How to Think Like a Computer Scientist: The PreTeXt Interactive Edition

As we explained in a previous lesson, GUI programs are best implemented as Python classes because it allows you to manage the scope of the variables in your GUI interface and callback functions. However, as GUI programs become more complex, it can become overwhelming to implement them as a single class. If a single class has over than 2,000 lines of code it is probably getting too big to effectively manage. What are some ways to effectively break down complex problems into manageable pieces?

One of the most widely used ways to break down a GUI program into manageable pieces is called the Model-View-Controller software design pattern. This is often abbreviated as MVC (Model-View-Controller). It divides a problem into three pieces:

To says this in more general terms, the controller manages the applications input, the model manages the application’s “state” and enforces application consistency, and the view updates the output, which is what the user sees on the screen. This is basically identical to what all computer processing is composed of, which is:

input --> processing --> output

The MVC design pattern renames the pieces and restricted which part of the code can talk to the other parts of code. For MVC design:

controller (input) --> model (processing) --> view (output)

From the perspective of a GUI program, this means that the callback functions, which are called when a user causes events, are the controller, the model should perform all of the application logic, and the building and modification of the GUI widgets composes the view.

Let’s develop a Whack-a-mole game program using this design strategy. Instead of creating one Python Class for the entire game, the code will be developed as a set of cooperating objects. So where should we begin? I would suggest that the same stages of development we used in the previous lesson are a good approach, but we will create a separate Python class for most of the stages. Let’s walk through the code development.

Let’s create a Python class that builds the user interface for a Whack-a-mole game. The emphasis for this code is the creation of the widgets we need to display to the user. For this version let’s allow the moles to be placed at random locations inside the left frame. To do this we must specify an exact size for the left frame. Otherwise the code is the same as the previous version.

Code

The model for this Whack-a-mole game is fairly simple. We need to keep a counter for the number of user hits on moles that are visible, and a counter for the number of times a user clicks on a mole that is not visible (or just clicks on the left frame and not a mole widget.)

The controller receives user events and sends messages to the controller to update the model’s state. For our Whack-a-mole game, we have the following four basic commands we need to send to the model:

The controller* needs to recognize these events and send them to appropriate methods in the model. The controller needs to define callback functions for these events and register the appropriate event with the appropriate callback. Therefore, the controller needs access to the widgets in the view object. This can easily be accomplished by passing a reference to the view object to the controller when it is created. Summary ——-