Poor UI performance when loading photos on iOS - ios

Question

I'm trying to make a photo slider similar to iOS's photos app. I've seen PhotoScroller, and I'm using initWithContentsOfFile.
CATiledLayer seems like a good idea, except I have no way of pre-generating tiles. The tiles also take up a lot of space. Images are part of a document bundle synced up with iCloud. Photos are typically JPEG. From hours of reading it seems like generating the tiles on the fly is slower than just loading the whole image.
It seems like a majority of the time is spent in decompressing the image anyway. And moving it to a background queue and displaying a smaller image should work well. So that's what I'm trying to do. And it works to a point, but if I slide without waiting for the image to load there's still somewhat of a stutter, which sometimes causes the scroll view to hang momentarily (when paging).
This is the function that sets the image:
- (void)setImage:(UIImage *)image placeholder:(UIImage *)placeholder
{
_image = image;
self.zoomScale = 1.0;
imageView.image = nil;
imageSize = image.size;
imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);
MachTimer *timer = [MachTimer new];
[timer start];
imageView.image = placeholder;
NSLog(#"placeholder: %f", [timer elapsedSeconds]);
//imageView.layer.contents = (id)placeholder.CGImage;
self.contentSize = image.size;
[self setMaxMinZoomScalesForCurrentBounds];
self.zoomScale = self.minimumZoomScale;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
MachTimer *timer = [MachTimer new];
[timer start];
UIGraphicsBeginImageContext(CGSizeMake(1, 1));
[image drawAtPoint:CGPointZero];
UIGraphicsEndImageContext();
NSLog(#"decode: %f", [timer elapsedSeconds]);
dispatch_sync(dispatch_get_main_queue(), ^{
if(_image == image) {
MachTimer *timer = [MachTimer new];
[timer start];
imageView.image = image;
NSLog(#"display: %f", [timer elapsedSeconds]);
//imageView.layer.contents = (id)image.CGImage;
}
});
});
}
My "placeholder" times are about 0.00005 - 0.00006 seconds (decompress and display), and they're 480px tall. My "decode" times (for full image) are about 0.8 to 1.2 seconds. "display" is about 0.0001 seconds (which is about 0.1 milliseconds).
So with those times the UI should be smooth as butter, but it isn't.
I've even tried to go as far as setting contents of a regular UIView to the CGImage. I've tried iOS 6.0's drawsAsynchronously, and that seems to make it a little worse.
What am I doing wrong?
EDIT:
I've pushed my sample project to GitHub:
https://github.com/lukescott/PhotoScroller

Answer 1

I would create a background thread for hard work (photo load or decode) and when its done then delegate UI work (draw or whatever) to main thread using one of tools (queues, msgs, etc).