Get value out of setCompletionBlockWithSuccess block - ios

Question

I want to get some data from web service and it works fine bu now I want to store the returned value from a variable from a block, but the variable's value is changed only inside the block and it returns null outside the block, how can I do it ? here is my code:
-(void)getDataFromServer:(NSString *) urlString{
__block id returnedData;
AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
NSURLCredential *credential = [NSURLCredential credentialWithUser:username password:password persistence:NSURLCredentialPersistenceNone];
NSMutableURLRequest *request = [manager.requestSerializer requestWithMethod:#"GET" URLString:urlString parameters:nil];
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
[operation setCredential:credential];
[operation setResponseSerializer:[AFJSONResponseSerializer alloc]];
[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
// NSLog(#"Success: %#", responseObject);
returnedData = responseObject;
NSLog(#"returned data%#" , returnedData); // here returnedData contains the value that I need to store
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(#"Failure: %#", error);
} ];
[manager.operationQueue addOperation:operation];
NSLog(#"returned data%#" , returnedData); // here it returns null value !!
}

Answer 1

The problem is not with access to the variable, it is with the timing.
Your second NSLog statement is running before the completion block is run. The operation runs asynchronously, and you're logging returnedData before it is set in the completion block.
Edit to add an example solution:
One solution would be to create the completion block in your calling class and past into the operation in the getDataFromServer: method.
- (void)getDataFromServer:(NSString *)urlString completion:(void (^)(AFHTTPRequestOperation *operation, id responseObject))completion {
AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
NSURLCredential *credential = [NSURLCredential credentialWithUser:username password:password persistence:NSURLCredentialPersistenceNone];
NSMutableURLRequest *request = [manager.requestSerializer requestWithMethod:#"GET" URLString:urlString parameters:nil];
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
[operation setCredential:credential];
[operation setResponseSerializer:[AFJSONResponseSerializer alloc]];
[operation setCompletionBlockWithSuccess:completion];
[manager.operationQueue addOperation:operation];
}