Swift Dynamic Font Size for Bounds

UIFont+BestFit.swift

extension UIFont {
    
    /**
     Will return the best font conforming to the descriptor which will fit in the provided bounds.
     */
    static func bestFittingFontSize(for text: String, in bounds: CGRect, fontDescriptor: UIFontDescriptor, additionalAttributes: [NSAttributedStringKey: Any]? = nil) -> CGFloat {
        let constrainingDimension = min(bounds.width, bounds.height)
        let properBounds = CGRect(origin: .zero, size: bounds.size)
        var attributes = additionalAttributes ?? [:]
        
        let infiniteBounds = CGSize(width: CGFloat.infinity, height: CGFloat.infinity)
        var bestFontSize: CGFloat = constrainingDimension
        
        for fontSize in stride(from: bestFontSize, through: 0, by: -1) {
            let newFont = UIFont(descriptor: fontDescriptor, size: fontSize)
            attributes[.font] = newFont
            
            let currentFrame = text.boundingRect(with: infiniteBounds, options: [.usesLineFragmentOrigin, .usesFontLeading], attributes: attributes, context: nil)
            
            if properBounds.contains(currentFrame) {
                bestFontSize = fontSize
                break
            }
        }
        return bestFontSize
    }
    
    static func bestFittingFont(for text: String, in bounds: CGRect, fontDescriptor: UIFontDescriptor, additionalAttributes: [NSAttributedStringKey: Any]? = nil) -> UIFont {
        let bestSize = bestFittingFontSize(for: text, in: bounds, fontDescriptor: fontDescriptor, additionalAttributes: additionalAttributes)
        return UIFont(descriptor: fontDescriptor, size: bestSize)
    }
}

UILabel+BestFit.swift

extension UILabel {
    
    /// Will auto resize the contained text to a font size which fits the frames bounds.
    /// Uses the pre-set font to dynamically determine the proper sizing
    func fitTextToBounds() {
        guard let text = text, let currentFont = font else { return }
    
        let bestFittingFont = UIFont.bestFittingFont(for: text, in: bounds, fontDescriptor: currentFont.fontDescriptor, additionalAttributes: basicStringAttributes)
        font = bestFittingFont
    }
    
    private var basicStringAttributes: [NSAttributedStringKey: Any] {
        var attribs = [NSAttributedStringKey: Any]()
        
        let paragraphStyle = NSMutableParagraphStyle()
        paragraphStyle.alignment = self.textAlignment
        paragraphStyle.lineBreakMode = self.lineBreakMode
        attribs[.paragraphStyle] = paragraphStyle
        
        return attribs
    }
}

Hi there!
Thanks for that awesome code!! But, it won't work.
It does reduces the font size to be fit (as it seems to be doing), but the text is still trimmed.
Made an empty ViewController and put a label with top, left and right constraints of 24 to the superview (viewcontroller itself).
Using latest xcode and swift.
Thanks :)

@eitanaviv - I agree, some instances it can be trimmed. I just made a small modification which should address this. Having centered text in a label, or depending on how the string looks, sometimes the string could be on two lines and return a false positive for its sizing. Should be better now!

NSAttributedStringKey is now NSAttributedString.Key

hi, this does not work, i am getting dots, in one line text

@joncardasis
Thanks your code :)
Can I use your code in my commercial app?
Please let me know how I can use the code for commercial distribution.

Thank you for sharing, it worked with me in iOS 11 & 14, Xcode 12

Thank you