An important constraint about the Model that we stressed throughout the book is that it should never be involved in GUI-related business. UI Retrieving Model breaks this rule. Credit for this approach goes to Scott Miller.
One of the tasks of the Model is to retrieve information from a data source, such as a file, memory, or database. In some cases, the Model may have to retrieve information that only a direct session with the User can provide. We can break the rules and let the Model pop up a GUI dialog to request this information. The Model is considering the User as a database, whose API involves the use of graphical widgets.
The interaction diagram design of a UI retrieving Model is depicted in Figure
The core of the design is the short-lived UserUI object. In its most
basic form this object is a modal dialog, prompted to the User and
awaiting for input. The modality of the dialog keeps the operation
synchronous, a requirement imposed by the synchronous nature of the
Model object.
Once the dialog is closed, the value is gathered and returned to the requesting client. Depending on the nature of the retrieved information, it might be useful for the Model to cache it instead of prompting the User again.
One trivial example of this pattern would be a Model object representing the User session, with a password field initially set to null. When the program starts the session, it will ask for the credentials to the Model layer. Asking the username will return the proper string, but asking the password may return null if no password has been set yet. The model is then authorized to retrieve this information from the User by a GUI dialog, maybe trying some non-interactive strategies first, like checking in a configuration file. When the user acts on the dialog, the Model stores the password and returns it to the Presenter layer, which then proceeds with the authentication logic. If the password is incorrect, the presenter will ask the Model user to reset the password field, and the user will be prompted again for the password, otherwise any additional request will use the cached password without additional requests to the user.