多线程

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        dispatch_group_t group = dispatch_group_create();
        dispatch_group_async(group, queue, ^{
            
        });
        dispatch_group_async(group, queue, ^{
            
        });
        dispatch_group_notify(group, queue, ^{
            dispatch_async(dispatch_get_main_queue(), ^{
                
            });
        });
    });

    //串行队列
    dispatch_queue_t serialQueue = dispatch_queue_create("myThreadQueue1", DISPATCH_QUEUE_SERIAL);
    //并发队列
    dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    for (int i = 0; i < count; ++i) {
//        异步加载
        dispatch_async(serialQueue, ^{
            [self loadImage:[NSNumber numberWithInt:i]];
        });
//        同步加载
        dispatch_sync(serialQueue, ^{
            [self loadImage:[NSNumber numberWithInt:i]];
        });
    }
    
    
    dispatch_barrier_async(globalQueue, ^{
        [self loadImage:[NSNumber numberWithInt:count - 1]];
    });
    for (int i = 0; i < count - 1; ++i) {
        dispatch_async(globalQueue, ^{
            [self loadImage:[NSNumber numberWithInt:i]];
        });
    }

    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    dispatch_sync(mainQueue, ^{
        [self updateImage:imageData];
    });
    
    dispatch_async(mainQueue, ^{
        [self updateImage:imageData];
    });

    1、Thread
    for (int i = 0; i < ROW_COUNT * COLUMN_COUNT; ++i) {
        [NSThread detachNewThreadSelector:@selector(loadImage:) toTarget:self withObject:[NSNumber numberWithInt:i]];
        NSThread * thread=[[NSThread alloc] initWithTarget:self selector:@selector(loadImage:) object:[NSNumber numberWithInt:i]];
        thread.name=[NSString stringWithFormat:@"myThread%i",i];
        [thread start];
    }





    [self performSelectorOnMainThread:@selector(updateImage:) withObject:imageData waitUntilDone:YES];
    [self performSelectorInBackground:@selector(updateImage:) withObject:imageData];

    NSOperationQueue * operationQueue = [[NSOperationQueue alloc] init];
    operationQueue.maxConcurrentOperationCount = 5;
    for (int i = 0; i < count; ++i) {
        [operationQueue addOperationWithBlock:^{
            [self loadImage:[NSNumber numberWithInt:i]];
        }];
    }
    
    依赖关系
    NSBlockOperation * lastBlockOperation= [NSBlockOperation blockOperationWithBlock:^{
        [self loadImage:[NSNumber numberWithInt:(count - 1)]];
    }];
    for (int i = 0; i < count - 1; ++i) {
    NSBlockOperation * blockOperation = [NSBlockOperation blockOperationWithBlock:^{
        [self loadImage:[NSNumber numberWithInt:i]];
    }];
        [blockOperation addDependency:lastBlockOperation];
        [operationQueue addOperation:blockOperation];
    }
    
    [operationQueue addOperation:lastBlockOperation];




    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        [self updateImage:imageData];
    }];