set the background Image for View using blocks in ios?

Question

When user taps on button im navigating to the another view,and set the background image for that view.But the image is from URL.To convert into NSData format its taking some time.So,How can i achieve this using blocks.

I have used below code but no luck..

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    [self setBackGroundImageView];
});


- (void)setBackGroundImageView {
    NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:sharedDelegate.fbAppScopeId]];

    dispatch_sync(dispatch_get_main_queue(), ^(void) {            
        if (imageData) {
            UIImageView *backgroundView = [[UIImageView alloc] initWithImage:[self squareImageWithImage:[UIImage imageWithData:imageData] scaledToSize:CGSizeMake(100, 100)]];

            CGFloat ratio = backgroundView.frame.size.width/screenWidth;

            CGRect imageFrame = backgroundView.frame;
            imageFrame.size.width = screenWidth;
            imageFrame.size.height = backgroundView.frame.size.height/ratio;

            if (imageFrame.size.height<screenHeight) {
                ratio = backgroundView.frame.size.height/screenHeight;
                imageFrame.size.height = screenHeight;
                imageFrame.size.width = backgroundView.frame.size.width/ratio;
            }

            backgroundView.frame = imageFrame;
            backgroundView.center = CGPointMake(screenWidth/2, backgroundView.center.y);


            backgroundView.alpha = 0.5;
            [self.view addSubview:backgroundView];
        }
    }
}

Answer 1

Your problem is in this line of code:

dispatch_sync(dispatch_get_main_queue()

The correct use of dispatch_async is

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
    //Background Thread
    dispatch_async(dispatch_get_main_queue(), ^(void){
        //Run UI Updates
    });
});

Please, change dispatch_sync on setBackGroundImageView to dispatch_async

Also I would recommend you to place all async code in one method, as i mention before. something like this:

    - (void)setBackGroundImageView {

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:sharedDelegate.fbAppScopeId]];
   // YOUR PROBLEM WAS HERE 
        dispatch_async(dispatch_get_main_queue(), ^(void) {            
            if (imageData) {
                UIImageView *backgroundView = [[UIImageView alloc] initWithImage:[self squareImageWithImage:[UIImage imageWithData:imageData] scaledToSize:CGSizeMake(100, 100)]];

                CGFloat ratio = backgroundView.frame.size.width/screenWidth;

                CGRect imageFrame = backgroundView.frame;
                imageFrame.size.width = screenWidth;
                imageFrame.size.height = backgroundView.frame.size.height/ratio;

                if (imageFrame.size.height<screenHeight) {
                    ratio = backgroundView.frame.size.height/screenHeight;
                    imageFrame.size.height = screenHeight;
                    imageFrame.size.width = backgroundView.frame.size.width/ratio;
                }

                backgroundView.frame = imageFrame;
                backgroundView.center = CGPointMake(screenWidth/2, backgroundView.center.y);


                backgroundView.alpha = 0.5;
                [self.view addSubview:backgroundView];
            }
        }
    });
    }

Answer 2

You can't do UI Events (adding or removing views) on another thread other than Main Thread. If you do , that might misbehave.

If you still want to do so , add imageView on main thread and set its image in block.

P.S. You can preload (on app startup) image data using block, once it done you can use it as imageView.image=preLoadImage;.