How to stop a time UILabel from resizing at every time increment?

Question

I have a stopwatch feature in my app that uses a centered attributed UILabel with a proportionally spaced font to render time. At every time increment the width

Answer 1

Use a monospaced font or pass parameters when creating the font which force monospaced numbers:

//kNumberSpacingType 6
//kMonospacedNumbersSelector 0
NSArray *setting = @[
                         @{
                             UIFontFeatureTypeIdentifierKey: @(6),
                             UIFontFeatureSelectorIdentifierKey: @(0)
                             }
                         ];

return [UIFont fontWithDescriptor:[[self fontDescriptor] fontDescriptorByAddingAttributes:@{UIFontDescriptorFeatureSettingsAttribute : setting}] size:0.0];

Answer 2

For future readers:

This is how to enable monospaced numbers on iOS 9 with San Francisco:

let originalFont = UIFont.systemFontOfSize(17)
let originalFontDescriptor = originalFont.fontDescriptor()

let fontDescriptorFeatureSettings = [
    [
    UIFontFeatureTypeIdentifierKey: kNumberSpacingType,
    UIFontFeatureSelectorIdentifierKey: kMonospacedNumbersSelector
    ]
]

let fontDescriptorAttributes = [UIFontDescriptorFeatureSettingsAttribute: fontDescriptorFeatureSettings]    
let fontDescriptor = originalFontDescriptor.fontDescriptorByAddingAttributes(fontDescriptorAttributes)
let font = UIFont(descriptor: fontDescriptor, size: 0)

Edit:

Also available on GitHub: https://github.com/salutis/swift-font-monospaced-digits