Virtualvfunc_Retrieve the secret value of this object.
Each retrievable object has a single secret which might be a password or some other secret binary value.
This function returns immediately and completes asynchronously.
optional cancellation object
called when the operation completes
Virtualvfunc_Complete asynchronous operation to retrieve the secret value of this object.
asynchronous result passed to callback
Interface for implementing Retrievable. Contains only the virtual methods that need to be implemented.