SGL: Scala Game Library

Scala Game Library (tentatively abbreviated as SGL) is a library for developing cross-platform 2D video games in Scala. It provides a high-level API for building 2D games, and can deploy to Desktop, Android, and HTML5. Other platforms, including iOS and consoles are on the roadmap.

SGL is still in development, but is intended to provide an extensive toolkit to build games, with a core abstraction on top of platform-specific features, as well as offering an out-of-the-box implementation for many common features needed in games, such as physics, collision detection, tilemaps, and scenes management.

Features

The main selling point of SGL is to provide a platform-independent Scala game framework to build games and then deploy them to any platform. You can get started by writing a core generic game implementation and configure any backend with a few lines of Scala. Then, you can iterate by running the AWT backend, which is as straighforward as:

sbt desktop-awt/run

You can quickly iterate on your game without the need to spend a lot of time on deploying to your final platform such as mobile or console.

The current implementation provides the following backends:

• Desktop with JVM and AWT. This is mostly convenient during development, but can also serve as a final release if you are able to distribute your game to people that have a JVM. It should be cross-platform across Windows, Mac, and Linux.
• Android. The Android backend is implemented with the native Android SDK for Java, which means that SGL supports Android natively.
• Web with Scalajs. The web backend is implemented with scalajs and uses the HTML5 canvas for graphics.
• Native. The native backend implemented with scala-native is able to generate a native executable that can then be run on the target platform without a JVM. The support for native is not complete yet, but the current implementation is a proof of concept. Further extensions to this backend should enable SGL to eventually target iOS and various consoles.

Work in Progress

This is a work in progress, so please don't hesitate to get in touch if you are interested in writing a game in Scala. This is in no case production ready, but I'm putting this project out there as I think it has a good potential, and I'm looking for feedback from people interested in such a library.

I'm developing new features on a need basis. I'm working on some Android games, and I started to use this library as it was much nicer to build and test the game on my Linux desktop, and only deploy to the phone for the final tests. I'm constantly adding new features to the library based on my needs for my games, but if you miss some other features, please let me know and I will add them! You're also very welcome to contribute :)

If you check out the latest master branch, and find out that some stuff is not working as expected, please understand that the project is evolving rapidly and I'm likely just breaking existing stuff to try to improve the overall design. The library does truly help in building actual games, and I successfully developed one published Android game with it. The library helped tremendously, by being entirely Scala-based and by allowing for transparent porting from the Desktop to the Android platform.

Depending on SGL

SGL is split across several sub-projects so that games built by the framework only packs the necessary dependencies. The organization is to provide a core sub-project which defines all the APIs and roughly one backend per platform. A game should then depend on both the core abstraction and the platform on which it will deploys. Cross-platform games can be further split into smaller units, with a cross-platform core logic that will only depends on the core SGL abstractions and various platform-specific implementations. The snake project demonstrates how you can organize a game to be cross platform.

SGL is currently spllited into the following sub-projects:

• coreJVM, coreJS, coreNative (the core abstractions compiled for JVM, scalajs, and scala-native).
• desktopAWT, depends on coreJVM and provide window and graphics with AWT.
• desktopNative, depends on coreNative and use scala-native and OpenGL to build a native executable.
• html5, depends on coreJS and use scala.js to generate a javascript game.
• coreAndroid and android, for the android platform.
• jvmShared, some non-core utilities shared by all JVM-based platform

These projects are defined in the built.sbt file and have their sources in each corresponding subdirectory.

Getting Started

There are minimal working projects in the examples folder. Cloning this directory and copy-pasting one of the example project is a good way to start working on your own game (keep the copy in the examples folder as the build file are pointing to the SGL root directory). I also developed a small, but complete, open-source game intended to demonstrate some of the features of the library.

Design Principles

• Games and only games. This is not a general media toolkit. The only things that should be build with this library are games.

• True Scala library. We want to expose a Scala-like library a much as possible. That means very clean and elegant design with type-safety. We are not going to compromise for compatibility with Java.

• Entirely cross-platform, no cheating. The core library should abstract everything and only exposes features that are truly cross-platform. Any platform-specific extensions should be provided in a type-safe way.

• Generic but pragmatic. We try to remain as generic as possible, in the sense that only features that would be useful for at least two different games would be integrated. However, we want to provide a very effective tool, so almost anything that is remotely useful for building games should be made available. Whenever a problem has many alternative implementations, we should try to provide an abstract interface, with each alternative implementation available and let the user select the one he prefers.

• 2D only. The library does not target 3D games. I believe there are an infinite number of wonderful games that can be build entirely in 2D, and I would rather focus on getting a great library to build 2D games than an average library to do everything.

• No magic build tricks. Everything is explicitly implemented in Scala. No additional code generator to handle the different platform, setting up a deployment platform should be simple enough to be done manually.

Gallery

This section is intended to list some actual published and commercial games, as well as games currently in production that are using the SGL. The list is quite short, but I'm hopeful that it will grow over time.

Published

• WinSmash, available for Android
• Scalavator, available for Android and for the Web. Code source available on GitHub.

In Developement

• A game based on the existing Android title Rat Trap

Code Organization

I heavily use the cake pattern as a means to abstract the different backends and to correctly modularize the system. A good article to introduce using the cake pattern for dependencies injection is this one. There is also a great talk that describes how to use the cake pattern, which closely ressembles our usage here.