Catroid is an on-device visual programming language for Android smartphones and tablets that is inspired by the Scratch programming language for PCs developed by the Lifelong Kindergarten Group at the MIT Media Lab.

Catroid allows kids and teens from age 8 up to easily write Android apps using only their mobile device, without the need for an additional PC. Catroid projects are written in a graphical “Lego”-type style as known from Scratch or AppInventor.

Paintroid is part of the Catroid project and is a graphical paint editor for the Android platform that, among others, allows setting parts of pictures to transparent (pinch zoom up to single pixel level), which is an important feature if one wants to extract, e.g., one’s own head from a photograph while deleting all the background, or some other part from any image, in order to use the resulting shape as a graphical element inside one’s app.

Catroid’s Web 2.0 community site allows kids and teens to share their apps in a way similar to Scratch's community site, including the sharing and remixing of Catroid project files under the AGPL license, the aim being to facilitate the learning of programming skills among kids and teens (and also older users).

Typical users of Catroid would be, e.g. an 11 year old girl wanting to share her Hannah Montana animation with her friends, or a 15 year old boy wanting to implement a multi-player multi-touch space invaders game. Thus, usability is a major aspect of the project and is accorded top priority.

The motivation behind the Catroid project is that programming is an important cultural technique on the same level as mathematics and physics, from a practical as well as from a philosophical point of view. Our aim thus is to popularize the skills needed to program from an early age on in a fun and engaging way that will facilitate the spread of its adoption among young people all over the world.

The Catroid team released a first public beta in March 2012. There are several subprojects involving, e.g. the bidirectional coupling with an AR.Drone quadcopter, with a Lego Mindstorms robot, with a Robocup soccer robot via the Arduino open-source electronics platform, a program that allows to package a Catroid project as an apk file (i.e., as a stand-alone Android app), an HTML5 player (though not supporting all Catroid features as not all sensors etc can be accessed), a localization / internationalization support site (we currently support several languages, with speakers of English, Mandarin, Cantonese, Hindi, Arabian, German, Turkish, French, Japanese, Urdu, and Malaysian in the team), and the building of an engaging tutorial game inspired by the funny tutorial of Nintendo’s Wario Ware: D.I.Y. programming game for the Nintendo DS.

The Catroid team currently is composed of more than 70 active developers. We use the test-driven development method (test-first style; tests used as declarative executable program documentation) with several additional practices from extreme programming such as ping pong pair programming in frequently changing pairs, kanban system, very clear self-explaining clean code (including test code; no comments in code unless not expressible in the code itself; no prose documentation), continuous usability consideration (paper prototyping on a daily basis, the personas method, usability expert feedbacks, and usability tests with children), continuous integration using Mercurial on Google Code, automated unit-, functional-, regression-, and monkey-testing, permanent refactoring, the YAGNI principle, and collective code ownership.

External contributors will need to follow our test-driven development method. While we minimize the amount of test code and are very pragmatic, please understand that we are concerned of producing high quality and highly maintainable code that can be extended and refactored without the need for direct human communication and without the need to refer to non-executable documents.