Creates a SimpleAsyncResult.
The common convention is to create the SimpleAsyncResult in the function that
starts the asynchronous operation and use that same function as the source_tag.
If your operation supports cancellation with Cancellable (which it probably should) then you should provide the user's cancellable to set_check_cancellable immediately after this function returns.
| source_object |
a Object, or null. |
| source_tag |
the asynchronous function. |
| callback | |
| user_data |
user data passed to |