Question

I want to keep a mutable collection of CGImageRefs. Do I need to wrap them in NSValue, and if so how do I wrap and unwrap them properly? Can I get away with using a C array? If so how do I construct it and how do I add elements to it later? Is it significantly more costly to use UIImages instead of CGImageRefs as the elements of the collection?

Answer 1

Getting the CGImageRef out of an UIImage via image.CGImage can be costly. From the documentation:

If the image data has been purged because of memory constraints, invoking this method forces that data to be loaded back into memory. Reloading the image data may incur a performance penalty.

If you feel comfortable with mixing C++ and Objective-C, you can use a std::vector for storing the CGImageRef. Rename your source file from .m to .mm and try this:

#include <vector>
...
CGImageRef i;
...
std::vector<CGImageRef> images;
images.push_back(i);

If you want to keep the vector as a member of a Objective-C class, you should allocate it on the heap, not the stack:

Header file:

#include <vector>
using std;

@interface YourInterface : ...
{
   vector<CGImageRef> *images;
}

and in the implementation file:

images = new std::vector<CGImageRef>();
images->push_back(i);
...
//When you're done
delete images;
images = NULL;