Programming iOS 4

Colors and Patterns

A color is a CGColor (actually a CGColorRef). CGColor is not difficult to work with, and is bridged to UIColor through UIColor’s colorWithCGColor: and CGColor methods.

A pattern, on the other hand, is a CGPattern (actually a CGPatternRef). You can create a pattern and stroke or fill with it. The process is rather elaborate. As an extremely simple example, I’ll replace the red triangular arrowhead with a red-and-blue striped triangle (Figure 15-7). To do so, remove this line:

Figure 15-7. A patterned fill

CGContextSetFillColorWithColor(con, [[UIColor redColor] CGColor]);

In its place, put the following:

CGColorSpaceRef sp2 = CGColorSpaceCreatePattern(NULL);
CGContextSetFillColorSpace (con, sp2);
CGColorSpaceRelease (sp2);
CGPatternCallbacks callback = {
    0, &drawStripes, NULL
};
CGAffineTransform tr = CGAffineTransformIdentity;
CGPatternRef patt = CGPatternCreate(NULL,
                      CGRectMake(0,0,4,4),
                      tr,
                      4, 4,
                      kCGPatternTilingConstantSpacingMinimalDistortion,
                      true,
                      &callback);
CGFloat alph = 1.0;
CGContextSetFillPattern(con, patt, &alph);
CGPatternRelease(patt);

That code is verbose, but it is almost entirely boilerplate. To understand it, it almost helps to read it backward. What we’re leading up to is the call to CGContextSetFillPattern; instead of setting a fill color, we’re setting a fill pattern, to be used the next time we fill a path (in this case, the triangular arrowhead). ...