1.4.1 The Model
Entities taking the Model role are responsible for holding the running state and business functionality of the application, either as a whole or as the part that is relevant to that specific MVC Triad, as either data (stored values) or activity (routines computing relevant data). They define the protagonists of the application’s domain, their mechanism of operation and cooperation. Model objects can represent, for example,
- An interface to a database, filesystem, low level driver, hardware machine
- Access to a computational backend.
- Proxies for a remote service
- A representation of business entities such as weather forecast in a specific area, people’s details in a phonebook, tracks information in a music CD, student grades
- In some designs, information related to the GUI, such as selected items, or the X-axis scale of a plot.
- A running process.
When implemented, a Model can go from a dictionary-like of key/value pairs to a complex network of objects with well defined interfaces. Regardless of the implementation, Models must provide the following three services:
Querying: to inquire about their current state, either represented by high-level domain objects (Object Oriented approach), or through an IO layer of routines providing access to the data (Data Oriented approach). In the Object Oriented approach, the Model objects generally represents an identifiable part of the domain of your application, and provide access to data through a well-defined object-oriented interface. The Model can also perform computation, generally of information derived or associated to the main data it represents. In the Data Oriented approach, the routines “speak the domain language” and have high-level semantics to access the data, generally from a data storage (e.g. disk).
Altering: to modify the current state. The Model interface provides setter
methods or routines to modify its state. The Model performs consistency
checks about the data it handles, enforcing fundamental integrity: for example,
it can raise an exception or ignore the passed data if a method
set_current_temperature is called passing a string instead of a float, or a
set_length is called with a negative value.
Notifying: to inform interested parties that a change has occurred in its state. The Model allows interested objects to listen for change notifications. When a change occurs, these objects will be notified of this fact and can act accordingly, normally by synchronizing themselves against the Model’s new contents.
Model objects should provide core application functionality through a clear and self-documenting interface, exposing what can be done with the program’s state. To operate, they can depend only on other Model objects or other components of the application that don’t involve presentation, like an IO or Service layer. The relationship among Model objects is that of a strong dependency.
On the other hand, a Model should not contain nor be dependent for its functionality on any graphical entity, nor contain formatting/visual logic for presentation (e.g. logic to make a negative value represented in red, or logic to present the date in US vs. ISO representation). Model objects should be completely unaware of how user interaction is handled by the application they live in, and should have a weak dependency toward its listeners via the notification generic interface.
For data modification, all the Model does is to process incoming requests in the form of method calls. Normally these requests are performed by Controllers, but a Model can also change due to requests from other subsystems (for example, a subprocess executing in the background), from another Model component or because it is monitoring a backend (e.g. a database, or a filesystem) and the monitored entity changes. The only entities never allowed to issue a change request to the Model are the Views.
The Model should enforce integrity of the data, but it does not necessarily enforce validity: data might be correct (for example, integers for min/max values) but overall invalid for computation (for example, if min > max). While integrity should be enforced, storing invalid data can be acceptable: depending on the application, invalid data may be marked as such in the Model by the part of the code that detects the invalidity, so that the View can represent it (for example, with a red font); An invalid state might be needed as a stepstone to reach a valid state at the end of a set of changes done by the User via the UI.
With the above guidelines and restrictions in place, the resulting implementation is robust, flexible and testable: Views and Controllers are the components that change the most as the application evolves, and a Model that is agnostic to these changes is easier to code and maintain. The Model can be tested independently from the rest of the application, and it opens itself to scripting, allowing the User to change the Model programmatically instead of through the GUI.