iPhone run animation while moving object on the screen smoothly - iphone

Question

currently I have an image on the screen that is swapped out every 5 seconds with another image and is using an animation to do so.
At the same time on the screen I have objects that the user can pick up and drag around (using panning gesture). During the .5 duration of the animation, if I am moving around an object the UI stutters. For example I have a brush that I pick up and move around the screen. the 5 second timer ends and the background image updates. while this updates the brush stutters while that animation occurs. I moved the Image loading the the UI thread and force it to load by using NSData.
Is there a way that I can prevent this stutter while the animation to change the image is running. Here is how I swap the image.
// Dispatch to the queue, and do not wait for it to complete
// Grab image in background thread in order to not block UI as much as possible
dispatch_async(imageGrabbingQueue, ^{
curPos++;
if (curPos> (self.values.count - 1)) curPos= 0;
NSDictionary *curValue = self.values[curPos];
NSString *imageName = curValue [KEY_IMAGE_NAME];
// This may cause lazy loading later and stutter UI, convert to DataObject and force it into memory for faster processing
UIImage *imageHolder = [UIImage imageNamed:imageName];
// Load the image into NSData and recreate the image with the data.
NSData *imageData = UIImagePNGRepresentation(imageHolder);
UIImage *newImage = [[UIImage alloc] initWithData:imageData];
dispatch_async(dispatch_get_main_queue(), ^{
[UIView transitionWithView:self.view duration:.5 options:UIViewAnimationOptionTransitionCrossDissolve|UIViewAnimationOptionAllowUserInteraction|UIViewAnimationOptionAllowAnimatedContent
animations:^{
[self.image setImage:newImage ];
// Temp clause to show ad logo
if (curPos != 0) [self.imagePromotion setAlpha:1.0];
else [self.imagePromotion setAlpha:0];
}
completion:nil];
});
});
Thanks,
DMan

Answer 1

The image processing libs on the iPhone are not magic, they do take CPU time to actually decode the image. This is likely what you are running into. Calling UIImage imageNamed would likely cache the image, but caches can always be flushed so that does not force the system to keep the image in memory. Your code to call initWithData is pointless because it still has to decompress the PNG into memory and that is the part that is causing the slowdown. What you could do is render the image out as decoded pixels and then save that into a file. Then, memory map the file and wrap the mapped memory up in a coregraphics image. That would avoid the "decode and render" step that is likely causing the slowdown. But, anything else may not actually do what you are expecting. Oh, and you should not hold the decoded bytes in memory, because image data is typically so large that it will take up too much room in the device memory.