To display bordered text (if I understand correctly what you want) you should set text drawing mode to kCGTextFillStroke
(and set appropriate values for text drawing parameters, such as stroke and fill colors etc)
// Choose appropriate text font
CGContextSelectFont(context, [[UIFont boldSystemFontOfSize:24].fontName UTF8String], (int)fontSize, kCGEncodingMacRoman);
// Set text drawing mode
CGContextSetTextDrawingMode(context, kCGTextFillStroke);
// Set appropriate color/line width values
// Here we're set to draw white letters with black border
CGContextSetRGBStrokeColor(context, 0, 0, 0, 1);
CGContextSetRGBFillColor(context, 1, 1, 1, 1);
CGContextSetLineWidth(context, 1);
// Set this transformations or text will be displayed upside-down
CGAffineTransform xform = CGAffineTransformMake(1.0, 0.0, 0.0, -1.0, 0.0, 0.0);
CGContextSetTextMatrix(context, xform);
// Display text
CGContextShowTextAtPoint(...);