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A Dockerfile for provisioning a build environment for Epidemic

Introduction

Epidemic is a game about exponential growth written in Haskell. (Here's a demo on YouTube.)

But building it for mobile devices ain't that easy.

Apart from requiring a GHC cross-compiler, you must cross-compile various C libraries and then build cross-compiled versions of all the Haskell libraries which, unfortunately, doesn't work out of the box for some libraries when installing them with Cabal.

So, with the aid of Docker I wrote a script to build a fully fledged Android build environment. This builds on earlier work that I did in the docker-build-ghc-android repo. docker-build-ghc-android just builds a GHC 7.8.3 cross-compiler targetting ARMv7, while this repo builds all the C and Haskell libraries required to build Epidemic.

In conjunction with android-build-epidemic-apk you can build an APK for installation on your Android device.

Installation

Please ensure that you are using at leaast Docker version 1.6. Check with docker version.

(Optional) Build debian-wheezy-ghc-android

You probably only want to do this if for some reason you can't download sseefried/debian-wheezy-ghc-android from the Docker Hub registry. It's rather large at 1.1G.

Follow the instruction in the README.md here.

Once you've done that you'll need to tag the resulting image as sseefried/debian-wheezy-ghc-android locally to build the image this Dockerfile specifies.

Build with Docker

At the command line simply type:

$ docker build .

This will take a while to build. First, unless you performed the previous step, Docker must download the image sseefried/debian-wheezy-ghc-android (about 1.1G). It will then download, clone and build a bunch of libraries. Go get a coffee, drink it slowly, go for a long walk and then come back. Once it's finished type:

$ docker images

You will get something like:

REPOSITORY                            TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
<none>                                <none>              3b16cf90e485        6 minutes ago       5.923 GB
...

You can tag the image with something memorable like:

docker tag <image id> epidemic-build-env

You now have two options for building and installing Epidemic.

Option A: Build, copy APK and install

You can simply run an interactive shell and build the APK inside a running container.

$ docker run -it epidemic-build-env /bin/bash
androidbuilder@283089ad80b9:~/build$ cd android-build-epidemic-apk

Now follow the instructions in the README.md here

The adb tool is not installed in the image so once you have built the APK you will want to copy the APK to your local machine (which presumably has adb installed in it).

Keep the container running. In a fresh shell (in another terminal window) type:

 $ docker ps

You'll get something like:

CONTAINER ID        IMAGE                       COMMAND             CREATED              STATUS                  PORTS               NAMES
d4a82703a3a9        epidemic-build-env:latest   "/bin/bash"         About a minute ago   Up 57     seconds                           dreamy_ptolemy

This will give you a container ID (d4a82703a3a9 or dreamy_ptolemy here).

 $ docker cp dreamy_ptolemy:/home/androidbuilder/build/android-build-epidemic-apk/bin/com.declarative.games.epidemic.beta-debug.apk .

You can now install this APK with

 $ adb install -r com.declarative.games.epidemic.beta-debug.apk

Option B: Share a local directory, build, and install

Another option to is checkout android-build-epidemic-apk locally and then share this directory with a running container.

$ git clone https://github.com/sseefried/android-build-epidemic-apk
$ docker run -v /local/path/to/android-build-epidemic-apk:/home/androidbuilder/build/android-build-epidemic-apk -it epidemic-build-env /bin/bash

(This will overwrite the directory in the Docker container.)

Now, inside the interactive shell in the running container, follow the instructions in the README.md here

Once you are done the APK will be in /local/path/to/android-build-epidemic-apk/bin, and you can install it with:

 $ adb install -r com.declarative.games.epidemic.beta-debug.apk

Want to help beta test the game?

Actually, the version of the game I've released will not be the final version. If you'd like to stay in the loop then subscribe here.

Guiding principles of the Dockerfile

Here I outline some of the guiding principles behind the design of the Dockerfile.

  • Download specific versions of libraries. Check them against a SHA1 hash.
  • cabal install specific versions of libraries
  • git clone specific commits of repositories

This way we increase the likelihood that Docker will complete the build into the future.

Why so many small scripts?

I call these scriptlets. Apart from logically structuring the Dockerfile so that each library is built in isolation, this also means I can take advantage of Docker's cache which is a form of filesystem checkpointing. See a blog post I wrote on this. Also see the next question.

Why do you ADD a script just before RUNning it?

This made developing this build script that much easier. While developing a specific scriptlet I didn't want to have to build from the beginning each time I made a small change. Docker's caching of sub-images meant that I could start building again from the point where a scriptlet changed and know with 100% certainty that the filesystem was in exactly the same state it was the last time I tried to build from that point. As a consequence the structure of "adding just before running" also makes this Dockerfile more maintainable.