How to create a category to extend CGPaths?

Question

I would like to create a category to "extend" the "class" of CGPath and add more functionality. Because CGPath is not a regular object of ObjectiveC how do I do that?

Excuse my lack of knowledge about c/c++

Answer 1

Simple, you just write C functions. You should be careful about name-collisions though, so add a prefix to the names of the function. If you need a function that creates a CGPath from NSData* or sth. like that, you would just declare and implement a C function named RDCGPathCreateWithData(NSData *data) (using RD-prefix for RubberDuck ;D).

In your header files, just declare the function, and implement them in the implementation files. For my example this would look something like this:

RDCGPath.h

#ifndef RDCGPATH_H
#define RDCGPATH_H

CGPathRef RDCGPathCreateWithData(NSData *data);
// any other function declarations

#endif

RDCGPath.m

#include "RDCGPath.h"

CGPathRef RDCGPathCreateWithData(NSData *data) {
    //...
}

// imlementation of other functions

The preprocessor directives in the .h file are called include guards. They protect you from errors due to having the same declaration multiple times.

Answer 2

You may want to look at UIBezierPath. It wraps an Objective-C API around CGPath, which you could extend.

Beyond that, if you look at how you modify CGPaths, you just call C functions to add points and move, so you could create your own functions, e.g.:

void myCGPathStarInRect(CGPathRef path, CGRect rect) {
    CGPathMoveTo(path, NULL, CGRectGetMidX(rect), CGRectGetMinY(rect));
    // ... draw the rest of the star.
}

Answer 3

You cannot add categories to C "objects" like CGPathRef. Categories are an ObjC feature. But there's no reason to anyway. Just make a new function that takes a CGPathRef.