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1 Basics of MVC: From Smart-UI to Traditional MVC

The aim of GUI programming is to provide an interactive and updated visual representation of the current state of the application. A typical set of requirements is the need to simultaneously represent this state graphically in various forms (i.e. a table of numbers and an XY plot), modify this state through an unpredictable sequence of mouse/keyboard events, and keep the state and its visual representation always synchronized and up-to-date. At the same time, the application state must respect business logic constraints and be kept within these constraints. At the code level, these requirements are generally translated into objects interacting through a communication network of various degrees of complexity. How can we satisfy these requirements, while at the same time providing a flexible design that keeps the object communication simple, understandable and organized as much as possible?

MVC addresses the above needs. It does so by clever subdivision of competences and roles in the code, while introducing constraints that keep a potentially chaotic communication well organized and streamlined. MVC is incredibly flexible and adaptable, as we will see, and using one of the many styles will be a matter of preference or constraints/best practices of the development framework of choice.

GUI Programming is a complex task. Many different levels of understanding and handling are needed: UI design and usability consideration, multithreading and multiprocessing for asynchronous evaluation, event notification coherence and balancing, adaptability of the GUI to unexpected requests and changes of style. There are plenty of dialogs, buttons, lists, all with different performance, presentation and visibility needs. In a sense, a GUI application develops emerging properties characteristic of a complex system where multiple entities interact. Keeping this system under strict control is the only way to maintain chaos at bay.

In this chapter, we will start from the most trivial implementation of a GUI application with both visual and non-visual logic: a single class responsible for everything. This approach, known as Smart UI, will be our foundation for a progressive refactoring into the three basic components of MVC: Model, View and Controller. We will call the result a “traditional MVC” design.

Note

Throughout this book, example code will be presented to clarify design ideas with concrete implementation. All implementation will make use of the python programming language, and all GUI rendering will be performed via the excellent Qt toolkit and its python bindings.

It is important to note that all the examples presented are not meant to act as a Qt programming tutorial. Qt provides mechanisms to address some MVC needs in an effective and idiomatic way, but in the upcoming code these mechanisms will be skipped on purpose to demonstrate concepts in a clear, simplified and unencumbered way.

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