Swift 如何从其字体计算一行文本的高度

Question

I ran into an issue where I needed to animate translating a label vertically the same distance of a textField's text height. In most cases just textField.bounds.heigt but if the textField's height is bigger than the text height it will not be any good for me. So I need to know: How to calculate the line height of the string text from its UIFont ?

Regarding the duplicate: There's a little bit different of what I need. that answer(which I've referenced in my answer) get the total height depending on 1) the string 2) the width 3) the font. What I needed is one line height dpending only on the font .

Answer 1

UIFont has a property lineHeight :

    if let font = _textView.font {
        let height = font.lineHeight
    }

where font is your font

Answer 2

I have been searching for a way to do that and find this answer where it has a String extension to calculate the size for the string and a given font. I have modified it to do what I want ( get the line height of text written using a font. ):

extension UIFont {
    func calculateHeight(text: String, width: CGFloat) -> CGFloat {
        let constraintRect = CGSize(width: width, height: CGFloat.greatestFiniteMagnitude)
        let boundingBox = text.boundingRect(with: constraintRect,
                                        options: NSStringDrawingOptions.usesLineFragmentOrigin,
                                        attributes: [NSAttributedStringKey.font: self],
                                        context: nil)
        return boundingBox.height
    }
}

I hope this helps someone looking for it. ( may be myself in the future ).

Answer 3

I've used this String extension in the past to draw some text as opposed to creating a UILabel somewhere. I don't like the fact that I can't seem to get the real height of the specific text I want to draw (not every string contains capital letters or characters with descenders, etc.) I've used a couple of enums for horizontal and vertical alignment around the given point. Open to ideas on the vertical height.

public func draw(at pt: CGPoint,
                 font: UIFont? = UIFont.systemFont(ofSize: 12),
                 color: UIColor? = .black,
                 align: HorizontalAlignment? = .Center,
                 vAlign: VerticalAlignment? = .Middle)
{
    let attributes: [NSAttributedString.Key : Any] = [.font: font!,
                                                      .foregroundColor: color!]
    
    let size = self.boundingRect(with: CGSize(width: 0, height: 0),
                                 options: [ .usesFontLeading ],
                                 attributes: [ .font: font! ],
                                 context: nil).size
    var x = pt.x
    var y = pt.y
    if align == .Center {
        x -= (size.width / 2)
    } else if align == .Right {
        x -= size.width
    }
    if vAlign == .Middle {
        y -= (size.height / 2)
    } else if  vAlign == .Bottom {
        y -= size.height
    }
    let rect = CGRect(x: x, y: y, width: size.width, height: size.height)
    draw(in: rect, withAttributes: attributes)
}