Different behavior between iPhone and iPad with the Assets Library

I'm using the Assets Library on an app to enumerate the Photos Events of a device.

My code works fine when i test it on my iPad. The Photos Events are enumerated and i can handle them perfectly. When I try the very same code on my iPhone, nothing happens (and I have Photos Events on this device too). It looks as if the enumeration code was not even called (ie no log appears in the console, cf. code).

Here is the code :

- (void)loadEvents {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
    [library enumerateGroupsWithTypes:ALAssetsGroupEvent
                           usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
                               if (group) {
                                   [photosEventsArray addObject:group];
                                   NSLog(@"Adding group");
                               } else {
                                   NSLog(@"End of the enumeration");
                               }
                           }
                         failureBlock: ^(NSError *error) {
                     NSLog(@"Failure while enumerating assets: %@", error);
                         }];
    [library release];

    NSLog(@"Found %d event开发者_C百科s", photosEventsFound);

    [self performSelectorOnMainThread:@selector(stopSpinner) withObject:nil waitUntilDone:YES];
    [pool drain];
}

My deployment target is iOS 4.1.

Any idea of what is going wrong here?

After more investigation, it seems that on iOS 4.3.5, the enumerateGroupsWithTypes method HAS to be called from the main thread.

I've patched my code this way (setting NO from iPhone and iPod Touch, and YES from iPad):

if (scanAssetsInBackground) {
    [self performSelectorInBackground:@selector(loadEvents) withObject:nil];
} else {
    [self performSelectorOnMainThread:@selector(loadEvents) withObject:nil waitUntilDone:YES];
}

Works fine with that patch.

There's not much information about this in Apple docs and there's no way to know which way (background or main thread) is the right way to scan assets libraries.