Unit Testing in Flutter: From Workflow Essentials to Complex Scenarios

By Prometteur solutions 17 Min Read

The amount of interest shown in Flutter has reached an all-time high, which is both welcome and long-needed. Android, iOS, macOS, web, Windows, and Linux are all supported by Google’s open-source software development kit (SDK). All of these are supported by a single codebase for Flutter. And unit testing is essential to provide a Flutter program that is dependable and consistent. This testing protects the app from errors, faults, and other types of defects by proactively improving the code quality before it is assembled.

What is automated testing?

The process of reviewing and verifying that a software application does what it is intended to do is referred to as software automated testing. It eliminates bugs and lowers the overall cost of development.

To understand there is something that needs to be understood. The sole purpose of automated testing is to prevent the software from already predetermined bugs. It does not mean that the software is entirely free of errors or bugs. It simply denotes that the software does not include typical bugs.

Following the development phase, we write on to writing the expected test cases (TDD is an exception). In most circumstances, we will ship the software once it has successfully passed all of the test cases. However, suppose a bug is discovered while the software is being used in production. In that case, we will correct it and then write the test cases to prevent the software from including the same bug in the future.

Testing is nothing more than the act of verifying something. In developing a new feature or repairing an existing bug, test cases assist us in determining whether or not all previously implemented features and units function as intended. It ensures that the ongoing development will not compromise any of the functions that are already in place.

“Testing demonstrates that there are bugs, not that there are none. — Edsger Dijkstra

Unit testing is implemented in Flutter in a manner that is very similar to how it is implemented in other technology stacks. In Flutter, you follow the following processes;

  • Analyze the code.
  • Set up data mocking.
  • Define the test for the group (s).
  • Define the test function signature (or signatures) for each test group.
  • Write the tests.

What is Unit Testing?

The automated testing process includes a step tested as “unit testing,” verifying the dependability of smaller chunks of code by running them through various use cases. A unit can be anything from a variable to a function to a method, class to a state.

The first and most fundamental level of testing is called unit testing. At this level, we test the functionality of each feature’s underlying components.

A unit test is comprised of three stages:

  • Arrange
  • Act
  • Assert

What needs to be done during the Arrange phase is to establish the object of the unit that needs to be tested and prepare the prerequisites for our test. These preparations include setting up the state variable and mocks, among other things. Of course, depending on the requirements, the Arrange phase might not be necessary at all.

During the Act phase, we put the unit through its paces by passing arguments and modifying some state before storing the outcome, if there is one.

During the Assert step, we check to see if the unit behaves as we would anticipate it to. For example, we might anticipate that a certain method will be called, or that the outcome will be the same as what was anticipated.

Is Unit Testing Important?

Writing and running unit tests is a simple process. This results in significant time savings.

Through unit testing, we are able to uncover bugs at an earlier stage. This results in significant cost and time savings.

Because we write about all of the possible outcomes of the unit, it is easy for everyone to understand what the unit is all about. As a result, this results in improved documentation.

Because doing so might cause the unit to become unusable, we don’t refactor your code too often. Instead, the presence of unit tests gives us the assurance necessary to rework our code.

Debugging is straightforward. Given that we have complete knowledge of the failing situations, we can zero in on the specific unit that is the root of the bug.

Simply glancing at the test cases gives us a clear picture of what the unit is meant to understand. As a result, maintenance over the long run is simplified.

What can we test with unit testing?

To build a reliable set of unit tests, we must first comprehend the elements that must be validated within each individual unit.

Unit testing often concentrates on the following elements:

  • State variables
  • Function/variable calls
  • The arguments to the function
  • Function returns

Variables that belong to the state are variables that exist outside of the local scope. This may be a class property or a global variable that more than one unit can access. In most cases, it maintains a state.

In a unit, we keep an eye on the following scenarios, among others:

  • Check to see if the value of the constant or the final variable is correct.
  • Values at the beginning of the state variables.
  • Check to see if a particular function is called 1…n times by the unit.
  • Check to see if the unit never calls a particular function.
  • Check that the state variables are being updated in the way that you anticipate.
  • The outcome of the unit was exactly the same as what was anticipated.
  • Be sure to check for empty situations whenever a string, list, or any other type of complicated DS is involved; this is especially important when we traverse through the DS.
  • Check for cases of null (Only for types that can be null). Dart is now safe against null)
  • Check the type of the variable or the argument (although this may not be necessary if we effectively use Dart’s type system).

A few points on the operation of Flutter: When a project is formed, the testing process is simplified by the framework thanks to the autoloading of the flutter test library. Flutter can now read, run, and evaluate unit tests due to the library’s contributions. Furthermore, the test folder used to store tests is also automatically created by Flutter. Therefore, it is of the utmost importance not to rename or relocate the test folder, since doing so will interfere with its functionality and, as a result, our capacity to execute tests. Additionally, it is vital to add the suffix _test.dart to the names of our test files. This is because Flutter uses this suffix to identify test files.

We will establish an orderly storage area for the test files that we will write, a system in which different groups of tests will each have their own “homes” that are immediately recognized. In view of the necessity imposed by Flutter to locate tests within the test folder, let’s replicate the folder structure of our source code and place it under test. After that, each time we write a new test, we will put it in the relevant subfolder, which are as follows: Unit tests of Model classes should be placed in a folder that is titled model, much like clean socks should be placed in the sock drawer of your dresser and folded shirts should be placed in the shirt drawer.

Adopting this file format brings openness to the project. It provides the group with a straightforward method of determining which parts of our code have corresponding tests.

What are some of the best practices for unit testing?

  • The unit tests need to be quick.
  • Unit tests ought to be straightforward Unit tests ought to be predetermined
  • The emphasis should be placed on unit tests.
  • In unit tests, it is acceptable to repeat code.
  • The description of the test must be straightforward.
  • Unit tests must be quick. Because we will be running the full test suite ourselves throughout the development phase, each unit test in our test suite should be able to run in under a minute. This aids in the early detection and correction of bugs. If it takes a longer time, we will typically incorporate that job into a pipeline of some type.

Unit tests ought to be straightforward. When we go through a unit test case, all of the required information should be contained within that test case. It is not acceptable for us to navigate the code to understand a single test case. Therefore, the unit test should not require an explanation.

Determinism should be applied to unit tests. Without making any changes to the source code, a unit test should always replicate the exact same behavior no matter where or when it is tested. The results of a unit test should not be dependent on any outside variables or conditions, such as the current time, a database, the web, or a native API. In most cases, we make fun of them.

The focus should be on the unit tests. A single unit should serve as the only focus of a unit test. Within the unit test context, we should not test the dependencies.

In unit tests, it is acceptable to repeat code. The goal of a unit test should be to ensure that the code being tested is as straightforward as possible. It should be possible for anyone to understand the test case without the need to investigate its dependencies or the present state of the unit. Therefore, repeating certain pieces of code is OK if doing so makes the program easier to comprehend and more straightforward.

The description of the unit test should be straightforward. A good description must contain four parts:

  • The unit that will be put to the test
  • The condition that the unit is in at the moment
  • The contribution that we will make
  • The reaction that we are preparing for.


The primary goal of unit testing is to compartmentalize and zero down on the unit that is now being evaluated instead of concentrating on testing any other dependencies that may be present. However, in most circumstances, we must rely on other dependencies such as database servers, web servers, platform APIs, and other external devices and components.

Let’s assume for the moment that our existing unit is reliant on a web API. The test completes slowly but successfully when run on a live server. However, the unit test will fail if the server is unavailable. This results in unpredictability in the unit test. Because we will no longer have control over the web server. When the web server experiences an outage, it is not our fault. The use of mocking occurs at this point.

In the context of unit testing, “mocking” refers to the process that is carried out whenever the unit being tested is dependent on external resources. Mocking is used to isolate and concentrate on the code being tested rather than on the behavior of any external dependencies.

MockTail is an abbreviation for “Without Code Generation.”

Mocktails primary goal is to provide a straightforward and well-known API for the development of mocks in Dart, complete with null safety, without requiring manual mock creation or code generation.

Felix Angelov, the same person who produced bloc, equatable, and other similar programs, is the author of this library. Even more impressive than his other packages is the fact that this library has complete coverage of its code.

For your best unit testing services, and to consult with unit testing experts, reach out to us now!


In Flutter, what is Unit Testing?

Unit tests are quite helpful when verifying the operation of a single method, class, or function. The test package provides the bare-bones infrastructure for developing unit tests, and the flutter test package contains supplementary utilities for testing widgets.

How many kinds of unit testing does the Flutter framework support?

Three distinct kinds of tests may be run with Flutter. First, a method or class’s operation is validated by a unit test. Without actually running the app itself, the functionality of Flutter widgets may be validated with the use of a widget test. Finally, the entire application is put through what is known as an integration test, which is also known as end-to-end testing or GUI testing.

In Flutter, what does it take to create a unit test?

The process for performing unit testing in Flutter is identical to the procedure that is used in most other frameworks. After establishing the classes and functions that need to be tested (the test cases), the next step is to evaluate the code, set up data mocking, define the test groups, define the test function signatures for each test group, write the tests, and run them.

Why is it vital to perform unit testing?

Unit testing can eliminate or significantly cut the number of bugs in an application, resulting in an improved user experience from the moment an app is made available to the public. In addition, reading unit tests is a great way to assist new developers in learning and understanding your code, which is an additional benefit.

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