A pattern for implementing a class managing a NSOperationQueue and NSOperations. The files attached show how to apply this pattern to downloading gists/gravatar images from github.com

GistCommunicationManager.h

//
//  GistCommunicationManager.h
//  GistExplorer
//
//  Created by Bernd Rabe on 28.05.13.
//  Copyright (c) 2013 RABE_IT Services. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "OperationDelegate.h"
#import "ProviderDelegate.h"

@interface GistCommunicationManager : NSObject

+ (instancetype)sharedManger;

- (void)loadContentForProvider:(id <ProviderDelegate>)provider success:(void(^)())sucess failure:(OperationFailure)failure;
- (void)loadGravatarForProvider:(id <ProviderDelegate>)provider avatarURL:(NSURL *)avatarURL success:(OperationSuccessWithData)success;

- (void)cancelAllOperationsForProvider:(id)provider;
- (void)cancelOperationForKey:(NSString *)key;

@end

GistCommunicationManager.m

//
//  GistCommunicationManager.m
//  GistExplorer
//
//  Created by Bernd Rabe on 28.05.13.
//  Copyright (c) 2013 RABE_IT Services. All rights reserved.
//

#import "GistCommunicationManager.h"
#import "JSONOperation.h"
#import "GravatarOperation.h"

@interface GistCommunicationManager()
@property (nonatomic, strong) NSOperationQueue      *operationQueue;
@property (nonatomic, strong) NSMutableDictionary   *operations;
@end

@implementation GistCommunicationManager

+ (instancetype)sharedManger
{
    static GistCommunicationManager *sharedManger = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedManger = [[GistCommunicationManager alloc] init];
    });
    return sharedManger;
}

- (NSOperationQueue *)operationQueue
{
    if (nil == _operationQueue) {
        _operationQueue = [[NSOperationQueue alloc] init];
        _operationQueue.name = @"Background Processing Queue";
    }
    return _operationQueue;
}

- (NSMutableDictionary *)operations
{
    if (nil == _operations) {
        _operations = [NSMutableDictionary dictionary];
    }
    return _operations;
}

- (void)loadContentForProvider:(id <ProviderDelegate>)provider success:(void(^)())sucess failure:(OperationFailure)failure
{
    JSONOperation *operation = [JSONOperation jsonOperationForProvider:provider
                                                               success:^(NSOperation *operation, NSString *objectNotation) {
                                                                   [self.operations removeObjectForKey:[(JSONOperation *)operation operationKey]];
                                                                   
                                                                   NSError *error = nil;
                                                                   [provider processData:objectNotation error:&error];
                                                                   if (nil == error) {
                                                                       if (sucess) {
                                                                           [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                                                                               sucess();
                                                                           }];
                                                                       }
                                                                   }
                                                                   else {
                                                                       if(failure) {
                                                                           [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                                                                               failure(operation, error);
                                                                           }];
                                                                       }
                                                                   }
                                                               }
                                                               failure:failure];
    
    [self.operations setObject:operation forKey:[operation operationKey]];
    
    [self.operationQueue addOperation:operation];
}

- (void)loadGravatarForProvider:(id <ProviderDelegate>)provider avatarURL:(NSURL *)avatarURL success:(OperationSuccessWithData)success
{
    GravatarOperation *operation = [GravatarOperation gravatarOperationForProvider:provider
                                                                         avatarURL:avatarURL
                                                                           success:^(NSOperation *operation, NSData *data) {
                                                                               [self.operations removeObjectForKey:[(GravatarOperation *)operation operationKey]];
                                                                               if (success) {
                                                                                   [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                                                                                       success(operation, data);
                                                                                   }];
                                                                               }
                                                                           }];
    [self.operations setObject:operation forKey:[operation operationKey]];
    
    [self.operationQueue addOperation:operation];
}

- (void)cancelAllOperationsForProvider:(id)provider
{
    [self.operationQueue cancelAllOperations];

    NSMutableArray *keys = [NSMutableArray array];
    [self.operations enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
        if ([obj provider] == provider) {
            [obj cancelOperation];
            [keys addObject:key];
        }
    }];
    [self.operations removeObjectsForKeys:keys];
}

- (void)cancelOperationForKey:(NSString *)key
{
    id operation = self.operations[key];
    if (operation && [operation conformsToProtocol:@protocol(OperationDelegate)]) {
        [operation cancelOperation];
        [self.operations removeObjectForKey:key];
    }
}

@end

GistProvider.h

//
//  GistProvider.h
//  GistExplorer
//
//  Created by Bernd Rabe on 28.05.13.
//  Copyright (c) 2013 RABE_IT Services. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "ProviderDelegate.h"

extern NSString *const gistsCellReuseIdentifier;

extern NSString *const GistsProviderErrorDomain;
extern NSString *const GistsProviderDidSelectGistNotification;

NS_ENUM(NSUInteger, GistsProviderError) {
    GistsProviderInvalidJSONError
};

@interface GistsProvider : NSObject <UITableViewDataSource, UITableViewDelegate, ProviderDelegate>

- (id)initWithURl:(NSURL *)url;

@end

GistProvider.m

//
//  GistProvider.m
//  GistExplorer
//
//  Created by Bernd Rabe on 28.05.13.
//  Copyright (c) 2013 RABE_IT Services. All rights reserved.
//

#import "GistsProvider.h"
#import "Gist.h"
#import "User.h"
#import "GistCommunicationManager.h"

NSString *const gistsCellReuseIdentifier = @"SubtitleStyle";
NSString *const GistsProviderErrorDomain = @"GistsProviderErrorDomain";
NSString *const GistsProviderDidSelectGistNotification = @"GistsProviderDidSelectGistNotification";

@interface GistsProvider()
@property (nonatomic, strong) NSArray   *data;
@property (nonatomic, strong) NSURL     *contentURL;
@property (nonatomic, strong) UIImage   *johnDoeImage;
@property (nonatomic, strong) NSCache   *avatarImageCache;
@end

@implementation GistsProvider

- (id)initWithURl:(NSURL *)url
{
    self = [super init];
    if (self) {
        self.contentURL = url;
    }
    return self;
}

#pragma mark - DataSource Part

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    NSParameterAssert(section == 0);
    
    return self.data.count;
}

- (Gist *)gistForIndexPath:(NSIndexPath *)indexPath
{
    return self.data[indexPath.row];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    NSParameterAssert(indexPath.section == 0);
    NSParameterAssert(indexPath.row < self.data.count);
    
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:gistsCellReuseIdentifier forIndexPath:indexPath];
    
    Gist *gist = [self gistForIndexPath:indexPath];
    cell.textLabel.text = [gist titleText];
    cell.detailTextLabel.text = [gist subtitleText];
    
    if ([gist hasUser]) {
        NSData *gravatarData = [self.avatarImageCache objectForKey:gist.user.gravatarID];
        if (gravatarData) {
            cell.imageView.image = [UIImage imageWithData:gravatarData];
        }
        else {
            __weak typeof(self) weakSelf = self;
            [[GistCommunicationManager sharedManger] loadGravatarForProvider:(GistsProvider *)tableView.dataSource
                                                                   avatarURL:gist.user.avatarURL
                                                                     success:^(NSOperation *operation, NSData *data) {
                                                                             [weakSelf.avatarImageCache setObject:data forKey:gist.user.gravatarID];
                                                                             UIImage *image = [UIImage imageWithData:data];
                                                                             [tableView cellForRowAtIndexPath:indexPath].imageView.image = image;
                                                                     }];
            cell.imageView.image = self.johnDoeImage;
        }
    }
    
    return cell;
}

- (void)tableView:(UITableView *)tableView didEndDisplayingCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath
{
    NSString *operationKey = [[self gistForIndexPath:indexPath] operationKey];
    if (operationKey) {
        [[GistCommunicationManager sharedManger] cancelOperationForKey:operationKey];
    }
}

- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (indexPath.row % 2 == 0) {
        cell.backgroundColor = [UIColor colorWithRed:163/255. green:204/255. blue:20/255. alpha:1.];
    }
}

#pragma mark - Delegate Part

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath;
{
    Gist *gist = [self gistForIndexPath:indexPath];
    NSNotification *notification = [NSNotification notificationWithName:GistsProviderDidSelectGistNotification object:gist];
    [[NSNotificationCenter defaultCenter] postNotification:notification];
}

#pragma mark - Lazy Init

- (UIImage *)johnDoeImage
{
    if (nil == _johnDoeImage) {
        _johnDoeImage = [UIImage imageNamed:@"JohnDoe.png"];
    }
    return _johnDoeImage;
}

- (NSCache *)avatarImageCache
{
    if (nil == _avatarImageCache) {
        _avatarImageCache = [[NSCache alloc] init];
    }
    return _avatarImageCache;
}

#pragma mark - ProviderDelegate

- (NSString *)title
{
    return @"Public Gist's";
}

- (void)processData:(NSString *)objectNotation error:(NSError **)error
{
    NSData *unicodeNotation = [objectNotation dataUsingEncoding:NSUTF8StringEncoding];
    NSError *localError = nil;
    
    id jsonObject = [NSJSONSerialization JSONObjectWithData:unicodeNotation
                                                    options:NSJSONReadingAllowFragments
                                                      error:&localError];
    NSDictionary *parsedObject = (id)jsonObject;
    if (nil == parsedObject) {
        if (error != NULL) {
            *error = [NSError errorWithDomain:GistsProviderErrorDomain code:GistsProviderInvalidJSONError userInfo:nil];
        }
    }
    else {
        NSMutableArray *gistsArray = [NSMutableArray array];
        for (NSDictionary *item in parsedObject) {
            Gist *gist = [[Gist alloc] init];
            if (item[@"id"]) {
                gist.gistID = [item[@"id"] integerValue];
            }
            
            if (item[@"created_at"]) {
                gist.createdAt = [gist dateFromISO8601String:item[@"created_at"]];
            }
            
            if (item[@"description"] && item[@"description"] != [NSNull null]) {
                gist.descriptionText = item[@"description"];
            }
            
            if (item[@"url"]) {
                gist.jsonURL = [NSURL URLWithString:item[@"url"]];
            }
            
            if (item[@"user"] != [NSNull null]) {
                gist.user = [User userFromUserDictionary:item[@"user"]];
            }
            [gistsArray addObject:gist];
        }
        self.data = [gistsArray copy];
    }
}

- (void)removeAllData
{
    _data = nil;
}

@end

GravatarOperation.h

//
//  GravatarOperation.h
//  GistExplorer
//
//  Created by Bernd Rabe on 28.05.13.
//  Copyright (c) 2013 RABE_IT Services. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "OperationDelegate.h"

@interface GravatarOperation : NSOperation <NSURLConnectionDataDelegate, OperationDelegate>

+ (instancetype)gravatarOperationForProvider:(id)provider avatarURL:(NSURL *)avatarURL success:(OperationSuccessWithData)success;

@end

GravatarOperation.m

//
//  GravatarOperation.m
//  GistExplorer
//
//  Created by Bernd Rabe on 28.05.13.
//  Copyright (c) 2013 RABE_IT Services. All rights reserved.
//

#import "GravatarOperation.h"
#import "ProviderDelegate.h"

@interface GravatarOperation()
@property (nonatomic, assign) BOOL executing;
@property (nonatomic, assign) BOOL finished;
@property (nonatomic, assign) BOOL cancelled;

@property (nonatomic, strong) NSURL *url;

@property (nonatomic, weak) id <ProviderDelegate> delegate;
@property (nonatomic, strong) NSURLConnection *connection;
@property (nonatomic, strong) NSMutableData *receivedData;

@property (nonatomic, copy) OperationSuccessWithData success;
@end

@implementation GravatarOperation

- (void)start
{
    if ([self isCancelled])
    {
        [self willChangeValueForKey:@"isFinished"];
        _finished = YES;
        [self didChangeValueForKey:@"isFinished"];
        
        return;
    }
    
    NSURLRequest *request = [NSURLRequest requestWithURL:self.url];
    self.connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:NO];
    [self.connection scheduleInRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];
    [self.connection start];
}

+ (instancetype)gravatarOperationForProvider:(id)provider avatarURL:(NSURL *)avatarURL success:(OperationSuccessWithData)success
{
    NSAssert(nil != provider, @"provider parameter can't be nil");
    
    GravatarOperation *operation = [[[self class] alloc] init];
    operation.delegate = provider;
    operation.url = avatarURL;
    operation.success = success;
    
    return operation;
}

- (BOOL)isConcurrent {
    return YES;
}

- (BOOL)isExecuting {
    return _executing;
}

- (BOOL)isFinished {
    return _finished;
}

- (BOOL)isCancelled {
    return _cancelled;
}

- (NSMutableData *)receivedData
{
    if (nil == _receivedData) {
        _receivedData = [NSMutableData data];
    }
    return _receivedData;
}

#pragma mark - NSURLConnectionDataDelegate

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    if (_success) {
        self.success(self, [self.receivedData copy]);
    }
    
    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];
    
    _executing = NO;
    _finished = YES;
    
    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    [self.receivedData appendData:data];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    [connection cancel];
    _connection = nil;
    _receivedData = nil;
    _url = nil;
    
    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];
    
    _executing = NO;
    _finished = YES;
    
    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

#pragma mark - OperationDelegate

- (NSString *)operationKey
{
    return [self.url absoluteString];
}

- (id)provider
{
    return _delegate;
}

- (void)cancelOperation
{
    _success = nil;

    [self.connection cancel];
    _connection = nil;
    
    _receivedData = nil;
    _url = nil;
    
    [self willChangeValueForKey:@"isCancelled"];
    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];
    
    _executing = NO;
    _finished  = YES;
    _cancelled = YES;
    
    [self didChangeValueForKey:@"isCancelled"];
    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

@end

JSONOperation.h

//
//  JSONOperation.h
//  GistExplorer
//
//  Created by Bernd Rabe on 28.05.13.
//  Copyright (c) 2013 RABE_IT Services. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "OperationDelegate.h"

@interface JSONOperation : NSOperation <NSURLConnectionDataDelegate, OperationDelegate>

+ (instancetype)jsonOperationForProvider:(id)provider success:(OperationSuccessWithString)success failure:(OperationFailure)failure;

@end

JSONOperation.m

//
//  JSONOperation.m
//  GistExplorer
//
//  Created by Bernd Rabe on 28.05.13.
//  Copyright (c) 2013 RABE_IT Services. All rights reserved.
//

#import "JSONOperation.h"
#import "ProviderDelegate.h"

@interface JSONOperation()
@property (nonatomic, assign) BOOL executing;
@property (nonatomic, assign) BOOL finished;
@property (nonatomic, assign) BOOL cancelled;

@property (nonatomic, strong) NSURL *url;

@property (nonatomic, weak) id <ProviderDelegate> delegate;
@property (nonatomic, strong) NSURLConnection *connection;
@property (nonatomic, strong) NSMutableData *receivedData;

@property (nonatomic, copy) OperationFailure failure;
@property (nonatomic, copy) OperationSuccessWithString success;
@end

@implementation JSONOperation 

- (void)start
{
    if ([self isCancelled])
    {
        [self willChangeValueForKey:@"isFinished"];
        _finished = YES;
        [self didChangeValueForKey:@"isFinished"];
        
        return;
    }
    
    NSURLRequest *request = [NSURLRequest requestWithURL:self.url];
    self.connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:NO];
    [self.connection scheduleInRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];
    [self.connection start];
    
    [self willChangeValueForKey:@"isExecuting"];
    _executing = YES;
    [self didChangeValueForKey:@"isExecuting"];
}

- (NSMutableData *)receivedData
{
    if (nil == _receivedData) {
        _receivedData = [NSMutableData data];
    }
    return _receivedData;
}

+ (instancetype)jsonOperationForProvider:(id <ProviderDelegate>)provider success:(OperationSuccessWithString)success failure:(OperationFailure)failure
{
    NSAssert(nil != provider, @"provider parameter can't be nil");

    JSONOperation *operation = [[[self class] alloc] init];
    operation.delegate = provider;
    operation.url = provider.contentURL;
    operation.failure = failure;
    operation.success = success;
    
    return operation;
}

- (BOOL)isConcurrent {
    return YES;
}

- (BOOL)isExecuting {
    return _executing;
}

- (BOOL)isFinished {
    return _finished;
}

- (BOOL)isCancelled {
    return _cancelled;
}

#pragma mark - NSURLConnectionDataDelegate

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    
    if (_success) {
        NSString *receivedText = [[NSString alloc] initWithData:self.receivedData encoding:NSUTF8StringEncoding];
        _receivedData = nil;
        
        self.success(self, receivedText);
    }

    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];
    
    _executing = NO;
    _finished = YES;
    
    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    [self.receivedData appendData:data];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    [connection cancel];
    _connection = nil;
    _receivedData = nil;
    _url = nil;
    
    if (_failure) {
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            self.failure(self, error);
        }];
    }
    
    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];
    
    _executing = NO;
    _finished = YES;
    
    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

#pragma mark - OperationDelegate

- (NSString *)operationKey
{
    return [self.url absoluteString];
}

- (id)provider
{
    return _delegate;
}

- (void)cancelOperation
{
    _failure = nil;
    _success = nil;

    [self.connection cancel];
    _connection = nil;
    
    _receivedData = nil;
    _url = nil;
    
    [self willChangeValueForKey:@"isCancelled"];
    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];
    
    _executing = NO;
    _finished  = YES;
    _cancelled = YES;
    
    [self didChangeValueForKey:@"isCancelled"];
    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

@end

OperationDelegate.h

//
//  OperationDelegate.h
//  GistExplorer
//
//  Created by Bernd Rabe on 28.05.13.
//  Copyright (c) 2013 RABE_IT Services. All rights reserved.
//

#import <Foundation/Foundation.h>

typedef void(^OperationSuccessWithData)(NSOperation *operation, NSData *data);
typedef void(^OperationSuccessWithString)(NSOperation *operation, NSString *objectNotation);
typedef void(^OperationFailure)(NSOperation *operation, NSError *error);

@protocol OperationDelegate <NSObject>
- (id)provider;
- (NSString *)operationKey;
- (void)cancelOperation;
@end

ProviderDelegate.h

//
//  ProviderDelegate.h
//  GistExplorer
//
//  Created by Bernd Rabe on 28.05.13.
//  Copyright (c) 2013 RABE_IT Services. All rights reserved.
//

#import <Foundation/Foundation.h>

@protocol ProviderDelegate <NSObject>
- (NSString *)title;
- (NSURL *)contentURL;
- (void)processData:(NSString *)objectNotation error:(NSError **)error;
- (void)removeAllData;
@end