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Truncate a String and Append an Ellipsis, Respecting the Font Size

Posted on July 26, 2010 by John Muchow in Cocoa


  

A number of the UI related controls will automatically truncate and append ellipsis with no effort required on your part. For example, with UILabel you can specify the linebreak mode to indicate how you would like the system to manage wrapping and truncating the label text.

However, they are times when having a method to truncate a string without using a UI control would be handy. In this post I’ll create a category which will add a method to the NSString class to do just that. You can read more about working with categories in this post: Introduction to Categories.

NSString Category

Start by creating a new interface file with the name: NSString+TruncateToWidth.h – appending the method name to the class is a common naming convention when working with categories.

The code below defines a method that will be available to all NSString objects. The method will truncate a string to a specific width, using the font size to properly determine the string size before truncating:

@interface NSString (TruncateToWidth)
 
- (NSString*)stringByTruncatingToWidth:(CGFloat)width withFont:(UIFont *)font;
 
@end
Code to Truncate NSString

The implementation of the stringByTruncatingToWidth method follows, add this code to the NSString+TruncateToWidth.m file.

#import "NSString+TruncateToWidth.h"
 
#define ellipsis @"…"
 
@implementation NSString (TruncateToWidth)
 
- (NSString*)stringByTruncatingToWidth:(CGFloat)width withFont:(UIFont *)font
{
  // Create copy that will be the returned result
  NSMutableString *truncatedString = [[self mutableCopy] autorelease];
 
  // Make sure string is longer than requested width
  if ([self sizeWithFont:font].width > width)
  {
    // Accommodate for ellipsis we'll tack on the end
    width -= [ellipsis sizeWithFont:font].width;
 
    // Get range for last character in string
    NSRange range = {truncatedString.length - 1, 1};
 
    // Loop, deleting characters until string fits within width
    while ([truncatedString sizeWithFont:font].width > width) 
    {
      // Delete character at end
      [truncatedString deleteCharactersInRange:range];
 
      // Move back another character
      range.location--;
    }
 
    // Append ellipsis
    [truncatedString replaceCharactersInRange:range withString:ellipsis];
  }
 
  return truncatedString;
}
 
@end

Notice that the desired final width of the string is passed in, as well as the font that will be used when calculating the size (width) of the string. The process is quite simple: change the incoming width to accommodate an ellipsis on the end of the string – loop through the string removing characters from the end until the desired width is reached – append an ellipsis onto the string.

Calling the Truncate NSString Method

You can now call the method stringByTruncatingToWidth:withFont: on any NSString object:

NSString *str = @"Lorem ipsum dolor sit amet, consectetuer adipiscing";
 
// Adjust the width parameter to change the final string width
str = [str stringByTruncatingToWidth:170 withFont:[UIFont systemFontOfSize:15]];
NSLog(@"%@", str);

The output from above would look as follows:

Lorem ipsum dolor sit…

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