CoreText and RTF

Sample Unicode: 魚もたばません

Cello has text so it has support for line layout and line breaking. It has classes that represent Paragraphs, Styles, Style Runs and Layout blocks etc. Down at the metal the text is serialized (written and read) as RTF. Cello has its own RTF reader and writer. All this has to mesh with CoreText - that is where the glyph curves ultimately come from. Also in the back of my mind is the thought that I need to support Unicode, bidirectional line layout - and ultimately (I hope) vertical text.

I am still exploring the best course of action - but currently the solution that seems to be optimal is to use CoreText for everything.

Styled text is represented in Core Text as a NS/CFAttributedString. The NS/CFAttributedString is a Unicode string with runs of attributes. In the all roads lead to Rome approach of Cocoa and Core Foundation attributes are expressed as NS/CFDictionaries. So, for example, to set a font you would essentially create a dictionary with the kCTFontAttributeName attribute set to the CTFont that expressed the font that you wanted. To make the world a simpler place the API makes it possible to set individual attributes without having to create a dictionary first.

Cocoa also uses NS/CFAttributedString to represent styled text and has the capability to create an attributed string directly from an RTF source. So the following code fragment will read an RTF file from a URL.

CFAttributedStringRef ReadRTF(CFURLRef url)
{
NSData *data = [NSData dataWithContentsOfURL:(NSURL *)url];
NSAttributedString *result = [[NSAttributedString alloc] initWithRTF:data documentAttributes:NULL];
return (CFAttributedStringRef) result;
}

I modified Apple's CoreTextTest sample code to import RTF using the code sinippet above. The resulting CFAttributedString works fine in Core Text except that NSColor is not suported. The console fills with messages:

CGContextSetFillColorWithColor: invalid context
CGContextSetStrokeColorWithColor: invalid context

The reason behind this seems to be that cocoa attributed string uses NSColor, Core Text uses CGColor the two objects are not toll free bridged and are not the same. This does no look like a hard thing to fix up after reading, and before writing.

One of the advantages of using the Cocoa RTF import is that, unlike the Cello import, it supports unicode.

Here is a screenshot of this post saved as RTF being drawn by Core Text with a little unicode.