How to merge UIImage's quickly?

Question

Objective-C

+ (UIImage *)imageByCombiningImage:(UIImage*)firstImage withImage:(UIImage*)secondImage {
    UIImage *image = nil;
    
    CGSize newImageSize = CGSizeMake(MAX(firstImage.size.width, secondImage.size.width), MAX(firstImage.size.height, secondImage.size.height));
    if (UIGraphicsBeginImageContextWithOptions != NULL) {
        UIGraphicsBeginImageContextWithOptions(newImageSize, NO, [[UIScreen mainScreen] scale]);
    } else {
        UIGraphicsBeginImageContext(newImageSize);
    }
    [firstImage drawAtPoint:CGPointMake(roundf((newImageSize.width-firstImage.size.width)/2),
                                        roundf((newImageSize.height-firstImage.size.height)/2))];
    [secondImage drawAtPoint:CGPointMake(roundf((newImageSize.width-secondImage.size.width)/2),
                                         roundf((newImageSize.height-secondImage.size.height)/2))];
    image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return image;
}

Runs too long on the main thread. What is the best way to parallelize and is it possible?

Answer 1

// Переходим в рабочий поток.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // Выполняем мерджинг.
    UIImage *resultImage = [UIImage imageByCombiningImage:firstImage withImage:secondImage];
    // Возвращаем картинку в главный поток.
    dispatch_sync(dispatch_get_main_queue(), ^{
        imageView.image = resultImage;
    });
});