在iOS中定期在后台线程中执行任务

我是iOS中的新手我正在研究应用程序需要运行一个任务,从后台线程中的服务器获取数据,因为我不想在主线程中锁定UI.这个任务需要很长时间,我尝试使用NSTimer,但它仍然锁定UI.我的任务是检查聊天屏幕中的新消息,我需要每5秒调用一次这个任务.如果我使用NSTimer,当输入文本时,文本似乎在此任务执行时冻结一会儿.有没有办法处理这个任务没有锁UI.请给我一些建议.非常感谢.

==更新代码==

- (void)performBACkgroundTask
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),^{
        //Do BACkground work

        if([[NSUserDefaults standardUserDefaults] boolForKey:@"LoggedIn"]) {
            NSDictionary * userDictionary = [[NSUserDefaults standardUserDefaults] DictionaryForKey:@"SessionDictionary"];

            NSString *authenKey= [userDictionary valueForKey:@"authToken"];

            NSString* limit = @"1000";

            [[LSDataManager sharedDataManager] getLatestmessagesWithAuthKey:authenKey andLimit:limit withBlock:^ (NSDictionary* responseDict)
             {
                 if (responseDict) {
                     [self loadDataFromServer:responseDict];

                     NSArray* lastmessageArray= nil;

                     //filter message data
                     if (self.messagesArray.count >0) {

                         if (!self.isSeller) {

                             lastmessageArray = [self filtermessageData:self.messagesArray withProductID:self.productID withSellerID:self.receiverID withBuyerID:self.senderID];
                         }
                         else
                         {
                             lastmessageArray = [self filtermessageData:self.messagesArray withProductID:self.productID withSellerID:self.senderID withBuyerID:self.receiverID];
                         }

                         NSLog(@"filter array %@",lastmessageArray);

                         if([lastmessageArray count] >0){
                             //[self loadmessages:lastmessageArray];
                             if (self.TemPDAtasource == nil) {
                                 self.TemPDAtasource = [NSMutableArray array];
                             }
                             else
                             {
                                 [self.TemPDAtasource removeAllObjects];
                             }

                             self.TemPDAtasource = [[[ContentManager sharedManager] generateConversation:lastmessageArray withSenderID:self.senderID] mutableCopy];

                         }
                     }
                 }
             }];
        }


        dispatch_async(dispatch_get_main_queue(),^{
            //update UI

            //compare 2 arrays
            if ([self.TemPDAtasource count] == [self.datasource count]) {
                NSLog(@"both are same");
            }
            else{
                NSLog(@"both are different");
                self.datasource = [self.TemPDAtasource mutableCopy];

                [self refreshmessages];
            }

        });
    });
}

- (void)viewDidLoad {
    [super viewDidLoad];    
    [self startTimedTask];
}

- (void)startTimedTask
{
    NSTimer *fiveSecondTimer = [NSTimer scheduledTimerWithTimeInterval:5.0 target:self SELEctor:@SELEctor(performBACkgroundTask) userInfo:nil repeats:YES];
}

- (void)performBACkgroundTask
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,^{
        //Do BACkground work
        dispatch_async(dispatch_get_main_queue(),^{
            //update UI
        });
    });
}