What are the best practices for unit testing in MVVM?

1 Choosing a testing framework

There are many testing frameworks available for different programming languages and platforms, such as NUnit, xUnit, MSTest, JUnit, and more. You should choose a framework that suits your needs and preferences, as well as the features and tools that it offers. Some of the criteria to consider are the syntax, the assertions, the runners, the integration with your IDE, the code coverage, and the mocking capabilities.

3 Testing the viewmodel layer

The viewmodel layer is the core of the MVVM pattern, as it contains the logic and state that bind the model and the view layers. You should test the viewmodel layer by verifying that it can manipulate the model data, expose the properties and commands for the view, and handle the user interactions and events. You can use mock objects or fake models to isolate the viewmodel layer from the model layer. You should also test the viewmodel's observability, meaning that it can notify the view of any changes in its state.

4 Testing the view layer

The view layer is the most challenging layer to test, as it involves the user interface elements and their interactions. You should test the view layer by verifying that it can display the data from the viewmodel, execute the commands from the viewmodel, and react to the user inputs and events. You can use UI testing frameworks or tools, such as Selenium, Espresso, Appium, and more, to automate the testing of the view layer. You should also test the view's binding, meaning that it can update itself according to the viewmodel's notifications.

5 Following the AAA pattern

The AAA pattern is a common structure for writing unit tests, where each test consists of three phases: Arrange, Act, and Assert. The Arrange phase is where you set up the initial conditions and inputs for the test, such as creating the objects, initializing the variables, and mocking the dependencies. The Act phase is where you execute the code or method that you want to test, such as calling a command, setting a property, or triggering an event. The Assert phase is where you verify that the expected results or outcomes are achieved, such as checking the values, states, or behaviors of the objects.

6 Applying the FIRST principles

The FIRST principles are a set of guidelines for writing effective and maintainable unit tests, where each letter stands for a quality: Fast, Isolated, Repeatable, Self-validating, and Timely. Fast means that the tests should run quickly and not take too long to execute. Isolated means that the tests should not depend on each other or on external factors, such as the network, the database, or the file system. Repeatable means that the tests should produce the same results every time they run, regardless of the environment or the order. Self-validating means that the tests should have a clear pass or fail criteria, and not require manual inspection or interpretation. Timely means that the tests should be written as soon as possible, preferably before or during the development of the code.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?