - don't want to have to do shotgun surgery each time

but can't seem to work out the syntax for inheritance of a function with an opaque type return - if I do this:


class Element
{
   func forDisplay() -> some View 
   { fatalError("this function needs to be overridden")}
}

then try to override it

class TextElement : Element
{
    ...

    override func forDisplay() -> some View
    { return Text( self.textValue ) };
}

It gives me a compile error - and if I just leave out the word override

class TextElement : Element
{
    ...

    func forDisplay() -> some View
    { return Text( self.textValue ) };
}

it compiles fine, runs fine - but then calls the underlying error function

SwiftUI is Protocol based, not Inheritance based. Element should be a Protocol that the others adopt. — Yrb
– Yrb, Commented Feb 27, 2022 at 21:46
elements are complex things - 80% of the code is shared - I don't see how I could use a protocol without duplicating that code — Darren Oakey
– Darren Oakey, Commented Feb 27, 2022 at 21:52
actually - I just tried it - and I can't even create a func that returns a view in a protocol - func for_display() -> View and func for_display() -> some View both don't work — Darren Oakey
– Darren Oakey, Commented Feb 27, 2022 at 21:54
The protocol would need an associated type, and forDisplay would return that associated type. — Sweeper
– Sweeper, Commented Feb 27, 2022 at 21:57
Undo what you just did and use an associated type. AnyView will mess you up later in SwiftUI. You need to read up on Protocols. — Yrb
– Yrb, Commented Feb 27, 2022 at 22:03

score 1 · Accepted Answer · 2022-02-28 09:51:32Z

1

You can't override opaque types. Better just to never subclass anything, really. It's a last resort. I bet you don't even want a reference type and you've only chosen a class because of inheritance, thinking it would model your solution well. Regardless…

Maybe you want a protocol with an associated type as has been recommended, or maybe you just want a generic Element<View: SwiftUI.View>. Trouble with those is, you won't be able to constrain extensions because you won't know what the opaque Views are. So I suspect you'll have more luck with phantom types.

protocol HeadingTag { }
enum H1: HeadingTag { }
enum H2: HeadingTag { }
enum H3: HeadingTag { }

class Element<Tag> {

extension Element where Tag: HeadingTag  {
  func forDisplay() -> some View {
    Text(textValue)
  }
}

extension Element where Tag == H2 {
  func forDisplay() -> some View {
    // An H2-specific view

edited Feb 28, 2022 at 9:51

answered Feb 28, 2022 at 6:33

user652038

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1 Comment

Darren Oakey Over a year ago

interesting - will think about this - thanks

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