iOS6 permissions [Contacts]

On iOS6, apple introduce new privacy control, user can control the accessment of contact and calender by each app. So, in the code side, you need to add some way to request the permission. In iOS5 or before, we can always call

ABAddressBookRef addressBook = ABAddressBookCreate();

to get the addressbook without any problem, but in iOS6, if you don't have permission, this call will just return empty pointer. That why we need to change the method to get ABAddressBookRef.

__block BOOL accessGranted = NO;
if (ABAddressBookRequestAccessWithCompletion != NULL) { // we're on iOS 6
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    ABAddressBookRequestAccessWithCompletion(addressBook, ^(bool granted, CFErrorRef error) {
        accessGranted = granted;
        dispatch_semaphore_signal(sema);
    });
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    dispatch_release(sema);  
}
else { // we're on iOS 5 or older
    accessGranted = YES;
}

if (accessGranted) {
    // Do whatever you want here.
}

In the code,semaphore is used for blocking until response, while ABAddressBookRequestAccessWithCompletion will ask for permission if the app didn't ask before. Otherwise it will just follow the settings in Settings-Privacy-Contact.