Function that returns any Shape (AnyShape?)

Using Swift
1

I'm pulling my hair out over here.
I'm just trying to produce a struct that has a member variable that can accept any shape, such as Circle(), Rectangle(), RoundedRectangle(), etc.

of course, the compiler is very mad about this, so I tried to write a function that takes an enum and returns 'some Shape' but that doesn't work because of opaque return types...

I'm thinking I need to build an 'AnyShape' using type erasure but it's super confusing to me. Does anyone know how to do this?

Looking for any helps / pointers you guys have. I've wasted hours on this.

2 Likes
2

If the underlying type doesn't change for each variable, you need generic struct:

struct Foo<SomeShape: Shape> {
  var shape: SomeShape
  ...
}

Say, if what you want is to store Circle, then change to Rectangle on the same variable, you need type erasure.

3

I have an AnyShape in one of my projects:

#if canImport(SwiftUI)

import SwiftUI

@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, *)
public struct AnyShape: Shape {
    public var make: (CGRect, inout Path) -> ()

    public init(_ make: @escaping (CGRect, inout Path) -> ()) {
        self.make = make
    }

    public init<S: Shape>(_ shape: S) {
        self.make = { rect, path in
            path = shape.path(in: rect)
        }
    }

    public func path(in rect: CGRect) -> Path {
        return Path { [make] in make(rect, &$0) }
    }
}

#endif

However, I've only ended up using the initializer that takes a closure, not the initializer that takes a shape.

4 Likes
4

Type erasure would look like this:

public struct AnyShape: Shape {
    private var base: (CGRect) -> Path
    
    public init<S: Shape>(shape: S) {
        base = shape.path(in:)
    }
    
    public func path(in rect: CGRect) -> Path {
        base(rect)
    }
}

Though I ask that you consider this carefully if you need type-erasure. In SwiftUI, chances are, you can do away with other mechanisms (like generic, if/else block).

3 Likes
5

Wow. Thank you so much for the help guys! I always feel weird stealing code but @mayoff your code really did work like a charm. Looks like it comes down to just passing along the path?

Thanks again guys

1 Like
6

You can use my AnyShape by providing a shape:

let aShape: AnyShape = AnyShape(Rectangle())

Or you can pass a closure that draws the path:

let aShape: AnyShape = AnyShape { rect, path in
    path.addEllipse(in: rect)
}
7

Just to note, you don't need to capture make since the closure doesn't escape, not that it makes any difference.

8

I’m struggling with something kind of similar. Anyone know how to let a ViewModifier know that it’s `contents is something that conforms to the Shape protocol ? I need a Shape stroked for a card game app ... and compiler complains that stroke() is not a member of Any view.

9

i use your AnyShape, works great.

just as you noted to use it as a last resort only, is it possible to get rid of this type erasure in the following fragment without introducing code duplication?

    let shape = isSquare ?
        AnyShape(RoundedRectangle(cornerRadius: size*0.3 + delta).inset(by: delta + extra)) :
        AnyShape(Circle().inset(by: delta + extra))
    
    return Group {
        if icon != nil {
            Image(uiImage: icon!)
                .resizable()
                .aspectRatio(contentMode: .fill)
                .background(Color.yellow)
                .padding(delta)
                .frame(width: w, height: w)
                .clipShape(shape)
        } else {
            Text("XXX")
                .frame(width: w, height: w)
                .font(Font.custom("Courier", size: size*0.4))
                .background(Color.yellow)
                .overlay(shape.stroke(lineWidth: bw).foregroundColor(Color.red))
        }
    }
10

One thing I would do, is to make Circle just a RoundedRectangle with infinite cornerRadius, or better yet, cornerRadius of width/2.

Swapping Shape like how you did it does lose the ability to animate shape transitions. It is probably fine otherwise to type-erase it since I just realise Shape does not participate in ViewBuilder, only View (which Shape conforms to).

Though I'd personally prefer to make it a proper shape

enum ImportantShape: Shape {
  case square(Float), circle

  func path(in rect: CGRect) -> Path { ... }
}

but that could just be me being paranoid about performance impact from type erasure, which used to be quite a problem in the past (not sure about now).

3 Likes