Calling a GCD Block else where in Main Thread - iphone

Question

I am using following Code to get response from a string query. There are a lot of queries all around in my app and i want to copy and paste this code again and again
Is there any way I can just make an instance of it , pass the urlString and then return the response..
I have tried creating a function
+(NSString*) myFunc{} in an NSObject Class but it seems that GCD Doesn't work except Main UI Threads. How can i fix this issue
__block__ NSString *response;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
//your server url and request. data comes back in this background thread
response; = [NSString stringWithContentsOfURL:[NSURL URLWithString:queryString] encoding:NSUTF8StringEncoding error:&err];
dispatch_async(dispatch_get_main_queue(), ^{
//update main thread here.
NSLog(#"%#",response); // NEED TO RETURN THIS
if (err != nil)
{
UIAlertView *alert = [[UIAlertView alloc]initWithTitle: #"Error"
message: #"An error has occurred."
delegate: self
cancelButtonTitle:#"Ok"
otherButtonTitles:nil];
[alert show];
[indicator stopAnimating];
}
});
});

Answer 1

I would separate out the request processing from the error reporting, using a completion block to provide the feedback to the caller.
First define the completion block semantics; we know we want the string response and an optional error descriptor:
typedef void (^COMPLETION_BLOCK)(NSString *response, NSString *error);
Second implement the method that will get the response in the background and then call the completion block in the main thread. This could be a class method in some global utility class if you wish:
- (void)responseFromURL:(NSURL *)url
completionBlock:(COMPLETION_BLOCK)completionBlock
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSError *error = nil;
NSString *response = [NSString stringWithContentsOfURL:url
encoding:NSUTF8StringEncoding
error:&error];
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock(response, error);
}
}
}
And finally call the method:
[someClass responseFromURL:[NSURL URLWithString:queryString]
completionBlock:^(NSString *response, NSError *error) {
NSLog(#"response='%#'", response);
if (error != nil)
{
UIAlertView *alert = [[UIAlertView alloc]initWithTitle:#"Error getting response"
message:[error localizedDescription]
delegate:self
cancelButtonTitle:#"Ok"
otherButtonTitles:nil];
[alert show];
[indicator stopAnimating];
}
}];
(this code is untested and will therefore doubtless contain some bugs)