Unified preview style for SwiftUI's PreviewProvider

ContentView.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        Text("Hello, World!")
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
            .previewStyle(.compact)
    }
}

PreviewStyle.swift

import SwiftUI

enum PreviewStyle {
    case compact
    case full
}

extension View {
    func previewStyle(_ style: PreviewStyle) -> some View {
        Group {
            if style == .compact {
                compactView()
            } else if style == .full {
                fullView()
            }
        }
    }

    private func compactView() -> some View {
        modifier(PreviewStyleCompact())
    }

    private func fullView() -> some View {
        modifier(PreviewStyleFull())
    }
}

struct PreviewStyleCompact: ViewModifier {

    func body(content: Content) -> some View {
        ForEach([ColorScheme.light, .dark], id: \.self) { scheme in
            content
                .background(Color(.systemBackground))
                .environment(\.colorScheme, scheme)
                .previewLayout(.sizeThatFits)
        }
    }
}

struct PreviewStyleFull: ViewModifier {

    func body(content: Content) -> some View {
        ForEach([ColorScheme.light, .dark], id: \.self) { scheme in
            content
                .background(Color(.systemBackground))
                .environment(\.colorScheme, scheme)
        }
    }
}