Big slowdown on ios7: CGContextDrawImage or UIView setImage?

Question

Ok, I have a serious problem with ios7. I have a code where the user drags a control, and as he drags, the interface is updated with a processed image. This was very fast until iOS 6.1, and now it's very slow.

Here's how my code works:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    //get initial position

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    //calculates new position

    applyEffects();

- (void)applyEffects {

    UIGraphicsBeginImageContext();
    //CGContextTranslateCTM, CGContextScaleCTM, CGContextDrawImage and stuff like that
    UIImage *blendedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    preview.image = blendedImage; //preview is a UIImageView

Now the thing is, after logging, I realized applyEffects is getting called every time I move the finger on screen, but the view is being updated very very few times, and I think that's why it appears so slow.

So, any idea on why "preview.image = blendedImage;" is not updating the view anymore? Am I missing something?

Again, this only happens on iOS 7.

Thank you in advance!

Answer 1

Changed my applyEffects to this:

dispatch_queue_t myQueue = dispatch_queue_create("My Queue",NULL);

if (!running) {
    running = true;

    dispatch_async(myQueue, ^{

        UIGraphicsBeginImageContext();
        //CGContextTranslateCTM, CGContextScaleCTM, CGContextDrawImage and stuff like that
        UIImage *blendedImage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();

        dispatch_async(dispatch_get_main_queue(), ^{
            // Update the UI
            preview.image = blendedImage;
            running = false;
        });
    });
}

Now it will create a separate thread to apply the effect, and update the UI when it's ready. This way I'm not clogging up the UI touch event processing.

The if (!running) is a dumb control to make it only run once at a time.

The ideal way is to throw away all that code and reimplement it with GPUImage, but it will require much more time, so it's a temporary solution.