DTCoreText Programming Guide

This document is meant to serve as a collection of programming questions related to DTCoreText.

Smoke Test

After having integrated DTCoreText and its dependencies into your project you should be able to build your app be able to use DTCoreText functionality. As a quick Smoke Test - to see if all is setup correctly - you can test your setup by adding this code to your app delegate:

#import "DTCoreText.h"

NSString *html = @"<p>Some Text</p>";
NSData *data = [html dataUsingEncoding:NSUTF8StringEncoding];

NSAttributedString *attrString = [[NSAttributedString alloc] initWithHTMLData:data documentAttributes:NULL];
NSLog(@"%@", attrString);

You should see that this executes and that the NSLog outputs a description of the generated attributed string.

Font Matching Performance

DTCoreText employs an internal lookup table which contains a font face name for each combination of font family and bold and italic traits. This lookup table can be initialized by including a DTCoreTextFontOverrides.plist in your app bundle and/or prepopulating it with all available system fonts. Which of these you want to use depends on your app.

If you only use a very limited number of fonts you should have the plist file contain only these.

For most normal use cases you can use the overrides plist that is part of the DTCoreText demo app. This contains most commonly used fonts on iOS.

If you don’t know the set of fonts used by your app you can trigger an asynchronous pre-loading of the internal lookup table. To start the loading process you add the following to your app delegate.

// preload font matching table
[DTCoreTextFontDescriptor asyncPreloadFontLookupTable];

Calling this does not replace entries already existing in the lookup table, for example loaded from the DTCoreTextFontOverrides.plist included in the app bundle.

Setting a Fallback Font Family

When encountering a font family in HTML that is not known to the system the fallback font family is used. This can be set like this:

[DTCoreTextFontDescriptor setFallbackFontFamily:@"Helvetica Neue"];

Note that the font family name must be valid on the system that this run on, either because it is a system font or a font you have installed at runtime. If you try to set an invalid font family name an exception will be thrown.

Getting a Tapped Word

To retrieve the word a user tapped on you get the closest cursor position to the tapped point. Then you iterate over the plain text’s words until you find the one that contains the cursor position’s string index.

- (void)handleTap:(UITapGestureRecognizer *)gesture
    if (gesture.state == UIGestureRecognizerStateRecognized)
        CGPoint location = [gesture locationInView:_textView];
        NSUInteger tappedIndex = [_textView closestCursorIndexToPoint:location];

        __block NSRange wordRange = NSMakeRange(0, 0);

        [plainText enumerateSubstringsInRange:NSMakeRange(0, [plainText length]) options:NSStringEnumerationByWords usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
            if (NSLocationInRange(tappedIndex, enclosingRange))
                *stop = YES;
                wordRange = substringRange;

        NSString *word = [plainText substringWithRange:wordRange];
        NSLog(@"%d: '%@' word: '%@'", tappedIndex, tappedChar, word);

Visible String Range

To retrieve the string range in the NSAttributedString you set on an DTAttributedTextView you have to get the scroll view bounds. Then you retrieve an array of lines visible in this rectangle from the DTCoreTextLayoutFrame. Finally you retrieve and create a union of the string ranges.

CGRect visibleRect = _textView.bounds;
NSArray *visibleLines = [_textView.attributedTextContentView.layoutFrame linesVisibleInRect:visibleRect];

NSRange stringRange = [visibleLines[0] stringRange];
stringRange = NSUnionRange([[visibleLines lastObject] stringRange], stringRange);

NSLog(@"visible string range: %@", NSStringFromRange(stringRange));

Determing Size Required for an Attributed String

When creating a DTCoreTextLayoutFrame you can specify the maximum width and height that should be filled with text. If you specify CGFLOAT_WIDTH_UNKNOWN for the frame size width then the needed with will be calculated. If you specify CGFLOAT_HEIGHT_UNKNOWN the height will be calculated. You can get the needed size from the layoutFrame’s frame property.

NSAttributedString *attributedString = ...
DTCoreTextLayouter *layouter = [[DTCoreTextLayouter alloc] initWithAttributedString:attributedString];

NSRange entireString = NSMakeRange(0, [attributedString length]);
DTCoreTextLayoutFrame *layoutFrame = [layouter layoutFrameWithRect:maxRect range:entireString];

CGSize sizeNeeded = [layoutFrame frame].size;

Displaying remote images

The best way to display remote images is to use DTLazyImageView. First you will need to return DTLazyImageView instance for your image attachments.

- (UIView *)attributedTextContentView:(DTAttributedTextContentView *)attributedTextContentView viewForAttachment:(DTTextAttachment *)attachment frame:(CGRect)frame
    if([attachment isKindOfClass:[DTImageTextAttachment class]])
        DTLazyImageView *imageView = [[DTLazyImageView alloc] initWithFrame:frame];
        imageView.delegate = self;

        // url for deferred loading
        imageView.url = attachment.contentURL;
        return imageView;
    return nil;

Then in the in delegate method for DTLazyImageView reset the layout for the affected DTAttributedContextView.

- (void)lazyImageView:(DTLazyImageView *)lazyImageView didChangeImageSize:(CGSize)size 
    NSURL *url = lazyImageView.url;
    NSPredicate *pred = [NSPredicate predicateWithFormat:@"contentURL == %@", url];

    // update all attachments that matching this URL
    for (DTTextAttachment *oneAttachment in [self.attributedTextContentView.layoutFrame textAttachmentsWithPredicate:pred]) 
        oneAttachment.originalSize = size;

    // need to reset the layouter because otherwise we get the old framesetter or cached layout frames
    self.attributedTextContentView.layouter = nil;

    // here we're layouting the entire string,
    // might be more efficient to only relayout the paragraphs that contain these attachments
    [self.attributedTextContentView relayoutText];