How to implement a type erased struct like AnyView in SwiftUI?

Question

I'm curious about the default implementation of AnyView in SwiftUI. How to put structs with different generic types into a protocol array?

For example:

let a = AnyView(Text("hello"))
let b = AnyView(Image(systemName: "1.circle"))
let genericViews = [a, b] // No compile error

And my implementation:

struct TypeErasedView<V: View>: View {
    private var _view: V
    init(_ view: V) {
        _view = view
    }
    var body: V {
        _view
    }
}

let a = TypeErasedView(Text("Hello"))
let b = TypeErasedView(Image(systemName: "1.circle"))
let genericViews = [a, b] // compile error

The compile error will be "Heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional".

Does anyone have any ideas?

Asperi Asperi · Accepted Answer · 2019-12-03T20:01:05

Here is a demo of possible approach. It is simplified, but shows the generic idea of how this might be done... or at least a direction.

Full compilable & working module. Tested on Xcode 11.2 / iOS 13.2

import SwiftUI

private protocol TypeErasing {
    var view: Any { get }
}

private struct TypeEraser<V: View>: TypeErasing {
    let orinal: V
    var view: Any {
        return self.orinal
    }
}

public struct MyAnyView : View {
    public var body: Never {
        get {
            fatalError("Unsupported - don't call this")
        }
    }

    private var eraser: TypeErasing
    public init<V>(_ view: V) where V : View {
        eraser = TypeEraser(orinal: view)
    }

    fileprivate var wrappedView: Any { // << they might have here something specific
        eraser.view
    }

    public typealias Body = Never
}


struct DemoAnyView: View {
    let container: [MyAnyView]
    init() {
        let a = MyAnyView(Text("Hello"))
        let b = MyAnyView(Image(systemName: "1.circle"))
        container = [a, b]
    }

    var body: some View {
        VStack {
            // dynamically restoring types is different question and might be
            // dependent on Apple's internal implementation, but here is
            // just a demo that it works
            container[0].wrappedView as! Text
            container[1].wrappedView as! Image
        }
    }
}

struct DemoAnyView_Previews: PreviewProvider {
    static var previews: some View {
        DemoAnyView()
    }
}

How to implement a type erased struct like AnyView in SwiftUI?

2 Answers