iOS 取消网络请求

第一点：不要initialize a new AFHTTPSessionManager object everytime 一定要把manager用成全局的
第二点：把请求返回的task对象丢进数组，下次触发的时候把遍历数组，把之前的所有任务[task cancel]

// somewhere in your class, let's say in ViewDidLoad you should init the AFHTTPSessionManager object  
- (void)viewDidLoad {  
    [super viewDidLoad];  
  
    /// create the AFHTTPSessionManager object, we're gonna use it in every request  
    self.manager = [[AFHTTPSessionManager alloc] init];  
    self.manager.responseSerializer = [AFJSONResponseSerializer serializer];  
  
    /// create an array that is going to hold the requests task we've sent to the server. so we can get back to them later  
    self.arrayOfTasks = [NSMutableArray new];  
    /// discussion:  
    /// an array holds multiple objects. if you just want to hold a ref to the latest task object  
    /// then create a property of NSURLSessionDataTask instead of NSMutableArray, and let it point to the latest NSURLSessionDataTask object you create  
  
}  
  
  
  
-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string;{  
    /// your code goes here  
    /// .....  
    /// .....  
    /// .....  
    /// .....  
    /// till we reach  
    if(stringLength>=3){  
        /// cancel all previous tasks  
        [self.arrayOfTasks enumerateObjectsUsingBlock:^(NSURLSessionDataTask *taskObj, NSUInteger idx, BOOLBOOL *stop) {  
            [taskObj cancel]; /// when sending cancel to the task failure: block is going to be called  
        }];  
  
        /// empty the arraOfTasks  
        [self.arrayOfTasks removeAllObjects];  
  
        /// init new task  
        NSURLSessionDataTask *task = [self.manager GET:urlString parameters:nil success:^(NSURLSessionDataTask *task, id responseObject){  
           /// your code  
        }failure:^(NSURLSessionDataTask *task, NSError *error){  
           /// your code  
        }];  
  
        /// add the task to our arrayOfTasks  
        [self.arrayOfTasks addObject:task];  
    }  
    return YES;  
}  

注意：
这里的task cancel亲测确实能把网络请求cancel掉，可以看下面的log，记住一点，这里cancel之后不代表就不回调
了，只是会回调到error的那个block里面，各位需要信息的可以测试下，在error打个断点就能调试出来了

参考文章链接：http://blog.csdn.net/deft_mkjing/article/details/51985232