Function invoke

pub fn invoke(helper: &mut Program, action: &Action) -> Result

Invoke the given helper with action in context.

Usually the first call is performed with Action::Get to obtain Some identity, which subsequently can be used if it is complete. Note that it may also only contain the username or password, and should start out with everything the helper needs. On successful usage, use NextAction::store(), otherwise NextAction::erase(), which is when this function returns Ok(None) as no outcome is expected.