ScaledValue

ScaledValue.swift

import SwiftUI
import UIKit

@propertyWrapper
struct ScaledValue: DynamicProperty {
    @Environment(\.dynamicTypeSize) private var dynamicTypeSize
    private var lastDynamicTypeSize: DynamicTypeSize!

    private(set) var wrappedValue: CGFloat

    var unscaledValue: CGFloat {
        didSet { forceUpdate() }
    }

    var textStyle: Font.TextStyle {
        didSet { forceUpdate() }
    }

    init(wrappedValue: CGFloat, textStyle: Font.TextStyle = .body) {
        self.unscaledValue = wrappedValue
        self.wrappedValue = .nan
        self.textStyle = textStyle
        self.forceUpdate()
    }

    init(unscaledValue: CGFloat, textStyle: Font.TextStyle = .body) {
        self.unscaledValue = unscaledValue
        self.wrappedValue = .nan
        self.textStyle = textStyle
        self.forceUpdate()
    }

    private func calculate() -> CGFloat {
        let traitCollection = UITraitCollection(preferredContentSizeCategory: UIContentSizeCategory(dynamicTypeSize))
        return UIFontMetrics(forTextStyle: UIFont.TextStyle(textStyle)).scaledValue(for: unscaledValue, compatibleWith: traitCollection)
    }

    mutating func update() {
        guard lastDynamicTypeSize != dynamicTypeSize else { return }
        forceUpdate()
    }

    private mutating func forceUpdate() {
        lastDynamicTypeSize = dynamicTypeSize
        wrappedValue = calculate()
    }
}

private extension UIFont.TextStyle {
    init(_ textStyle: Font.TextStyle) {
        switch textStyle {
        case .largeTitle:
            self = .largeTitle
        case .title:
            self = .title1
        case .title2:
            self = .title2
        case .title3:
            self = .title3
        case .headline:
            self = .headline
        case .subheadline:
            self = .subheadline
        case .body:
            self = .body
        case .callout:
            self = .callout
        case .footnote:
            self = .footnote
        case .caption:
            self = .caption1
        case .caption2:
            self = .caption2
        @unknown default:
            self = .body
        }
    }
}