How to fetching image from server asynchronously - iphone

Question

I am doing following thing to load my image from server. But problem is that imageview is showing up 5-7 seconds later when alertview is closing.
how to solve this ?
- (void)showFullImage :(id) sender
{
// for loading view
self.loading_alert = [[UIAlertView alloc] initWithTitle:NSLocalizedString(#"LOADING",nil) message:nil delegate:self cancelButtonTitle:nil otherButtonTitles: nil];
[self.loading_alert show];
UIActivityIndicatorView *indicator = [[UIActivityIndicatorView alloc] initWithFrame: CGRectMake(125, 65, 30, 30)];
indicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyleWhiteLarge;
//indicator.center = CGPointMake(loading_alert.bounds.size.width / 2, loading_alert.bounds.size.height - 50);
[indicator startAnimating];
[self.loading_alert addSubview:indicator];
self.image_url = [self.question_set valueForKey:#"image_url"];
//--------Asyncronously fetch image----------------------------------------
NSURL *URL = [NSURL URLWithString:image_url];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL cachePolicy:NSURLCacheStorageAllowedInMemoryOnly
timeoutInterval:60.0];
[NSURLConnection sendAsynchronousRequest:request queue:[[NSOperationQueue alloc] init] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error){
NSLog(#"1",nil);
imgView= [[UIView alloc] initWithFrame:CGRectMake(0,0,320,480) ];
UIImage* queImage = [UIImage imageWithData:data];
UIImageView *iv = [[UIImageView alloc] initWithFrame:CGRectMake(0,42,320,420)];
iv.image = queImage;
[imgView addSubview:iv];
[self.view addSubview:imgView];
//dismiss alert view on main thread
dispatch_async(dispatch_get_main_queue(), ^(void) {
// dismiss alert view...
NSLog(#"2",nil);
[self.loading_alert dismissWithClickedButtonIndex:0 animated:YES];
});
}];
}
Help will be appreciated.

Answer 1

Forget GCD, you can pass [NSOperationQueue mainQueue] in queue parameter of NSURLConnection's sendAsynchronousRequest:queue:completionHandler: method, so you dont need to think much about main thread and background thread, it'll automatically handle call request in background and return data on main thread.
And below is how your code will look like:
NSURL *URL = [NSURL URLWithString:image_url];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL cachePolicy:NSURLCacheStorageAllowedInMemoryOnly timeoutInterval:60.0];
// request is called asynchronously and response completion handler will be called on main thread
[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error){
// do your whatever work here as you do normally, because it's in main thread
NSLog(#"1",nil);
imgView= [[UIView alloc] initWithFrame:CGRectMake(0,0,320,480) ];
UIImage* queImage = [UIImage imageWithData:data];
UIImageView *iv = [[UIImageView alloc] initWithFrame:CGRectMake(0,42,320,420)];
iv.image = queImage;
[imgView addSubview:iv];
[self.view addSubview:imgView];
}

Answer 2

You need to download images in background queue and UI updation in main queue.
Using GCD like this,
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
NSURL *URL = [NSURL URLWithString:image_url];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL cachePolicy:NSURLCacheStorageAllowedInMemoryOnly
timeoutInterval:60.0];
[NSURLConnection sendAsynchronousRequest:request queue:[[NSOperationQueue alloc] init] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error){
NSLog(#"1",nil);
UIImage* queImage = [UIImage imageWithData:data];
dispatch_async(dispatch_get_main_queue(), ^{
imgView= [[UIView alloc] initWithFrame:CGRectMake(0,0,320,480) ];
if([receivedView isKindOfClass:[UIImageView class]])
{
UIImageView *iv = [[UIImageView alloc] initWithFrame:CGRectMake(0,42,320,420)];
iv.image = queImage;
[imgView addSubview:iv];
[self.view addSubview:imgView];
}
// dismiss alert view...
NSLog(#"2",nil);
[self.loading_alert dismissWithClickedButtonIndex:0 animated:YES];
});
}
});

Answer 3

SDWebmage -> This is a nice third party library for loading images.