Efficiently determine how much text can fit in a UILabel in IOS - optimization

Question

I have an NSString and I want to know how much of that string will fit into a UILabel.
My code builds a test string by adding one character at a time from my original string. Each time I add a character I test the new string to see if it will fit into my label:
CGRect cutTextRect = [cutText boundingRectWithSize:maximumLabelSize options:NSStringDrawingUsesLineFragmentOrigin attributes:stringAttributes context:nil];
Then I compare the height of that rect to the height of my label to see if the string overflowed.
This works, but instruments shows me that the loop is taking up all my cpu time.
Can anyone think of or know of a faster way to do this?
Thanks!

Answer 1

While not the prettiest:
- (NSString *)stringForWidth:(CGFloat)width fullString:(NSString *)fullString
{
NSDictionary *attributes = #{NSFontAttributeName: label.font};
if ([fullString sizeWithAttributes:attributes].width <= width)
{
return fullString;
}
// Might be worth researching more regarding 'average' char size
CGFloat approxCharWidth = [#"N" sizeWithAttributes:attributes].width;
NSInteger approxNumOfChars = (NSInteger)(width / approxCharWidth);
NSMutableString *resultingString = [NSMutableString stringWithString:[fullString substringToIndex:approxNumOfChars]];
CGFloat currentWidth = [resultingString sizeWithAttributes:attributes].width;
if (currentWidth < width)
{
// Try to 'sqeeze' another char.
while (currentWidth < width && approxNumOfChars < fullString.length)
{
approxNumOfChars++;
[resultingString appendString:[fullString substringWithRange:NSMakeRange(approxNumOfChars - 1, 1)]];
currentWidth = [resultingString sizeWithAttributes:attributes].width;
}
}
// String might be oversized
if (currentWidth > width)
{
while (currentWidth > width)
{
[resultingString deleteCharactersInRange:NSMakeRange(resultingString.length - 1, 1)];
currentWidth = [resultingString sizeWithAttributes:attributes].width;
}
}
// If dealing with UILabel, it's safer to have a smaller string than 'equal',
// 'clipping wise'. Otherwise, just use '<=' or '>=' instead of '<' or '>'
return [NSString stringWithString:resultingString];
}
There are a couple of loops, but each one is a 'fine tuning' and should only run a small number of times.
One way to improve efficiency is to get a better starting point than calculating be how many N's can fit in a given width.
I'm open to suggestions about that.
-Edit:
Regarding multiline label, once I know a given text for width, I can expect the following text (if any) will go to the next line.
In other words, getting 'text for width' is the tricky part, 'width for text' we get for free.