AsyncOperationProgressCallback<P> typedef
Callback which called periodically in the implementation specific frequency
when async operation is progress some degree.
The parameter is P which represents some object to represent the progress.
It may tell degree of progress with percentage int, double representing
completion ratio, or String describing qualitative status.
It might have additional information to help to discrimination among
concurrent operations by caller.
Implementation
@optionalTypeArgs
typedef AsyncOperationProgressCallback<P> = void Function(P progress);