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 code 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 structure and design of data, and/or objects and classes that represent that data.
  • If your application stores data in a database or structured files such as XML (extensible markup language) or JSON (JavaScript Object Notation), then you have a specific structure for that data (e.g. field/column names, the data types of those fields, whether or not a field is allowed null values, which field is the primary key, etc).
  • This also includes code/classes with properties or data members and methods or functions. These classes are independent of the rest of the components. You should be able to use the model in any application.
  • The model models data, nothing else! There should be no other code/logic in the model. For example, a class that models an Employee object should only contain data members for employee data and methods/functions that model employee behaviours (constructors, accessors, calculate weekly pay, etc). The Employee class should not contain code that creates a user interface for viewing an Employee object or code that reads/writes an employee object's data directly to a data store.
• View - displays data/output to the user, provides a means for the user to manipulate the data, trigger operations, etc.
  • Views are often specific to the model they're allowing a user to manipulate and the functions that the program contains. For example, a View that displays an employee's complete record might not be useful in an application that allows a user to view all the student's grades in a college course. However, a view that simply lists the records in a database table might be useful to any part of the application that lists records, maybe even other applications that list records (it really depends on how modular and re-usable the view's code is).
  • 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. A view can be a container with a search text field and button, and another view on the same screen can be a container that lists the results of the search.
• Controller - the code/logic that executes behind the scenes, it "makes things work".
  • 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, and if appropriate, it executes specific methods/functions to handle those actions.
  • Views send user inputs to the controller, and the controller processes those inputs. For example, when you enter a new employee's data in the Employee Information view, then click a Submit button, the view sends the input data to the controller, and the controller executes a specific function associated with the Submit button (perhaps it performs data validation on the input data and then saves that new data to a file).
  • The controller sends outputs back to the view so the user can see them. For example, if there are errors in the input data, the controller sends error messages to the view. Or, if there are no errors, the controller can insert the new record into the data store and then retrieve a list of all the records, and send those records to the view so they can be displayed.

An example of MVC used in an object-oriented programming language:

• 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 user interface (UI) that allows a registrar clerk to update the student's personal information, select the courses the student wants to register for, and enter grade data for transcripts.
• Controller: The code that retrieves Student data from a database, updates the Student's record in the database when the clerk enters a grade or registers the student for a course, and displays the student's information on the Student Information screen (a view).

MVC is often used in web applications:

• Model: we might have classes/objects (depending on the language) used to represent data objects in formats like JSON or XML: both of these are structured with properties (JSON) or elements (XML) that represent data fields. For example, if your web application sold socks, then you might have different socks stored as JSON objects. A sock object might have properties for colour, size, type, pattern, and material. If you were using an XML file, you might have a <socs></socks> element that contained <sock></sock> objects for each type of sock you sold. Each <sock></sock> might have child elements for <colour>, <size>, <type>, <pattern>, and <material>.
• Views: we have views (HTML pages) for displaying information and gathering input data (such as forms, or pages that display information with tables and lists). You might even have placeholders in a view for dynamic data. For example, a heading might be defined as <h1>Welcome <%=username%></h1>, where <%=username%> is a placeholder that will be replaced with a user's name when they load the page.
• Controller: usually we have server-side code that handles incoming requests and routes those requests to various handlers, which then produce responses that are sent back to the client. There could also be client-side code to perform simple tasks like client-side validation. For example, when a user clicks the Checkout button on their shopping cart, the client sends a request to the server: the server executes code that totals up the purchases and calculates taxes. Then the server code looks up the shipping options and their costs from a database, based on the user's location. Then the server sends back a response with the totals and a list of shipping options to choose from.

In both scenarios, data is a big part of the application. Data can potentially be stored in databases or database servers, or even in file formats like JSON or XML. In the MVC design pattern, the Views are how the user triggers data operations such as saving new or edited data, or requesting a search for a specific value in the data store. The controller interacts directly with the data store to save/retrieve data. The model is the template or pattern the data takes. For example, a user can enter some new data into an HTML form and then press the Submit button. This triggers a request to the server, and that request is processed by the Controller. The Controller then performs the requested operation on the data store and sends any results back to the client via HTTP response. The result is displayed in a 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/edit 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.