How to tell when a UILabel will be truncated an/or its line break position will change

Question

I have a multi-line UILabel (numberOfLines = 0). It's width can change at runtime, and sometimes this leads to truncation and/or re-wrapping. Some examples illustrate this best:

Example 1: the reduction in width leads to a different line break point

Example 2: the reduction in width leads to truncation

Example 3: the reduction in width leads to both truncation and a different line break position

Example 4: the reduction in width does not have any effect on truncation or line break position

Since this change in formatting can be quite jarring, I intend to mask it behind some animation (probably a fade in/fade out). However, the first hurdle is identifying when I need to do this. I don't want to apply the animation whenever the label re-sizes - only when it will cause a change in either truncation or line break positions.

How might I test this? The test should return YES for example 1, 2, and 3, but NO for example 4.

Note: the resizing will never alter the number of lines in my example.

Note 2: if anyone has some better tags related to text formatting I'd love to know them - feel free to edit.

Note 3: if you are interested in seeing this behavior accomplished, try Apple's mail.app on the iPhone. When viewing the inbox, swipe an email and watch the summary line fade-in/out as it re-wraps and/or truncates (but not when it doesn't need to).

Answer 1

Swift 3 solution

You can count the number of lines after assigning the string and compare to the max number of lines of the label.

import Foundation
import UIKit

extension UILabel {

    func countLabelLines() -> Int {
        // Call self.layoutIfNeeded() if your view is uses auto layout
        let myText = self.text! as NSString
        let attributes = [NSFontAttributeName : self.font]

        let labelSize = myText.boundingRect(with: CGSize(width: self.bounds.width, height: CGFloat.greatestFiniteMagnitude), options: NSStringDrawingOptions.usesLineFragmentOrigin, attributes: attributes, context: nil)
        return Int(ceil(CGFloat(labelSize.height) / self.font.lineHeight))
    }

    func isTruncated() -> Bool {

        if (self.countLabelLines() > self.numberOfLines) {
            return true
        }
        return false
    }
}