Running a task in background thread periodically in iOS

Question

I'm a new in iOS. I'm working on the app needs to run a task for getting data from Server in the background thread as i don't want to lock the UI in main thread. This task will take a long time, I tried using the NSTimer but it is still lock the UI. My task is to check new messages in Chat screen, I need to call this task every 5s. If I use NSTimer, when input text , the text seems to freeze a moment while this task is performing. Is there any way to process this task without lock UI. Please give me some advices. Thanks so much.

== UPDATE CODE ==

 - (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];
            }

        });
    });
}

Answer 1

Scheduling the task using an NSTimer is indeed the right way to go. You just need to make sure you're running your heavy non-UI code on a background thread. Here's an example

- (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, 0), ^{
        //Do background work
        dispatch_async(dispatch_get_main_queue(), ^{
            //Update UI
        });
    });
}