Using CGDataProviderCreateWithData callback - iphone

Question

I'm using CGDataProviderCreateWithData to (eventually) create a UIImage from a malloced array of bytes. I call CGDataProviderCreateWithData like this:
provider = CGDataProviderCreateWithData(NULL, dataPtr, dataLen, callbackFunc);
where
dataPtr is the previously malloced array of data bytes for the image,
dataLen is the number of bytes in the dataPtr array, and
callbackFunc is as described in the CGDataProviderCreateWithData documentation:
void callbackFunc(void *info, const void *data, size_t size);
The callback function is called when the data provider is released so I could free() dataPtr there, but I may want to continue using it (dataPtr) and at some later stage free it. This block of code will be called multiple times, and the flow will look something like:
malloc(dataPtr)
create image (call CGDataProviderCreateWithData etc)
display image
release image (and so release data provider created by CGDataProviderCreateWithData)
continue to use dataPtr
free(dataPtr)
so 1..6 may be executed multiple times. I don't want dataPtr hanging around for the entire execution of the program (and it may change in size anyway), so I want to malloc/free it as necessary.
The problem is that I can't free(dataPtr) in the callback from CGDataProviderCreateWithData because I still want to use it, so I want to free it some time later - and I can't free it until I know that the data provider no longer needs it (as far as I can tell CGDataProviderCreateWithData uses the array I pass, it doesn't take a copy).
I can't do (1) above until I know it is ok to free and re-malloc dataPtr, so what I really want to do is block waiting for the data provider to be freed (well, I want to know whether I should re-enter the 1..6 block of code, which I can't do until the data provider is freed). It will be - I create the data provider, create the image and immediately display it and release the data provider. The trouble is that the data provider isn't actually released until the UIImage is released and is finished with it.
I'm reasonably new to objective-c and iOS. Am I missing something obvious?

Answer 1

If you malloc the memory for the provider's data, you really want to to free it in the callback. Do not be tempted to try to circumvent this in any manner. It be too easy to leak and would be susceptible to simple memory issues.
Having said that, there are two simple solutions that address your question. Either:
Make your own copy of the data that you'll manage separately from the provider; or
Instead of using the void * renditions of the CGDataProvider methods, use the CFData rendition (e.g. CGDataProviderCreateWithCFData) and then you can maintain your own strong reference to this data object (or if in non-ARC code, do your own retain of the data object). The object will not be deallocated until all strong references are resolved (or, in non-ARC code, all of your manual retain calls are resolved with a corresponding release or autorelease call).
With either of these approaches, you can continue to let CGProviderRef manage the memory as it sees fit, but you can continue to use the data object for your own purposes, too.

Answer 2

I ran into a similar problem as well. I wanted to use CGImageCreateWithJPEGDataProvider, so used CGDataProviderRef with a malloc'd byte array. I was getting the same problem, when trying to free the byte array at some point after creating the Data Provider Reference.
It occurs because the data provider reference takes over the ownership of the byte array, so can only be freed by the reference, when it is done with it.
I agree with #TheBlack in that if you need the data elsewhere, make a copy of it.

Answer 3

What I'm doing is essentially the same to what you want to achieve, I just took different route.
This whole process is wrapped into NSOperation so you have control over scheduling memory usage.
CGImageRef imageRef = [image CGImage];
NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
CGColorSpaceRef colorSpace = CGImageGetColorSpace(imageRef);
NSUInteger bitsPerComponent = CGImageGetBitsPerComponent(imageRef);
UInt8 *rawData = calloc((height * width * bitsPerComponent), sizeof(UInt8));
NSUInteger bytesPerRow = CGImageGetBytesPerRow(imageRef);
CGContextRef context = CGBitmapContextCreate(rawData, width, height,
bitsPerComponent, bytesPerRow, colorSpace,
kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Host);
if (context)
{
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
// This is the place you get access to data trough rawData. Do whatever you want with it, when done, get image with
CGImageRef newImg = CGBitmapContextCreateImage(context);
}
OR
subclass CALayer, override drawInContext and use CGBitmapContextGetData to get raw data. Don't have much experience with this though, sorry, but if you want to see instant changes
on image based on user input, I'd do it this way:
Subclass UIView and CALayer which becomes layer for view and displays image. View gets input and image data (from CGBitmapContextGetData) in layer class is manipulated based on input.
Even CATiledLayer can be used for huge images in this way. When done with image, just release UIView and replace it with new one. I'll gladly provide help if you need any, just ping here.