UILabel with a (custom) CGFont

Thu, 26. Aug 2010

Categories: en development Tags: CGFont Cocoa iOS iPhone Objective C OTF TrueType TTF UIFont UILabel

UILabel’s font property accepts UIFonts – but strange enough there’s no way to get a custom loaded CGFont (from a ttf or otf file)Β converted into such an UIFont. You’re stuck with the iPhone’s pre-installed fonts – at least when you have to support iOS 3.0 devices.

After googling a bit and searching Stackoverflow I found the solutions presented there not ideal or great, but too heavy weight.

So I inherited UILabel with a very lean custom class UILabelWithCGFont and overloaded it’s drawTextInRect: method like this:

-(void)drawTextInRect:(CGRect)rect
{
  MRLogD(@"(%f,%f) (%f,%f)", rect.origin.x, rect.origin.y, rect.size.width, rect.size.height);
  if ( _CGFont == NULL ) {
    [super drawTextInRect:rect];
    return;
  }
  NSAssert(_mapping != NULL, @"Mapping function pointer not set.");

  // prepare the target graphics context.
  const CGContextRef ctx = UIGraphicsGetCurrentContext();
  CGContextSaveGState(ctx);
  {
    // prepare the glyphs array to draw
    const NSString *txt = self.text;
    const size_t glyphCount = txt.length;
    CGGlyph glyphs[glyphCount];
    {
      // turn the string txt into glyphs (indices into the font):
      // give non-allocating unicode character retrieval a try:
      const UniChar *raw_unichars = CFStringGetCharactersPtr( (CFStringRef)txt );
      const UniChar *unichars = raw_unichars == NULL ? malloc( glyphCount * sizeof(UniChar) ) : raw_unichars;
      NSAssert(unichars != NULL, @"unichars not allocated");
      if ( raw_unichars == NULL )
        CFStringGetCharacters( (CFStringRef)txt, CFRangeMake(0, txt.length), (UniChar *)unichars );
      for ( int i = glyphCount - 1; i >= 0; i-- )
        glyphs[i] = _mapping(unichars[i]);
      if ( raw_unichars == NULL )
        free( (void *)unichars );
    }

    CGContextSetFont(ctx, _CGFont);
    CGContextSetFontSize(ctx, self.font.pointSize);
    CGContextSetTextMatrix( ctx, CGAffineTransformMake(1.0, 0.0, 0.0, -1.0, 0.0, 0.0) );

    // first print 'invisible' to measure size:
    CGContextSetTextDrawingMode(ctx, kCGTextInvisible);
    const CGPoint pre = CGContextGetTextPosition(ctx);
    CGContextShowGlyphs(ctx, glyphs, glyphCount);
    const CGPoint post = CGContextGetTextPosition(ctx);
    // restore text position
    CGContextSetTextPosition(ctx, pre.x, pre.y);

    // centered horizontal + vertical:
    NSAssert( (int)rect.origin.x == 0, @"origin.x not zero" );
    NSAssert( (int)rect.origin.y == 0, @"origin.y not zero" );
    NSAssert(self.baselineAdjustment == UIBaselineAdjustmentAlignCenters, @"vertical alignment not 'center'");
    NSAssert(self.textAlignment == UITextAlignmentCenter, @"horizontal alignment not 'center'");
    const CGPoint p = CGPointMake( ( rect.size.width - (post.x - pre.x) ) / 2, (rect.size.height + self.font.pointSize + pre.y) / 2 );

    // finally render it to the graphics context:
    CGContextSetTextDrawingMode(ctx, kCGTextFill);
    CGContextSetFillColorWithColor(ctx, self.textColor.CGColor);
    CGContextShowGlyphsAtPoint(ctx, p.x, p.y, glyphs, glyphCount);
  }
  CGContextRestoreGState(ctx);
}

Usage: Just turn the UILabel instances in Interface Builder into UILabelWithCGFont and implement the UIViewController::viewDidLoad method like this:

CGGlyph unicode2glyphDeutscheDruckschrift(UniChar c)
{
  if ( '0' < = c && c <= '9' )
    return c + (16 - '0');
  if ( 'A' <= c && c <= 'Z' )
    return c + (32 - 'A');
  if ( 'a' <= c && c <= 'z' )
    return c + (58 - 'a');
  return 0;
}

-(void)viewDidLoad
{
  [super viewDidLoad];
  ...
  [fontLabel setFontFromFile:@"DeutscheDruckschrift" ofType:@"ttf" mapping:unicode2glyphDeutscheDruckschrift];
  ...
}

See this github gist for the complete implementation.

The mapping from Unicode character codes to glyph indices (inside the font description) currently is done via a C mapping function you have to provide a function pointer for. A later implementation could map the unicode character code to the glyph name and leverage Β CGFontGetGlyphWithGlyphName and render the custom mapping function obsolete.