How to force screen update for list view from non-UI thread (iPhone)

Question

I have a background thread listening to network, using a callback to process and store data. When it's ready (UI) controller gets a sync Notification, requests data, updates list and refreshes screen ...except screen doesn't update - until user scrolls the screen!

void aMessageArrivedCBack (const std::string text)
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        NSString *msg = [[[NSString alloc] initWithCString:(const char *)text.c_str() encoding:NSUTF8StringEncoding] autorelease];
    [[AMessageStore sharedInstance] aStoreMessage:msg];
    [pool drain];

    [[NSNotificationCenter  defaultCenter]
     postNotificationName:NOTIF_MessageReceived object:nil];
}

Here's the (UI) controller routine for Notification handling, the problem area:

- (void)aMessageWasReceived:(NSNotification*)aNotification
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    [self.msgListView reloadData];
    [self.view setNeedsDisplay];
    [pool drain];
}

Question: how can I force screen update, when new message arrives? I have tried so many things I've lost count.. and just found out I'm trying same old things for fifth time. Still doesn't work.

What I already tried:

Note: "didn't work" below means that screen didn't update.

Data callback is running in non-UI thread, thus Notification is sent & received also in non-UI thread. Distributed notification are not available in iPhone OS, cannot use NSDistributedNotificationCenter.

[self.view setNeedsDisplay] and [self.msgListview setNeedsDisplay] don't work. Didn't really expect, not any more...

[self.msgListView performSelectorOnMainThread:@selector(setNeedsDisplay) withObject:nil waitUntilDone:NO]; really should work, it didn't. I couldn't find object to send it to, I recall. At least "self" didn't work: unrecognized selector sent to instance at 0x14036f0. Terminating app due to uncaught exception 'NSInvalidArgumentException'. Self.msgListView works, but doesn't update screen.

[self.msgListView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationTop]; didn't work.

[self.msgListView insertRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationTop]; didn't work

[self.msgListView visibleCells]; works ok, but what to do with it?

I am positive it's a small thing to fix, but I just can't find it.. Is there any ways to kick the (nice) UI thread or tableView for update? Could I call some _tableView didSelectRowAtIndexPath or anything? Can I do anything with appDelegate? How could I emulate "user scrolls the screen" event myself?

Answer 1

When I do this in my apps I set a delegate for the background thread to send performSelectorOnMainThread:... to.

performSelectorOnMainThread is really the way to go here, most of those other things you list that you have tried do not work.

When you want to update a tableview you send it reloadData and it will in turn send setNeedsDisplay for the view to update. Have you tried that?

Answer 2

It looks like -aMessageWasReceived: is issued in a non-UI thread, and it contains code that attempts to update the UI.

Try performing that method (or its UI-updating contents in a separate method) on the main thread, instead, to get the UI to update in a timely fashion.