Using multithreading to capture image of UIView - iphone

Question

I am using Grand Central Dispatch to queue a task to capture a UIView as an image. Everything is working fine with this except that the image capturing running on a queue is taking quite a long time.
Is there any way to speed this up or improve my technique. Here is the code to capture and scale the image, which is then set to a UIImageView's image for displaying.
(void)captureScrollViewImageForLayoverView
{
// capture big map image
CGSize size = mainView.bounds.size;
UIGraphicsBeginImageContext(size);
[[mainView layer] renderInContext:UIGraphicsGetCurrentContext()];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
// scale map
size = myLocationOverlay.bounds.size;
UIGraphicsBeginImageContext(size);
[newImage drawInRect:CGRectMake(0,0,size.width,size.height)];
UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
myLocationOverlay.imageMap.image = scaledImage;
}
And Here is the code that is queueing the task.
...
mapImageDrawQueue = dispatch_queue_create("mapImage.drawQueue", NULL);
(void)captureScrollViewImageForLayoverViewWrapper {
// multithreaded approach to draw map
dispatch_async(mapImageDrawQueue, ^{ [self captureScrollViewImageForLayoverView]; });
}

Answer 1

I used to load images at queue in my app with same issue you mentioned. I checked official programming guide, remembered it told that it can't guarantee the queue run immediately after you call it. So I used NSThread do the job, and it's working well:
NSThread* myThread = [[NSThread alloc] initWithTarget:self
selector:#selector(captureScrollViewImageForLayoverView)
object:nil];
[myThread start];
Remember add these lines to the function captureScrollViewImageForLayoverView:
At top : NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
At end : [pool release];
Check this doc to know more - Threading Programming Guide
Hope it helps.