Asynchronously looks up for credentials for source.
When the operation is finished, callback will be called. You can then call lookup.end to get the
result of the operation.
| this | |
| source |
an Source, to lookup credentials for |
| cancellable |
optional Cancellable object, or |
| callback |
a TaskReadyCallback to call when the request is satisfied |
| user_data |
data to pass to the callback function |