主要精髓在于 
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]

- (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

// 查看网络任务的状态

[articleInterface.articleArrayTask enumerateObjectsUsingBlock:^(NSURLSessionDataTask *taskObj, NSUInteger idx, BOOLBOOL *stop) {

    DDLogVerbose(@"当前的文章删除前网络任务状态是==================>>>>>%ld",taskObj.state);

    [taskObj cancel]; /// when sending cancel to the task failure: block is going to be called

    DDLogVerbose(@"当前的文章删除后网络任务状态是==================>>>>>%ld",taskObj.state);

}];

typedef NS_ENUM(NSInteger, NSURLSessionTaskState) {

    NSURLSessionTaskStateRunning = 0,                     /* The task is currently being serviced by the session */

    NSURLSessionTaskStateSuspended = 1,

    NSURLSessionTaskStateCanceling = 2,                   /* The task has been told to cancel.  The session will receive a URLSession:task:didCompleteWithError: message. */

    NSURLSessionTaskStateCompleted = 3,                   /* The task has completed and the session will receive no more delegate notifications */

} NS_ENUM_AVAILABLE(NSURLSESSION_AVAILABLE, 7_0);  

转自:http://blog.5ibc.net/p/89130.html