Perform fetch without freezing the UI

Question

I'm making a small chat app. In my app I'm using NSFetchedResultsController. There are 2 tableViews, 1 for lobby and 1 for the chat room. The problem is that whenever I enter a chat room I have to wait for NSFetchedResultsController to perform fetch and load all the data before I can start typing anything. I was wondering if it's possible to preform fetch in background or somehow let the user start typing before the last messages are loaded.

So if I set that part in viewDidLoad method, my UI just freezes until all the data is loaded:

NSError *_error;
if (![self.fetchedResultsController performFetch:&_error]) {
    NSLog(@"Unresolved error %@, %@", _error, [_error userInfo]);
}

and if I set that piece of code into a separate method and then call that method in viewDidLoad in background

[self performSelectorInBackground:@selector(fetchIt) withObject:nil];

tableView is just not updating, so I have to go back to the first tableView and then come back to get any results.

Thank you.

Any help would be appreciated

update

I did try a few other ways to do it.

Way 1:

-(void)reloadTheTable
{
  [self.tableView reloadData];
}

-(void)fetchIt
{
  NSError *_error;
  if (![self.fetchedResultsController performFetch:&_error]) {
    NSLog(@"Unresolved error %@, %@", _error, [_error userInfo]);
  }
  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc]init];
  [self performSelectorOnMainThread:@selector(reloadTheTable) withObject:nil waitUntilDone:NO];
  [pool release];
}

- (void)viewDidLoad {
  //some code
  [self performSelectorInBackground:@selector(fetchIt) withObject:nil];
  //some other code
}

result: tableView doesn't show any data, need to re-open that view controller

Way 2:

- (void)viewDidLoad {
  //some code
  dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
    NSError *_error;
    if (![self.fetchedResultsController performFetch:&_error]) {
        NSLog(@"Unresolved error %@, %@", _error, [_error userInfo]);
    }
  });
  //some other code
}

result: tableView updates, but UI freezes

Way 3:

-(void)fetchIt
{
  NSError *_error;
  if (![self.fetchedResultsController performFetch:&_error]) {
    NSLog(@"Unresolved error %@, %@", _error, [_error userInfo]);
  }
}

- (void)viewDidLoad {
  //some code
  [self performSelectorOnMainThread:@selector(fetchIt) withObject:nil waitUntilDone:NO];
  //some other code
}

result: UI freezes, table view updates.

As I read somewhere, everything that has to do something with UI must be done on a main thread. But I still can't figure out how to do it without freezing the tableView.

Thank you for reading.

Any help is appreciated

I think you are looking for [THIS](http://developer.apple.com/library/mac/#documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html). — Kjuly, Jan 11 '12 at 12:57
@Kjuly thanks, but it still is kinda unresponsive. Using dispatch_async(dispatch_get_main_queue(), ^{ ... }); — Novarg, Jan 11 '12 at 13:16
Don't use `dispatch_get_main_queue` this will block your app either, as the UI run in main thread, you need to fetch data in other threads. — Kjuly, Jan 11 '12 at 13:54
@Kjuly I did try both dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0) and dispatch_queue_create(nil, nil), but the tableView just doesn't update — Novarg, Jan 11 '12 at 14:59
@Kjuly just tried dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{ ... });, tableView updates, but it still freezes — Novarg, Jan 11 '12 at 15:42
Hi @Novarg, does it work now? If not, please paste your code in your question. — Kjuly, Jan 12 '12 at 03:14
@Kjuly hi, nope, it doesn't work yet the way I want it to. I updated the question, thank you — Novarg, Jan 13 '12 at 08:13

score 15 · Accepted Answer · answered Jan 13 '12 at 09:21

15

I'm not sure whether this'll work or not, but you can have a try:

- (void)methodThatloadYourData {
  NSError *_error;
  if (![self.fetchedResultsController performFetch:&_error]) {
    NSLog(@"Unresolved error %@, %@", _error, [_error userInfo]);
  }
}

- (void)viewDidLoad {
  //some code

  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),
                 ^{
                   [self performSelector:@selector(methodThatloadYourData)];
                   dispatch_async(dispatch_get_main_queue(),
                                    ^{
                                      [self.tableView reloadData];
                                    });
                 });

  //some other code
}

answered Jan 13 '12 at 09:21

Kjuly

34,476
22
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118

I used this for similar issue. It works as expected except that when the table reloads it is BLAM, all the records appear at once somewhat abruptly. I think I flinched the first time I saw this it surprise me that much. Can anyone think of a way to make them flow a bit more organically into place without scaring the user? – Dean Davids Mar 20 '12 at 19:40
1

@DeanDavids you can create a method (e.g. `reloadDataAnimated:(BOOL)animated`) with fade animation & put `[self.tableView reloadData];` into this method. Then use `dispatch_async(dispatch_get_main_queue(), ^{ [self reloadDataAnimated:YES]; });` I have not tried, just a suggestion. :) – Kjuly Mar 21 '12 at 04:27
Downvoted. This is an old answer but this is completely wrong. This will return in Threading issues with NSManagedObjectContext. Turn on Multithreading log and you will get CoreData`+[NSManagedObjectContext __Multithreading_Violation_AllThatIsLeftToUsIsHonor__]:. – Jun 13 '17 at 05:12

score 2 · Answer 2 · answered Jan 11 '12 at 18:22

2

Set the fetch request's fetchBatchSize so that you're not actually fetching all the data at once. You probably only need enough records to fill the screen, plus some more to keep initial scrolling responsive. If the data in question is, say, chat messages, I'd think that limiting the fetch to the most recent 50 or 100 messages should speed things up considerably.

answered Jan 11 '12 at 18:22

Caleb

124,013
19
183
272

thanks for the answer, but it's already only fetching last 50 – Novarg Jan 13 '12 at 08:13

Ilanchezhian · Answer 3 · 2012-01-11T18:27:14.527

1

Once the fetch is done in fetchIt method, call the reloadData method in UITableView. For more reference, see Apple documentation on UITableView

Edit:

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView reloadData];
}

For more info, visit the Apple documentation

edited Jan 11 '12 at 18:27

answered Jan 11 '12 at 12:51

Ilanchezhian

17,426
1
53
55

Can you please tell me from where you are reloading the table? – Ilanchezhian Jan 11 '12 at 13:43
I put it in as the last line of "fetchIt" method. The whole method looks like that: -(void)fetchIt{ NSError *_error; if (![self.fetchedResultsController performFetch:&_error]) { NSLog(@"Unresolved error %@, %@", _error, [_error userInfo]); } [self.tableView reloadData]; } – Novarg Jan 11 '12 at 13:50
Implement `NSFetchedResultsControllerDelegate` and implement the method as in Edited part of the answer. – Ilanchezhian Jan 11 '12 at 18:25

score 1 · Answer 4 · answered Jan 11 '12 at 16:48

1

Dispatch your fetch request onto a new thread:

dispatch_queue_t coreDataThread = dispatch_queue_create("com.YourApp.YourThreadName", DISPATCH_QUEUE_SERIAL);

    dispatch_async(YourThreadName, ^{

Your Fetch Request

    });

    dispatch_release(YourThreadName);

answered Jan 11 '12 at 16:48

Nathan

1,609
4
25
42

thanks for the answer, but tableView doesn't reload unless I re-open that view controller – Novarg Jan 13 '12 at 08:56

score 0 · Answer 5 · answered Sep 10 '15 at 11:52

First of all, index your entity properties properly so the queries run faster, especially if sorting. Then look at optimising your fetch request, by setting batch size, limiting the data to the bare minimum necessary. Use the SQL debugger to examine the query, and even test the queries agains the sqlite file yourself using an app like Base. If you try to use multi-threading unnecssarily for such a simple app it will only make matters worse.

FYI in your code, viewDidLoad is already on the main thread. If you are trying to delay the fetch until after the view is loaded then you can use performSelector afterDelay:0.

Perform fetch without freezing the UI

5 Answers5