SelfSizingSheet.swift

import SwiftUI

struct SelfSizingSheet: ViewModifier {
    @State private var height: CGFloat = .zero
    
    private struct InnerHeightPreferenceKey: PreferenceKey {
        static let defaultValue: CGFloat = .zero
        static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
            value = nextValue()
        }
    }
    
    func body(content: Content) -> some View {
        content
            .overlay {
                GeometryReader { geometry in
                    Color.clear.preference(key: InnerHeightPreferenceKey.self, value: geometry.size.height)
                }
            }
            .onPreferenceChange(InnerHeightPreferenceKey.self) { newHeight in
                height = newHeight
            }
            .presentationDetents([.height(height)])
    }
}

public extension View {
    func selfSizingSheet() -> some View {
        modifier(SelfSizingSheet())
    }
}

// Usage

#Preview {
    Color.white
        .sheet(isPresented: .constant(true)) {
            VStack {
                RoundedRectangle(cornerRadius: 8)
                    .fill(Color.red)
                    .frame(height: 30)
                
                RoundedRectangle(cornerRadius: 8)
                    .fill(Color.green)
                    .frame(height: 60)
                
                RoundedRectangle(cornerRadius: 8)
                    .fill(Color.blue)
                    .frame(height: 120)
            }
            .padding(16)
            .selfSizingSheet()
        }
}