ScalableFonts.swift

import SwiftUI

enum ScalableFont {

    case system(size: CGFloat, weight: Font.Weight = .regular, design: Font.Design = .default)

    case custom(_ name: String, size: CGFloat)

    var size: CGFloat {
        switch self {
        case let .system(size: size, weight: _, design: _):
            return size
        case let .custom(_, size: size):
            return size
        }
    }

    func makeFont(withScaledSize scaledSize: CGFloat) -> Font {
        switch self {
        case let .system(size: _, weight: weight, design: design):
            return .system(size: scaledSize, weight: weight, design: design)
        case let .custom(name, size: _):
            return .custom(name, size: scaledSize)
        }
    }
}

struct ScaledFontModifier: ViewModifier {
    @Environment(\.sizeCategory) var sizeCategory
    var scalableFont: ScalableFont

    func body(content: Content) -> some View {
        let scaledSize = UIFontMetrics.default.scaledValue(for: scalableFont.size)
        return content.font(scalableFont.makeFont(withScaledSize: scaledSize))
    }
}

extension View {
    func scalableFont(_ scalableFont: ScalableFont) -> some View {
        return self.modifier(ScaledFontModifier(scalableFont: scalableFont))
    }
}