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.