Why not ALAssetLibrary -enumerateGroupsWithTypes: usingBlock: "stop" when I say?

Question

I noticed that although I set the pointer *stop BOOLto YES, my enum block is executed twice. I thought the installation *stop = YESwould stop after the first?

[lib enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
    NSLog(@"Enumerating a group");

    //should stop after first?
    *stop = YES; 
} failureBlock:^(NSError *error) {
}];

Magazines:

2014-03-05 12:27:29:363 AppName[3625:1547] Enumerating a group
2014-03-05 12:27:29:379 AppName[3625:1547] Enumerating a group

From the documentation, it -enumerateGroupsWithTypes:usingBlock:is asynchronous, but also does it automatically parallelize? Both enumerations happen in the main thread, so maybe I should do my own stop? It seems strange to suggest stopif it is not fully respected in the way the documentation suggests.

EDIT

For what it's worth, I added my own coercion, and it works. But why do I need this?

    __block BOOL stopItForReal = NO;
    [lib enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
        NSLog(@"Enumerating a group");

        //does not stop enumeration
        *stop = YES;

        //actually stops enumeration. I win, runtime!
        if (stopItForReal)
            return;
        stopItForReal = YES;

    } failureBlock:^(NSError *error) {
    }];

Answer 1

, ALAssetsLibraryGroupsEnumerationResultsBlock group = nil. - :

if (group != nil) 
{
    // your code.
}