How to cancel NSURLSessionDownloadTask with AFURLSessionManager?

Question

I wanna download some pictures with AFURLSessionManager in method @selector(collectionAction):

NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
AFURLSessionManager *manager = [[AFURLSessionManager alloc] initWithSessionConfiguration:configuration];
  for (int i = 0; i < _collectionArr.count; ++i) {

    NSURL *URL = [NSURL URLWithString:URL_ADDRESS(portNumber,_collectionArr[i])];
    NSURLRequest *request = [NSURLRequest requestWithURL:URL];

    NSURLSessionDownloadTask *downloadTask = [manager downloadTaskWithRequest:request progress:nil destination:^NSURL *(NSURL *targetPath, NSURLResponse *response) {
        NSURL *documentsDirectoryURL = [[NSFileManager defaultManager] URLForDirectory:NSDocumentDirectory inDomain:NSUserDomainMask appropriateForURL:nil create:NO error:nil];
        return [documentsDirectoryURL URLByAppendingPathComponent:[response suggestedFilename]];
    } completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {

        NSLog(@"File downloaded to: %@", filePath);
        if (!filePath) {
            [manager invalidateSessionCancelingTasks:YES];
//          [manager.operationQueue cancelAllOperations];

            [self performSelector:@selector(collectionAction) withObject:nil afterDelay:REQUEST_AGAIN_TIME];
            return ;
        }
        if (i == _collectionArr.count - 1) {
            [SVProgressHUD showImage:nil status:@"Done"];
        }

    }];
    [downloadTask resume];
}

If the task has failed, it will perform the selector automatically [self performSelector:@selector(collectionAction) withObject:nil afterDelay:REQUEST_AGAIN_TIME]; If I do nothing the output like this

2014-12-02 11:34:09.711 MoneyPower[11216:1204579] File downloaded to: (null)
2014-12-02 11:34:12.936 MoneyPower[11216:1204579] File downloaded to: (null)
2014-12-02 11:34:12.951 MoneyPower[11216:1204579] File downloaded to: (null)

and the number is rolling and rolling, then my iPhone is almost stop working. So I want to cancel the tasks if the any of these tasks failed. But with[manager invalidateSessionCancelingTasks:YES]; or [manager.operationQueue cancelAllOperations]; I have another error

2014-12-02 11:34:13.040 MoneyPower[11216:1204788] *** Terminating app due to uncaught     exception 'NSInternalInconsistencyException', reason: 'An instance 0x17947310 of class __NSCFLocalDownloadTask was deallocated while key value observers were still registered with it. Current observation info: <NSKeyValueObservationInfo 0x179449e0> (<NSKeyValueObservance 0x17944b40: Observer: 0x1785d3a0, Key path: state, Options: <New: YES, Old: YES, Prior: NO> Context: 0x3992cc, Property: 0x179462a0>)'
*** First throw call stack:(0x227ddc1f 0x2ffc4c8b 0x227ddb65 0x23485c25 0x2ffded5f 0x2229cb89 0x2ffded5f 0x3055fae1 0x3055fae1 0x836e29 0x8312c9 0x838567 0x839891 0x30685e31 0x30685b84)libc++abi.dylib: terminating with uncaught exception of type NSException

How should I deal with it?