设置唯一属性

arrangedObjects

valueForKey:

userName

NSArray *userNames = [[arrayController arrangedObjects] valueForKey:@\"userName\"];
if([userNames containsObject:@\"theNewUsername\"]) {
    // user name taken
} else {
    // user name available
}

indexOfObjectPassingTest:

NSArray *objects = [arrayController arrangedObjects];
NSUInteger index = [objects indexOfObjectPassingTest:^(id object, NSUInteger index, BOOL *stop) {
    if([[object valueForKey:@\"userName\"] isEqualToString:@\"theNewUsername\"]) {
        *stop = YES;
        return YES;
    }
    return NO;
}];
if(index == NSNotFound) {
    // user name available
} else {
    // user name taken by the object at \"index\" within \"objects\"
}