Asynchronously connects to the message bus specified by bus_type.
When the operation is finished, callback will be invoked. You can then call @get.end to get the
result of the operation.
This is an asynchronous failable function. See get_sync for the synchronous version.
| bus_type |
a BusType |
| cancellable |
a Cancellable or null |
| callback |
a TaskReadyCallback to call when the request is satisfied |
| user_data |
the data to pass to |