SwiftUI - Get size of child?

Question

Is there any way to get the size of a child view in SwiftUI?

I\'m basically looking to do the UIKit equivalent of:

Answer 1

Here's a reusable variant of the accepted answer:

protocol SizeReaderKey: PreferenceKey where Value == CGSize {}

extension SizeReaderKey {
    static func reduce(value _: inout CGSize, nextValue: () -> CGSize) {
        _ = nextValue()
    }
}

struct SizeReader: ViewModifier {
    func body(content: Content) -> some View {
        content
            .background(
                GeometryReader { geo in
                    Color.clear
                        .preference(key: Key.self, value: geo.size)
                }
            )
    }
}

extension View {
    func onSizeChanged(
        _ key: Key.Type,
        perform action: @escaping (CGSize) -> Void) -> some View
    {
        self
            .modifier(SizeReader())
            .onPreferenceChange(key) { value in
                action(value)
            }
    }
}

Usage:

struct Example: View {
    var body: some View {
        Text("Hello, World!")
            .onSizeChanged(CustomViewSizeKey.self) { size in
                print("size: \(size)")
            }
    }

    struct CustomViewSize: SizePreferenceKey {
        static var defaultValue: CGSize = .zero
    }
}