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:

1. malloc(dataPtr)
2. create image (call CGDataProviderCreateWithData etc)
3. display image
4. release image (and so release data provider created by CGDataProviderCreateWithData)
5. continue to use dataPtr
6. 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

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.