NSOperationを使って並列処理をするシンプルなサンプル ref: http://qiita.com/edo_m18/items/c121d0cb42a6e20cefbd

file0.m

// NOTOperation.m

@interface NOTOperation ()


@property (assign, nonatomic) BOOL isFinished;
@property (assign, nonatomic) BOOL isExecuting;

@end

////////////////////////////////////////////////////

@implementation NOTOperation

// KVO対象のキーを指定
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
{
    if ([key isEqualToString:@"isExecuting"] ||
        [key isEqualToString:@"isFinished"]) {
        return YES;
    }

    return [super automaticallyNotifiesObserversForKey:key];
}

// オペレーション実行
- (void)start
{
    // 処理中のフラグ
    self.isExecuting = YES;

    // スレッド処理開始

    // 分かりやすいようにスリープを入れる
    [NSThread sleepForTimeInterval:2.0];

    // 処理中フラグをオフ
    self.isExecuting = NO;

    // 処理終了フラグ
    self.isFinished = YES;
}

@end

file1.m

// NOTClient.m

@interface NOTClient ()

@property (strong, nonatomic) NOTOperation *operation;
@property (strong, nonatomic) NSOperationQueue *queue;

@end

//////////////////////////////////////////////////////////

@implementation NOTClient

- (instancetype)init
{
    if (self = [super init]) {
        self.operation = [[NSOperation alloc] init];
        self.queue     = [[NSOperationQueue alloc] init];
    }
    return self;
}

- (void)performMethod
{
    [self.operation addObserver:self
                       forKeyPath:@"isFinished"
                           options:NSKeyValueObservingOptionNew
                           context:nil];

    [self.queue addOperation:self.operation];
}

- (void)observeValueForKeyPath:(NSString *)keyPath
                         ofObject:(id)object
                           change:(NSDictionary *)change
                          context:(void *)context
{
    // do something.
}

@end

file2.m

[self.queue addObserver:self
              forKeyPath:@"operationCount"
                 options:NSKeyValueObservingOptionNew
                 context:nil];