Monthly Archives: February 2012

Automated Testing of Qt apps with Testability Driver

Testing is hugely important to us at Canonical. We all strive to have Ubuntu reliable, consistent and fast. But we’re human, and we make mistakes. Sometimes a bugfix will break something else, and for something as complex as a desktop shell, it’s easy to miss these breakages. While manual tests can help reduce these regressions, realistically we need an automated system to emulate the users inputs and verify our software works as it should – and scream bloody murder if it doesn’t!

In Unity 2D (my project!), we have just introduced an automated User Experience testing system, based on a test framework called Testability Driver (I’ll just call it ‘Testability’ from now on). First off, a clarification:

Testability is for Qt-based applications only!

A limitation: yes, but this requirement comes with a great reward: Testability allows inspection of the tree of QObjects in a Qt application while it is running!

It can read and write object properties, call methods and slots, verify signals are emitted, as well as fake mouse/keyboard/gesture inputs, grab visual outputs, and measure performance.

And best of all, Testability is open source and maintained by Nokia! That means everyone can run and contribute tests! 🙂

To show it off, here is a screengrab of the Testability Visualizer application which allows you to connect to a running application, dig into the QObject tree and investigate what’s going on (here, the Unity 2D Launcher – it will be good to zoom in):


On the left is an interactive snapshot of the UI, in the centre is the QObject tree which you can navigate, and on the right is the list of the selected QObject’s properties, methods and signals you can interact with.

On display in that screengrab are some of the attributes of the QDeclarativeImage object which draws the Terminal icon in the launcher (this is defined in QML!) – you can read off the source of the image, the x,y coordinates, width & height, and a whole lot more.

All these properties, methods, signals, etc, are scriptable via Ruby. Thus we can emulate every interaction that a user can make with the application, and determine the reaction matches our expectation.

And all this power comes with almost no changes* to the source code! (* = just need to add object names, see later)

This forms the foundation for the Unity 2D User Experience testing suite.

How Testability Works

First some definitions:
“target” = the machine with the software being tested
“host” = the machine running the test suite, and controlling the target

Testability works as follows

  • any Qt application using Qt4.6+ can be tested by executing it with the “-testability” switch.
  • a standalone server “qttasserver” runs in the background.
  • With the -testability switch, the Qt library tries to load a “” plugin which establishes a connection between the application and qttasserver, giving qttasserver access to the root node of the QObject tree.
  • qttasserver then climbs the QObject tree, reads all the info it can and converts it to an XML format. It also can receive commands and cause the application to react upon them (click here, type, etc..).
  • A series of Ruby scripts connect to qttasserver, receive and parse this XML and allow us to script tests and interactions with the application.


Note that there is a clear divide between the machine of the target and of the host.

This means Testability is great for testing software on embedded devices too. (Indeed Meego has been using it for that exact task!). It also reduces the risk that packages used to run the test suite could interfere with the software being tested.

However there’s nothing stopping you having the target and host the same machine.

[The Unity 2D test framework also includes an extra helper library to control the X server, to control the mouse and keyboard, and to manipulate windows on the desktop. As far as X is concerned, a human is controlling it.]

With the ability to fake any form of user input, and directly read the output from the shell applications
themselves, we can test almost every behaviour of the desktop shell. And as a result, the quality of the user’s experience will only go up.

How to get Testability and start writing tests

Unfortunately Testability is not available in Ubuntu’s repositories just yet, but I have it packaged in a PPA. Installation takes a few steps, so I suggest that interested people consult this wiki page.

Tests are just Ruby scripts, so running tests is just a matter of running a Ruby script!

I think it easiest to show how to use Testability with an example:

# Example test for Unity 2D with Testability Driver.
# Note: this probably won't succeed on your installation. Only works
# on unity-2d trunk *right now*, something that will change soon.# This is only a taster!require 'tdriver'
include TDriverVerify

# Establish connection to 'qttasserver' on target
@sut = TDriver.connect_sut(:Id => 'sut_qt')

# Execute the application to test, supplying '-testability' flag
@app = :name => 'unity-2d-launcher',
                 :arguments => '-testability' )
# ------------- Start of Testing Code ----------------

# Check Launcher is 65 pixels wide
verify_true(1, 'Launcher is not 65 pixels wide') {
  @app.LauncherView()['width'] == '65'

# Check that Terminal tile is using correct icon source
verify_true(1, 'Terminal icon in Launcher is wrong') {
  @app.LauncherList( :name => 'main' ) \
      .QDeclarativeItem( :name => 'Terminal' ) \
      .QDeclarativeImage( :name => 'icon' )['source']     == 'image://icons/utilities-terminal'
# this is a bad test, as the name "Terminal" can be # localised, meaning a fail if you're using a non-English # installation. Choosing good objectNames is important!

# ------------------- Clean up -----------------------
# This closes (kills actually) the launcher when we're done.

There is some boiler-plate code there, but the bits I want to point out are:


This causes Testability to search for an object of type “LauncherView()” in the QObject tree, and if found reads its “width” property and returns it. Then in Ruby, we can check this value matches what we expect (65).

The second test does a similar operation, but needs a little more help. As there are many tiles, we need to help Testability to find the exact tile we want. We do this by adding some object names (“main”, “Terminal”) to the C++/QML.

By consulting with the Visualizer image above, maybe you can see how Testability navigates the tree to find the icon source for the Terminal tile!

Once you can track down objects uniquely, you can then start interacting with them, send mouse clicks, set properties, call methods, etc. This power gives you great flexibility in testing your application!


As a demo, here is a video of a part of the Unity 2D test suite in action. On the right is Ubuntu Oneiric running inside VirtualBox, where the Launcher will be tested. On the left is my host machine terminal running the test suite.

Various hide/show tests are being performed, with windows in the way, mouse being controlled, keyboard shortcuts being pressed etc.

Our policy is that every new feature and bug fix in Unity 2D will now be tested like this. You can see our growing test suite here. This will ensure Unity 2D remains reliable, consistent and fast. Thanks to Testability!

Tests will improve your project’s quality too. Will you give it a try?

– Gerry Boland


Posted by on February 1, 2012 in Uncategorized


Tags: , , ,