Larry Price

And The Endless Cup Of Coffee

Writing Go/QML Convergent Ubuntu Apps

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Thinking about writing an Ubuntu application that will work in Unity 8? You’ll be writing a “convergent” app, which is an application that can respond to touch or mouse, and will adapt appropriately to a phone, tablet, or desktop screen. It will even be able to update its display on-the-fly if, say, you plug your phone into a monitor or bluetooth keyboard.

There are lots of ways to write a convergent application with the Ubuntu SDK. You can build apps in pure QML, C++ with QML, QtQuick, pure HTML5/Javascript, or even Golang with QML. We’ll be focusing on go apps for the rest of this post.

Go is a young language, and the recent release of go 1.6 has introduced some nasty changes particularly involving cgo. It has also introduced vendoring by default, which is a welcome change.

I’m using go 1.6.2. For me, the current project template provided by the Ubuntu SDK feels quite broken. I can’t get things to build locally, and furthermore I see no options for pushing an ARM build to my device.

Fortunately, I found this ubuntu-go-qml-template which is a template to enable running/building a go/qml application locally while also supporting building/installing onto an arm device. The kicker? This tool was designed to work for go 1.3.3. Sigh! Since I’m unwilling to compromise and use old technology, I forked the project and updated it to fit my more modern needs.

Because our go template will depend on QML, we depend on the go-qml project to create the necessary C bindings to allow us to use QML. However, with the update to go 1.6, the current version (revision 2ee7e5f) of go-qml will give a runtime error from cgo with the wonderfully helpful panic: runtime error: cgo argument has Go pointer to Go pointer. Another thorn in our side. Fortunately, this issue is in previously tread-upon ground and there is a fork of go-qml with enough of the cgo issues fixed to run our application with no problems. In the ubuntu-go-qml-template I forked above, I’ve gone ahead and vendored this fork of go-qml. It all works because of the default vendoring available in go 1.6.

With that background out of the way, let’s run through getting a project started:

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$ sudo apt-get install golang g++ qtdeclarative5-dev qtbase5-private-dev \
                       qtdeclarative5-private-dev libqt5opengl5-dev \
                                           qtdeclarative5-qtquick2-plugin
# installing dependencies...
$ git clone https://github.com/larryprice/ubuntu-go-qml-template.git your-project-name
# cloning repo...
$ cd your-project-name
$ chroot-scripts/setup-chroot.sh
# building a chroot for ubuntu-sdk-15.04
# distro=vivid, arch=arm
# with go 1.6.2 with armhf compilation support

You may need to have other dependencies including click or phablet-tools. The above commands installed dependencies, cloned the repo, and built and/or updated a chroot for building our source for arm.

Next you’ll want to setup the project for your own needs. The original template creator included a nifty setup script to get this done:

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$ ruby setup.rb -v -n your-project-name -a "Your Name" -e "[email protected]" \
                   -d "your-developer-namespace"

This will do some fancy gsub and file renaming to make the template your own.

If you check src/, you’ll find a main.go file ready for your use. You’ll also find a main.qml file in share/qml/. You can vendor all of your dependencies in vendor/, where you’ll already find the qml package.

As far as getting your application to work, there are more scripts available:

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$ ./build.sh
# this will build your project locally
$ ./run.sh
# this will build your project locally and then run it on the desktop

The best part about this template is the ability to build for arm and load your applications onto a device:

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$ ./build-in-chroot.sh
# builds the package using the vivid+armhf chroot we set up previously
$ ./install-on-device.sh
# builds for vivid+armhf and installs the click directly on
# the first USB-connected Ubuntu Touch device

Now you can run and install go/qml applications on desktop or on devices. Time to go build something cool!

Disclaimer: In the future, this template will likely need to be updated for new go versions or new default versions of the Ubuntu SDK. Don’t be afraid to make a comment below or submit a PR.