Overview of This Lesson

Model View Controller is a common design pattern used in most applications that have a graphical user interface. You can use this pattern in pretty much any kind of application: a web application, a Java application, a smart phone application, etc. The premise is that you separate the code for the data model, the actual interface, and the logic that works behind the scenes. This allows you to create more modular and re-usable cold while following the SOLID principles.

The Model-View-Controller (MVC) Pattern

A standard Graphical User Interface (GUI) has the following functionality:

• Allows the user to enter inputs into a program.
• Processes those user inputs in some way (e.g. calculations, searching, sorting, comparing).
• Presents meaningful output to the user that contains results of the processing of the user inputs.

There are three categories of code that make up a GUI in most object-oriented programming languages:

1. Presentation code: code that creates the UI components (e.g. text boxes, buttons, and radio buttons) and lays them out on the screen (layout managers).
2. Event logic: code that executes when the user manipulates the interface components (e.g. searching for an employee in an employee list when the user clicks a "Search" button).
3. Classes: OOP classes that model data the application needs, such as the Employee class that models employee records in a database.

Model-View-Controller, or MVC, is the most common design pattern used to create GUIs. MVC separates the presentation, event logic, and classes by defining these three design components:

• Model - the data and/or object/classes that model that data.
  • This should be code/classes that are independent of the rest of the components. You should be able to use the model in any application.
  • Models data, nothing else! There should be no other code/logic in the model.
• View - displays data/output to the user, provides a means for the user to manipulate the data, etc.
  • Views are specific to the model they're allowing a user to manipulate.
  • There are often multiple views on a single screen (the view is not necessarily the whole screen: a button is a view, a text field is a view).
• Controller - the code/logic that executes behind the scenes
  • Each View is paired with a Controller; they stay in sync with each other.
  • The controller interprets user actions on the view, such as button clicks or typing in a field.
  • Views send user inputs to the controller, and the controller processes those inputs.
  • The controller sends outputs back to the view so the user can see them.

MVC is actually the design pattern used in web applications today:

• Model: the HTML code, which defines the structure and content of the page.
• View: the CSS code, which defines the formatting and layout of the page.
• Controller: the browser, which provides a means for the user to manipulate the elements on the web page.

An example of MVC used in Java:

• Model: An Student class that models a student in a college. Students have an id, first name, last name, and Contact information. Students can register for courses and receive grades.
• View: The UI that allows a registrar clerk to update the student's personal information, select the courses for which the student wants to register, and enter grade data for transcripts.
• Controller: The Java code that retrieves Student data to display on the View, updates the Student information when the clerk enters a grade or registers the student for a course, and displays information about a Student object on the View.

The idea behind MVC is that it allows you to create modular and re-usable components. For example, you might create a UI that allows a user to enter data for players on their fantasy hockey team. You would likely have a Player class that models a single player. You can re-use that Player class in several applications. You could even use the Player UI in other applications that needed to provide a means to input player information. You can also re-use the UI in different parts of the application: not just to input player data, but also to search for or edit player data. By separating the event logic (validate player input data, use data to construct a player object, save a player object to a database, get a collection of players, get a single player by jersey number, etc) you can use that same UI for several different functions in the application.