Objective-C blocks and Grand Central Dispatch. Using GCD and blocks effectively.

EmailRepository.h

#import <Foundation/Foundation.h>

@interface EmailRepository : NSObject

+ (id)sharedEmailRepository;

- (void)getDataFromServer:(NSString *)emailAddress onComplete:(void (^)(NSString *response))completionBlock onFailure:(void (^)(NSString *response))failureBlock;
@end

EmailRepository.m

#import "EmailRepository.h"

@interface EmailRepository()

@property (nonatomic, copy) void (^onTransactionCompleted)();
@property (nonatomic, copy) void (^onTransactionFailure)();

@end

@implementation EmailRepository

@synthesize onTransactionFailure;
@synthesize onTransactionCompleted;

+ (id)sharedEmailRepository {
    static EmailRepository *emailRepository = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        emailRepository = [[self alloc] init];
    });
    return emailRepository;
}

- (void)getDataFromServer:(NSString *)emailAddress onComplete:(void (^)(NSString *response))completionBlock onFailure:(void (^)(NSString *response))failureBlock
{
    // No GCD.
    //self.onTransactionCompleted = completionBlock;
    //self.onTransactionFailure = failureBlock;
    //[self getData];
    
    self.onTransactionCompleted = completionBlock;
    self.onTransactionFailure = failureBlock;
    
    // GCD
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        // Compute big gnarly thing that would block for really long time.
        [self getData];
     });

}

- (void) getData {
    
    // Do something with the response. Handle from server for example.
    BOOL success = YES;
    NSString *resp = [NSString stringWithFormat:@"%", @"Some server response."];
    
    // Now it is done. Tell the main thread.
    dispatch_async(dispatch_get_main_queue(), ^{
        
        if(success == true){
            if(self.onTransactionCompleted){
                self.onTransactionCompleted(resp);
            }
        }else{
            if(self.onTransactionFailure){
                self.onTransactionFailure(resp);
            }
        }
    
    });   
}
@end

EmailRepositoryExample.m

EmailRepository *sharedEmail = [EmailRepository sharedEmailRepository];

[sharedEmail getDataFromServer:@"test"
      onComplete:^(NSString *response){
        NSLog(@"success");
      } onFailure:^(NSString *response){
        NSLog(@"failure");
}];