Это должно вывести вас на правильный путь.См. Справочник по структуре библиотеки активов
- (void)logVideoSizes {
void (^assetEnumerator)(ALAsset *asset, NSUInteger index, BOOL *stop) {
if(asset != nil){
ALAssetsRepresentation* representation = [asset defaultRepresentation];
NSLog(@"Size = %d", [representation size]);
}
}
void (^assetGroupEnumerator)(struct ALAssetsGroup *, BOOL *) = ^(ALAssetsGroup *group, BOOL *stop) {
if(group != nil) {
[group setAssetsFilter:[ALAssetsFilter allVideos]];
[group enumerateAssetsUsingBlock:assetEnumerator];
}
};
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
[library enumerateGroupsWithTypes:ALAssetsGroupAll
usingBlock:assetGroupEnumerator
failureBlock:^(NSError *error) {
NSLog(@"A problem occured");
}];
[library release];
}