Adjust letter spacing in iOS 7

For Swift 4+ the syntax is as simple as:

let text = NSAttributedString(string: "text", attributes: [.kern: 1.4])

You can adjust letter spacing like this, using NSAttributedString.

In Objective-C:

NSMutableAttributedString *attributedString = [[NSMutableAttributedString alloc] initWithString:@"The Clash"];
[attributedString addAttribute:NSKernAttributeName
                         value:@(1.4)
                         range:NSMakeRange(0, 9)];
    
self.label.attributedText = attributedString;

In Swift 3:

let attributedString = NSMutableAttributedString(string: "The Clash")
attributedString.addAttribute(NSKernAttributeName, value: CGFloat(1.4), range: NSRange(location: 0, length: 9))

label.attributedText = attributedString

In Swift 4 and later:

label.attributedText = NSAttributedString(string: "The Clash", attributes: [.kern: 1.4])

More info on kerning is available in Typographical Concepts from the Text Programming Guide.

I don't think there's a TextKit feature that will automatically match font spacing between bold and regular text.

With Swift 5.3 and iOS 14, NSAttributedString.Key has a static property called kern. kern has the following declaration:

static let kern: NSAttributedString.Key

The value of this attribute is an NSNumber object containing a floating-point value. This value specifies the number of points by which to adjust kern-pair characters. Kerning prevents unwanted space from occurring between specific characters and depends on the font. The value 0 means kerning is disabled. The default value for this attribute is 0.

The following Playground code shows a possible implementation of kern in order to have some letter spacing in your NSAttributedString:

import PlaygroundSupport
import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        let string = "Some text"
        let paragraph = NSMutableParagraphStyle()
        paragraph.alignment = .center
        let attributes: [NSAttributedString.Key: Any] = [
            NSAttributedString.Key.kern: 10,
            NSAttributedString.Key.paragraphStyle: paragraph
        ]
        let attributedString = NSMutableAttributedString(
            string: string,
            attributes: attributes
        )

        let label = UILabel()
        label.attributedText = attributedString

        view.backgroundColor = .white
        view.addSubview(label)

        label.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            label.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            label.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        ])
    }
}

PlaygroundPage.current.liveView = ViewController()