Tutorials - QML unit testing

In this tutorial you will learn how to write a unit test to strengthen the quality of your Ubuntu QML application. It builds upon the Currency Converter Tutorial.


  • Ubuntu 14.04 or later - Get Ubuntu
  • The Currency Converter tutorial - if you haven't already, complete the Currency Convertertutorial
  • The QML test runner tool - open a terminal with Ctrl+Alt+T and run these commands to install all required packages:
$ sudo apt-get install qtdeclarative5-dev-tools qtdeclarative5-test-plugin

What are unit tests?

To help ensure your application performs as expected it’s important to have a nice suite of unit tests. Unit tests are the foundation of a good testing story for your application. Let’s learn more about them.

A unit test should generally test a specific unit of code. It should be able to pass or fail in only one way. This means you should generally have one and only one assertion or assert for short. An assertion is a statement about the expected outcome of a series of actions. By limiting yourself to a single statement about the expected outcome, it is clear why a test fails.

Unit tests are the base of the testing pyramid. The testing pyramid describes the three levels of testing an application, going from low level tests at the bottom and increasing to high level tests at the top. As unit tests are the lowest level, they should represent the largest number of tests for your project.

In Ubuntu, unit tests for your QML application:

  • Are written in JavaScript within an Ubuntu Testcase type. This makes it easy to write tests with only a few lines of JavaScript
  • Are executed with the qmltestrunner tool, which will determine whether they pass or fail

Testing with qmltestrunner

QML makes developing applications easy. Fortunately, it makes testing those applications easy too! The qmltestrunner tool allows you to execute QML files as testcases. As we will learn later, these files should contain test_ functions and use the Ubuntu Testcase type.

Here’s an example of a very basic unit test:

import QtTest 1.0
import Ubuntu.Test 1.0
UbuntuTestCase {
   name: "MathTests"
   function test_math() {
       compare(2 + 2, 4, "2 + 2 = 4")

Running the example

To help you see what unit tests look like in real life, grab a branch of the currency converter code from the tutorial. Run this command in a terminal:

bzr branch lp:ubuntu-sdk-tutorials

This creates a new folder called ubuntu-sdk-tutorials. The code we'll be looking at inside the branch is under getting-started/CurrencyConverter. On the terminal, now switch to the tutorial folder:

cd ubuntu-sdk-tutorials/getting-started/CurrencyConverter

If you navigate to that folder with the file browser, you can click on the CurrencyConverter.qmlproject file and open it with the Ubuntu SDK IDE:

Inside you will notice a tests folder that contains three subfolders named unit, integration, and functional. You’ll notice this corresponds to the testing pyramid.

Since we are interested in the unit tests, navigate to the unit folder. Inside you’ll find the tst_convert.qml file, which is a QML unit test.

So let’s run it! Switch back to your terminal and run:

qmltestrunner -input tests/unit

If everything went successfully, you should see a small printout displaying all tests as passing.

What to test

Unit tests are a great way to ensure our code and functions react as we expect them to. The currency converter project has a convert function, so let’s test it! First a quick look at the QML function in the Main.qml file:

function convert(from, fromRateIndex, toRateIndex) {
       var fromRate = currencies.getRate(fromRateIndex);
       if (from.length <= 0 || fromRate <= 0.0)
               return "";
       return currencies.getRate(toRateIndex) * (parseFloat(from) / fromRate);

The function converts currencies given a currency and the rate indexes to convert from and to. With that in mind, let’s write some tests to test and ensure it’s working properly.

Our first test case

Since we want to test the convert function, let’s feed it some specific input and ensure it returns the proper results. Let’s start simple enough and pass in a value of 1. You can see this test case written out in test_convert1() inside of tst_convert.qml.

function test_convert1() {
       // convert 1.00 from currency 5 to currency 10
       var value = currencyConverter.convert("1.00", 5, 10)
       verify(value > 0)

This shows the basic format for a unit test. We call the function with a known value and then assert our expectations about the result. If for some reason the result is different than our expectations, the test will fail.

Another great use of unit tests is to explore how your code will react in edge cases. For instance, what would happen (and what should happen!) if 0 is passed to the function? What about -1? How about a string? Explore these edge cases and test them!

Running a testcase

After you’ve written your set of test cases, it’s important to understand how they will be executed. Ubuntu Testcase contains some built in methods that control execution. For example, here’s the actual order of execution for our example unit test suite in tst_convert.qml.


If you need to execute some code before or after running a test that is common to all tests, put it in init () / cleanup (). If you have a bit of code that needs to execute before any tests are run, or after the test suite is complete, place it in initTestCase() / cleanupTestCase() respectively.

For our test suite you can see we do have some code in both initTestCase() and cleanupTestCase(). Since our app requires an Internet connection, initTestCase() accounts for the data loading time. Loading data for an application is a common use case of something that might need to be added to initTestCase().

Finally, you will also notice that the test cases run in ascending order, sorted by name. However, generally tests should be self contained and independent, so don’t be tempted by assuming a test will run after a prior test based on ordering.


You've just learned how to write unit tests for a form-factor-independent Ubuntu application for the phone. But there is more information to be learned about how to write qml tests. Check out the links below for more documentation and help.