IPhone SDK - Leaking Memory with performSelectorIn

Question

Maybe someone can help me with this strange thing:

If a user clicks on a button, a new UITableView is pushed to the navigation controller. This new view is doing some database querying which takes some time. Therefore I wanted to do the loading in background.

What works WITHOUT leaking memory (but freezes the screen until everything is done):

WorkController *tmp=[[WorkController alloc] initWithStyle:UITableViewStyleGrouped];
self.workController=tmp;
[tmp release];

[self.workController loadList]; // Does the DB Query
[self.workController pushViewController:self.workController animated:YES];  

Now I tried to do this:

    // Show Wait indicator
    ....

    WorkController *tmp=[[WorkController alloc] initWithStyle:UITableViewStyleGrouped];
    self.workController=tmp;
    [tmp release];

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

-(void) getController {
    [self.workController loadList]; // Does the DB Query
    [self.navigationController pushViewController:self.workController animated:YES];
}

This also works but is leaking memory and I don't know why ! Can you help ?

By the way - is it possible for an App to get into AppStore with a small memory leak ? Or will this be checked first of all ?

Thanks in advance !

Answer 1

Btw, you're calling pushViewController: from a background thread. This is bad.

You should only do things to the UI - like pushing view controllers and changing UI items - from the main thread. If you don't, things break.

See the Cocoa Fundamentals Guide section titled "Are the Cocoa Frameworks Thread Safe?": it says "All UIKit objects should be used on the main thread only."

Answer 2

No, small memory leaks will not (most likely) you application to be rejected from appstore.

In your example as you run your method in separate thread you should create and dispose NSAutoreleasePool object for that thread to handle autoreleased objects. Following changes to getController method should do the trick:

-(void) getController {
    NSAutoreleasedPool *pool = [[NSAutoreleasedPool alloc] init];

    [self.workController loadList]; // Does the DB Query
    [self.navigationController pushViewController:self.workController animated:YES];

    [pool release];
}

For more details see Autorelease Pools section in memory management guide. Relevant quote from there:

If you spawn a secondary thread, you must create your own autorelease pool as soon as the thread begins executing; otherwise, you will leak objects. (See “Autorelease Pools and Threads” for details.)