Core Text - NSAttributedString line height done right?

Objective-C

NSInteger strLength = [myString length];
NSMutableParagraphStyle *style = [[NSMutableParagraphStyle alloc] init];
[style setLineSpacing:24];
[attString addAttribute:NSParagraphStyleAttributeName
                  value:style
                  range:NSMakeRange(0, strLength)];

Swift 5

let strLength = myString.length()
var style = NSMutableParagraphStyle()
style.lineSpacing = 24
attString.addAttribute(.paragraphStyle, value: style, range: NSRange(location: 0, length: strLength))

I'm still not 100% confident in my following statements, but it seems to make sense. Please correct me where I am wrong.

The line height (leading) refers to the distance between the baselines of successive lines of type. The baseline here can be interpreted as the imaginary line which the text sits on.

Spacing is the space between lines. The space appears after the line of text.

I ended up using the following solution to my problem:

// NOT SURE WHAT THE THEORY BEHIND THIS FACTOR IS. WAS FOUND VIA TRIAL AND ERROR.
    CGFloat factor = 14.5/30.5;
    CGFloat floatValues[4];
    floatValues[0] = self.lineHeight * factor/(factor + 1);
    floatValues[1] = self.lineHeight/(factor + 1);
    floatValues[2] = self.lineHeight;

This matrix is used with the paragraph style parameter for NSAttributedString:

CTParagraphStyleSetting paragraphStyle[3];

paragraphStyle[0].spec = kCTParagraphStyleSpecifierLineSpacing;
paragraphStyle[0].valueSize = sizeof(CGFloat);
paragraphStyle[0].value = &floatValues[0];

paragraphStyle[1].spec = kCTParagraphStyleSpecifierMinimumLineHeight;
paragraphStyle[1].valueSize = sizeof(CGFloat);
paragraphStyle[1].value = &floatValues[1];

paragraphStyle[2].spec = kCTParagraphStyleSpecifierMaximumLineHeight;
paragraphStyle[2].valueSize = sizeof(CGFloat);
paragraphStyle[2].value = &floatValues[2];

CTParagraphStyleRef style = CTParagraphStyleCreate((const CTParagraphStyleSetting*) &paragraphStyle, 3);
[attributedString addAttribute:(NSString*)kCTParagraphStyleAttributeName value:(id)style range:NSMakeRange(0, [string length])];
CFRelease(style);

Hope this helps someone. I'll update this answer as I discover more relevant information.

In Swift 3:

    let textFont = UIFont(name: "Helvetica Bold", size: 20)!
    let textColor = UIColor(white: 1, alpha: 1)      // White
    let paragraphStyle = NSMutableParagraphStyle()
    paragraphStyle.paragraphSpacing = 20             // Paragraph Spacing
    paragraphStyle.lineSpacing = 40                  // Line Spacing

    let textFontAttributes = [
        NSFontAttributeName: textFont,
        NSForegroundColorAttributeName: textColor,
        NSParagraphStyleAttributeName: paragraphStyle
        ] as [String : Any]

You can set/update line spacing and line height multiple from storyboard as well as programatically.

From Interface Builder:

Programmatically:

SWift 4

extension UILabel {

    // Pass value for any one of both parameters and see result
    func setLineSpacing(lineSpacing: CGFloat = 0.0, lineHeightMultiple: CGFloat = 0.0) {

        guard let labelText = self.text else { return }

        let paragraphStyle = NSMutableParagraphStyle()
        paragraphStyle.lineSpacing = lineSpacing
        paragraphStyle.lineHeightMultiple = lineHeightMultiple

        let attributedString:NSMutableAttributedString
        if let labelattributedText = self.attributedText {
            attributedString = NSMutableAttributedString(attributedString: labelattributedText)
        } else {
            attributedString = NSMutableAttributedString(string: labelText)
        }

        // Line spacing attribute

// Swift 4.2++


attributedString.addAttribute(NSAttributedString.Key.paragraphStyle, value:paragraphStyle, range:NSMakeRange(0, attributedString.length))

// Swift 4.1--
attributedString.addAttribute(NSAttributedStringKey.paragraphStyle, value:paragraphStyle, range:NSMakeRange(0, attributedString.length))

        self.attributedText = attributedString
    }
}

Now call extension function

let label = UILabel()
let stringValue = "How to\ncontrol\nthe\nline spacing\nin UILabel"

// Pass value for any one argument - lineSpacing or lineHeightMultiple
label.setLineSpacing(lineSpacing: 2.0) .  // try values 1.0 to 5.0

// or try lineHeightMultiple
//label.setLineSpacing(lineHeightMultiple = 2.0) // try values 0.5 to 2.0

Or using label instance (Just copy & execute this code to see result)

let label = UILabel()
let stringValue = "How to\ncontrol\nthe\nline spacing\nin UILabel"
let attrString = NSMutableAttributedString(string: stringValue)
var style = NSMutableParagraphStyle()
style.lineSpacing = 24 // change line spacing between paragraph like 36 or 48
style.minimumLineHeight = 20 // change line spacing between each line like 30 or 40

// Swift 4.2++
// Line spacing attribute
attrString.addAttribute(NSAttributedString.Key.paragraphStyle, value: style, range: NSRange(location: 0, length: stringValue.characters.count))

// Character spacing attribute
attrString.addAttribute(NSAttributedString.Key.kern, value: 2, range: NSMakeRange(0, attrString.length))


// Swift 4.1--
// Line spacing attribute
attrString.addAttribute(NSAttributedStringKey.paragraphStyle, value: style, range: NSRange(location: 0, length: stringValue.characters.count))

// Character spacing attribute
attrString.addAttribute(NSAttributedStringKey.kern, value: 2, range: NSMakeRange(0, attrString.length))

label.attributedText = attrString

Swift 3

let label = UILabel()
let stringValue = "How to\ncontrol\nthe\nline spacing\nin UILabel"
let attrString = NSMutableAttributedString(string: stringValue)
var style = NSMutableParagraphStyle()
style.lineSpacing = 24 // change line spacing between paragraph like 36 or 48
style.minimumLineHeight = 20 // change line spacing between each line like 30 or 40
attrString.addAttribute(NSParagraphStyleAttributeName, value: style, range: NSRange(location: 0, length: stringValue.characters.count))
label.attributedText = attrString

I tried all these answers, but to really get the EXACT line height that usually comes in design files from Sketch or Zeplin then you need to:

let ps = NSMutableParagraphStyle()
ps.minimumLineHeight = 34
ps.maximumLineHeight = 34
let attrText = NSAttributedString(
    string: "Your long multiline text that will have exact line height spacing",
    attributes: [
        .paragraphStyle: ps
    ]
)
someLabel.attributedText = attrText
someLabel.numberOfLines = 2
...