UILabel subclass.

gistfile1.swift

import UIKit

/// `Label` is an `UILabel` subclass that simplifies line height and letter spacing manipulation.
public final class Label: UILabel {

    /// Since line spacing is always absolute in iOS, we need to compute the correct line spacing for given
    /// font and scale.
    public enum LineHeight {
        case absoluteSpacing(CGFloat)
        case fraction(CGFloat)

        func lineSpacing(for font: UIFont) -> CGFloat {
            switch self {
            case .absoluteSpacing(let value):
                return value
            case .fraction(let value):
                return value * font.pointSize - font.pointSize
            }
        }
    }

    /// Set to `true` in case you want to use dynamic scaling. Default `false`.
    public var scaleFont = false {
        didSet {
            reformatAndStore(string: stylableText)
        }
    }

    /// Set a non-nil value in case you want to use custom letter spacing.
    public var letterSpacing: CGFloat? {
        didSet {
            reformatAndStore(string: stylableText)
        }
    }

    /// Set a non-nil value in case you want to use custom line height.
    public var lineHeight: LineHeight? {
        didSet {
            reformatAndStore(string: stylableText)
        }
    }

    /// Font of the label, replacing the `font` property. Do not pass a scaled font.
    public var unscaledFont: UIFont = .systemFont(ofSize: UIFont.systemFontSize) {
        didSet {
            reformatAndStore(string: stylableText)
        }
    }

    public var additionalAttributes: [([NSAttributedString.Key : Any], Range<String.Index>)] = [] {
        didSet {
            reformatAndStore(string: stylableText)
        }
    }

    /// The text if the label, replaces the `text` and `attributedText` properties.
    ///
    /// It is not possible in the current implementation to display an attributed string.
    public var stylableText: String? {
        get {
            super.attributedText?.string
        }
        set {
            additionalAttributes = []
            reformatAndStore(string: newValue)
        }
    }

    @available(*, unavailable, message: "This property is not available for Label, use 'unscaledFont'.")
    override public var font: UIFont! {
        get { super.font }
        set { super.font = newValue }
    }

    @available(*, unavailable, message: "This property is not available for Label, use 'stylableText'.")
    override public var text: String? {
        get { super.text }
        set { super.text = newValue }
    }

    @available(*, unavailable, message: "This property is not available for Label, use 'stylableText'.")
    override public var attributedText: NSAttributedString? {
        get { super.attributedText }
        set { super.attributedText = newValue }
    }

    // Observing is required so font scales correctly
    override public func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)
        reformatAndStore(string: stylableText)
    }

    @objc
    private func buttonShapesEnabledStatusDidChange() {
        reformatAndStore(string: stylableText)
    }

    private func reformatAndStore(string: String?) {
        super.attributedText = string.flatMap { text in
            var attrStr = NSMutableAttributedString(string: text, attributes: attributes)
            additionalAttributes.forEach { (custom, range) in
                attrStr.addAttributes(custom, range: NSRange(range, in: text))
            }
            return attrStr
        }
    }

    private var attributes: [NSAttributedString.Key: Any] {
        var attributes = [NSAttributedString.Key: Any]()

        let designatedFont = scaleFont ? unscaledFont : UIFontMetrics.default.scaledFont(for: unscaledFont)
        super.font = designatedFont
        attributes[.font] = designatedFont
        if let letterSpacing = letterSpacing {
            attributes[.kern] = letterSpacing
        }

        let paragraphStyle = NSMutableParagraphStyle()
        paragraphStyle.alignment = textAlignment
        paragraphStyle.lineBreakMode = .byTruncatingTail
        if let lineHeight = lineHeight {
            paragraphStyle.lineSpacing = lineHeight.lineSpacing(for: designatedFont)
        }
        attributes[.paragraphStyle] = paragraphStyle

        return attributes
    }
}