Fetches the extra data previously set for uid, either with
set_contact_extra or when adding contacts.
| this |
An BookCache |
| uid |
The uid of the contact to fetch the extra data for |
| out_extra |
Return location to store the extra data |
| cancellable |
optional Cancellable object, or null |
|
true on success, otherwise false is returned and throws is set appropriately. |