CTFontGetGlyphsForCharacters always returns false

Question

Please help me understand the problem with the following code:

NSString *fontName = @"ArialMT";
CGFloat fontSize = 20.0;
CTFontRef fontRef = CTFontCreateWithName((CFStringRef)fontName, fontSize, NULL);
NSString *characters = @"ABC";
NSUInteger count = characters.length;
CGGlyph glyphs[count];
if (CTFontGetGlyphsForCharacters(fontRef, (const unichar*)[characters cStringUsingEncoding:NSUTF8StringEncoding], glyphs, count) == false)
    NSLog(@"*** CTFontGetGlyphsForCharacters failed.");

Any help is appreciated.

Answer 1

You are getting a C string containing UTF-8 encoded characters and then casting it to unichar *. That won't work. A unichar is a 16-bit UTF-16-encoded character. A simple C cast won't convert the character encoding.

You need to ask the string for its characters as an array of unichar. In Objective-C:

NSString *fontName = @"ArialMT";
CGFloat fontSize = 20.0;
CTFontRef fontRef = CTFontCreateWithName((CFStringRef)fontName, fontSize, NULL);
NSString *string = @"ABC";
NSUInteger count = string.length;
unichar characters[count];
[string getCharacters:characters range:NSMakeRange(0, count)];
CGGlyph glyphs[count];
if (CTFontGetGlyphsForCharacters(fontRef, characters, glyphs, count) == false) {
    NSLog(@"*** CTFontGetGlyphsForCharacters failed.");
}

In Swift, you can get the UTF-16 encoding of a String by asking for its utf16 property. That returns a UTF16View, which you then need to convert to an Array.

import Foundation
import CoreText

extension CTFont {
    func glyphs(for string: String) -> [CGGlyph]? {
        let utf16 = Array(string.utf16)
        var glyphs = [CGGlyph](repeating: 0, count: utf16.count)
        guard CTFontGetGlyphsForCharacters(font, utf16, &glyphs, utf16.count) else {
            return nil
        }
        return glyphs
    }
}

let font = CTFontCreateWithName("ArialMT" as CFString, 20, nil)
print(font.glyphs(for: "Hello"))