This whole situation creates a demand for a universal, multi-platform way to program. The problem isn't exactly new: one solution to it, created by Sun in 1995, is the Java programming language. Its marketing promise—write once, run anywhere—was never fulfilled and the language itself is unreasonably cumbersome to use. This led to many mocking variations of the slogan, culminating with write once, run away that refers to many developers abandoning Java in favor of better programming languages, including Python.

Not coincidentally, Kivy—the main topic of this book—is a graphical user interface library facilitating easy creation of multi-platform Python applications. The main features of Kivy toolkit are as follows:

Speaking of third-party packages, Kivy can be seen as a superset of many battle-tested components: a large part of its functionality relies on well-known libraries such as Pygame, SDL, and GStreamer. The API that Kivy exposes, however, is very high-level and unified.

It's worth mentioning that Kivy is free and open source MIT licensed software. In practice, this means that you can use it commercially without paying licensing fees. Its full source code is hosted on GitHub, so you can also patch bugs or add new features to it.

Chapter 1, Building a Clock App provides a gentle introduction to writing applications with Kivy. It covers the Kivy language, layouts, widgets and timers. By the end of the chapter we build a simple Clock app, similar to the one found in your cellphone.

Chapter 2, Building a Paint App is a further exploration of the Kivy framework's components and functionality. The resulting Paint app showcases the customization of built-in widgets, drawing arbitrary shapes on canvas and handling multi-touch events.

Chapter 3, Sound Recorder for Android serves as an example of writing a Kivy-based Android app. It shows how to use the Pyjnius interoperability layer to load Java classes into Python, which enables us to mix Android API calls with a Kivy-based user interface.

Chapter 4, Kivy Networking is a hands-on guide to building a network application from the ground up. In covers a number of topics, from creating a simple protocol to writing server and client software in Python, and culminates with the Kivy Chat application.

Chapter 5, Making a Remote Desktop App exemplifies another way of writing client-server apps. This chapter's program is based on the HTTP protocol—the one that powers the Internet. We develop a command-line HTTP server first, and then build the Remote Desktop client app with Kivy.

Chapter 6, Making the 2048 Game walks you through building a playable replica of the 2048 game. We demonstrate more complex Kivy functionality, such as creating custom widgets, using Kivy properties for data binding, and processing touch screen gestures.

Chapter 7, Writing a Flappy Bird Clone introduces another Kivy-based game, this time it's an arcade game similar to the well-known Flappy Bird title. Over the course of this chapter we discuss the use of texture coordinates and sounds effects, implement arcade physics and collision detection.

Chapter 8, Introducing Shaders demonstrates the use of GLSL shaders in the context of a Kivy application. In this tutorial you will learn about OpenGL primitives such as indices and vertices, and then write incredibly fast low-level code that runs directly on the GPU.

Chapter 9, Making a Shoot-Em-Up Game continues where the previous chapter left off: we use the knowledge of GLSL in order to build a side-scrolling shooter. A reusable particle system class is developed along the way. This project concludes the series and capitalizes on many techniques that were explained throughout the book, such as collision detection, touch screen controls, sound effects and so on.

Appendix, The Python Ecosystem, gives you more on Python libraries and tools.

This section briefly discusses the requirements needed to effectively follow the narrative, implement, and run Kivy applications. Personal computer running a modern operating system—a Mac, Linux, or Windows box—is implied.

Installing and running Kivy

Kivy can be downloaded from the official site (http://kivy.org/); just choose an appropriate version and follow the instructions. This whole procedure should be pretty straightforward and simple.

Kivy downloads

To check whether the installation is working, follow these instructions:

• On a Mac:

  1. Open Terminal.app.

  2. Run kivy.

  3. The Python prompt, >>>, should appear. Type import kivy.

  4. The command should complete with no errors, printing a message along the lines of [INFO] Kivy v1.8.0.

• On a Linux machine:

  1. Open a terminal.

  2. Run python.

  3. The Python prompt, >>>, should appear. Type import kivy.

  4. The command should print a message similar to [INFO] Kivy v1.8.0.

• On a Windows box:

  1. Double-click kivy.bat inside the Kivy package directory.

  2. Type python at the command prompt.

  3. Type import kivy.

  4. The command should print a message similar to [INFO] Kivy v1.8.0.

  

  A terminal session

Running a Kivy application (basically, a Python program) is achieved similarly:

• On a Mac, use kivy main.py

• On Linux, use python main.py

• On Windows, use kivy.bat main.py (or drag-and-drop the main.py file on top of kivy.bat).

When learning a new programming language or technology, the first thing demonstrated to students is traditionally a "hello, world" program. This is how it looks in Python:

print('hello, world')

The Kivy version of a "hello, world" is a little lengthier and consists of two files, namely, a Python module and a .kv layout definition.

from kivy.app import App

class HelloApp(App):
    pass

if __name__ == '__main__':
    HelloApp().run()

Layout

A layout file is typically named after the application class, in this case HelloApp, sans the App suffix and in lowercase: hello.kv. It contains the following lines:

Label:
    text: 'Hello, Kivy'

This is a very simple Kivy layout definition consisting of a single widget, Label, with the desired text inside. Layout files allow building complex widget hierarchies in a concise, declarative fashion, which is clearly not displayed here, but will be heavily used over the course of this book.

If we run the program now (refer to the Installing and running Kivy section for details), this is what we'll get:

Our first application powered by Kivy

Now you're ready to move on to the first chapter and start writing real programs.

This book is intended for programmers who are comfortable with the Python language and who want to build desktop and mobile applications with a rich graphical user interface in Python with minimal hassle. Knowledge of Kivy, while certainly won't hurt, is not required—every aspect of the framework is described when it's first used.

At various points in this book, we will make an analogy between Kivy and web development practices. However, an extensive knowledge of the latter also isn't required to follow the narrative.

In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.

Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "Kivy application's entry point is customarily called main.py."

A block of code is set as follows:

from kivy.app import App

class HelloApp(App):
    pass

if __name__ == '__main__':
    HelloApp().run()

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

# In Python code
LabelBase.register(name="Roboto",
    fn_regular="Roboto-Regular.ttf",
    fn_bold="Roboto-Bold.ttf",
    fn_italic="Roboto-Italic.ttf",
    fn_bolditalic="Roboto-BoldItalic.ttf")

Any command-line input or output is written as follows:

pip install -U twisted

New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "The first event handler is for the Start and Stop buttons."

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