UIKit may be used on the main thread only. Your code is therefore technically invalid, since you use UIImage from a thread other than the main thread. You should use CoreGraphics alone to load (and non-lazily decode) graphics on a background thread, post the CGImageRef to the main thread and turn it into a UIImage there. It may appear to work (albeit with the stutter you don’t want) in your current implementation, but it isn’t guaranteed to. There seems to be a lot of superstition and bad practice advocated around this area, so it’s not surprising you’ve managed to find some bad advice…
Recommended to run on a background thread:
// get a data provider referencing the relevant file
CGDataProviderRef dataProvider = CGDataProviderCreateWithFilename(filename);
// use the data provider to get a CGImage; release the data provider
CGImageRef image = CGImageCreateWithPNGDataProvider(dataProvider, NULL, NO,
kCGRenderingIntentDefault);
CGDataProviderRelease(dataProvider);
// make a bitmap context of a suitable size to draw to, forcing decode
size_t width = CGImageGetWidth(image);
size_t height = CGImageGetHeight(image);
unsigned char *imageBuffer = (unsigned char *)malloc(width*height*4);
CGColorSpaceRef colourSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef imageContext =
CGBitmapContextCreate(imageBuffer, width, height, 8, width*4, colourSpace,
kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);
CGColorSpaceRelease(colourSpace);
// draw the image to the context, release it
CGContextDrawImage(imageContext, CGRectMake(0, 0, width, height), image);
CGImageRelease(image);
// now get an image ref from the context
CGImageRef outputImage = CGBitmapContextCreateImage(imageContext);
// post that off to the main thread, where you might do something like
// [UIImage imageWithCGImage:outputImage]
[self performSelectorOnMainThread:@selector(haveThisImage:)
withObject:[NSValue valueWithPointer:outputImage] waitUntilDone:YES];
// clean up
CGImageRelease(outputImage);
CGContextRelease(imageContext);
free(imageBuffer);
There’s no need to do the malloc/free if you’re on iOS 4 or later, you can just pass NULL as the relevant parameter of CGBitmapContextCreate, and let CoreGraphics sort out its own storage.
This differs from the solution you post to because it:
- creates a CGImage from a PNG data source — lazy loading applies, so this isn’t necessarily a fully loaded and decompressed image
- creates a bitmap context of the same size as the PNG
- draws the CGImage from the PNG data source onto the bitmap context — this should force full loading and decompression since the actual colour values have to be put somewhere we could access them from a C array. This step is as far as the forceLoad you link to goes.
- converts the bitmap context into an image
- posts that image off to the main thread, presumably to become a UIImage
So there’s no continuity of object between the thing loaded and the thing displayed; pixel data goes through a C array (so, no opportunity for hidden shenanigans) and only if it was put into the array correctly is it possible to make the final image.