NSOperation

NSOperation是苹果在gcd的基础上封装的一层，更好的处理和控制大量的并发操作，但是速度比gcd慢。NSOperationqueue会自动处理线程的优先级，在复杂的线程中，NSOperation更加方便。

使用方式：1.系统提供的子类，2.自定义子类

1.系统提供的子类

 NSOperationQueue *queue = [[NSOperationQueue alloc]init];
    NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        NSData *data = [[NSData alloc]initWithContentsOfURL:[NSURL URLWithString:@"http://d.hiphotos.baidu.com/image/pic/item/9922720e0cf3d7ca60432a7df41fbe096b63a907.jpg"]];
        dispatch_async(dispatch_get_main_queue(), ^{
            _imageback.image = [UIImage imageWithData:data];
        });
    }];
    //添加第二个队列
    [operation addExecutionBlock:^{
        NSData *data = [[NSData alloc]initWithContentsOfURL:[NSURL URLWithString:@"http://d.hiphotos.baidu.com/image/pic/item/9922720e0cf3d7ca60432a7df41fbe096b63a907.jpg"]];
        dispatch_async(dispatch_get_main_queue(), ^{
            _imageback2.image = [UIImage imageWithData:data];
        });
    }];
    [queue addOperation:operation];//异步线程
//    [operation start];//同步
//    [operation cancel];//同步
    [operation setCompletionBlock:^{
        NSLog(@"over");//执行完成之后的回调
    }];

2.自定义子类

通过在另一个页面中完成操作，将完成的操作结果通过delegate来发送到主页面。

其他操作：

 NSOperationQueue *queue = [[NSOperationQueue alloc]init];
    NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        NSData *data = [[NSData alloc]initWithContentsOfURL:[NSURL URLWithString:@"http://d.hiphotos.baidu.com/image/pic/item/9922720e0cf3d7ca60432a7df41fbe096b63a907.jpg"]];
        dispatch_async(dispatch_get_main_queue(), ^{
            _imageback.image = [UIImage imageWithData:data];
        });
    }];
   
    [queue addOperation:operation];//异步线程

    [operation setCompletionBlock:^{
        NSLog(@"over");//执行完成之后的回调
    }];
    
    NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{
        NSData *data = [[NSData alloc]initWithContentsOfURL:[NSURL URLWithString:@"http://d.hiphotos.baidu.com/image/pic/item/9922720e0cf3d7ca60432a7df41fbe096b63a907.jpg"]];
        dispatch_async(dispatch_get_main_queue(), ^{
            _imageback2.image = [UIImage imageWithData:data];
        });
    }];
    [queue addOperation:operation2];
    
    [operation addDependency:operation2];//添加依赖，操作1依赖于操作2，操作2完成后开始操作1

//修改操作的优先级
    operation.queuePriority = NSOperationQueuePriorityLow;
    operation2.queuePriority = NSOperationQueuePriorityVeryHigh;
    //修改最大并发数
    queue.maxConcurrentOperationCount = 1;
    
    [operation waitUntilFinished];//等待线程完成之后继续执行，会阻塞线程
    
    queue.suspended = YES;//暂停队列  只会暂停后面新添加的队列，但是对前面已经添加的队列无效。
    queue.suspended = NO;//继续队列