Thus, subsequent calls to it will produce different results. Such non-deterministic behavior makes it impossible to test the internal logic of the GetTimeOfDay method without actually changing the system date and time.
Tests like this would violate a lot of the rules discussed earlier. It would be expensive to write because of the non-trivial setup and teardown logic , unreliable it may fail even if there are no bugs in the system under test, due to system permission issues, for example , and not guaranteed to run fast.
And, finally, this test would not actually be a unit test — it would be something between a unit and integration test, because it pretends to test a simple edge case but requires an environment to be set up in a particular way. The result is not worth the effort, huh?
In its current form, this method suffers from several issues:. It is tightly coupled to the concrete data source. It is not possible to reuse this method for processing date and time retrieved from other sources, or passed as an argument; the method works only with the date and time of the particular machine that executes the code.
Tight coupling is the primary root of most testability problems. The method has multiple responsibilities; it consumes the information and also processes it. Another indicator of SRP violation is when a single class or method has more than one reason to change. From this perspective, the GetTimeOfDay method could be changed either because of internal logic adjustments, or because the date and time source should be changed. It lies about the information required to get its job done.
Developers must read every line of the actual source code to understand what hidden inputs are used and where they come from. It is hard to predict and maintain. The behavior of a method that depends on a mutable global state cannot be predicted by merely reading the source code; it is necessary to take into account its current value, along with the whole sequence of events that could have changed it earlier. In a real-world application, trying to unravel all that stuff becomes a real headache.
Fortunately, this is much easier than discussing all of its flaws — we just need to break the tightly coupled concerns. Now the method requires the caller to provide a DateTime argument, instead of secretly looking for this information by itself.
From the unit testing perspective, this is great; the method is now deterministic i. Notice that this simple refactor also solved all the API issues discussed earlier tight coupling, SRP violation, unclear and hard to understand API by introducing a clear seam between what data should be processed and how it should be done.
Excellent — the method is testable, but how about its clients? Say we continue working on the smart home system, and implement the following client of the GetTimeOfDay DateTime dateTime method — the aforementioned smart home microcontroller code responsible for turning the light on or off, based on the time of day and the detection of motion:. We have the same kind of hidden DateTime. Now input problem — the only difference is that it is located on a little bit higher of an abstraction level.
To solve this issue, we can introduce another argument, again delegating the responsibility of providing a DateTime value to the caller of a new method with signature ActuateLights bool motionDetected, DateTime dateTime. Inversion of Control is a simple, but extremely useful, technique for decoupling code, and for unit testing in particular. After all, keeping things loosely coupled is essential for being able to analyze them independently from each other.
The key point of IoC is to separate decision-making code when to do something from action code what to do when something happens. This technique increases flexibility, makes our code more modular, and reduces coupling between components. Then, make SmartHomeController reference an IDateTimeProvider implementation, and delegate it the responsibility of obtaining date and time:.
Now we can see why Inversion of Control is so called: the control of what mechanism to use for reading date and time was inverted , and now belongs to the client of SmartHomeController , not SmartHomeController itself.
Thereby, the execution of the ActuateLights bool motionDetected method fully depends on two things that can be easily managed from the outside: the motionDetected argument, and a concrete implementation of IDateTimeProvider , passed into a SmartHomeController constructor.
Why is this significant for unit testing? It means that different IDateTimeProvider implementations can be used in production code and unit test code. In the production environment, some real-life implementation will be injected e.
With the help of this class, it is possible to isolate SmartHomeController from non-deterministic factors and perform a state-based unit test. A test like this was not possible before refactoring. Despite the fact that we solved the problems caused by the non-deterministic hidden input, and we were able to test certain functionality, the code or, at least, some of it is still untestable! As we can see, SmartHomeController delegates the responsibility of turning the light on or off to a BackyardLightSwitcher object, which implements a Singleton pattern.
To fully unit test the ActuateLights bool motionDetected method, we should perform interaction-based testing in addition to the state-based testing; that is, we should ensure that methods for turning the light on or off are called if, and only if, appropriate conditions are met.
Unfortunately, the current design does not allow us to do that: the TurnOn and TurnOff methods of BackyardLightSwitcher trigger some state changes in the system, or, in other words, produce side effects. The only way to verify that these methods were called is to check whether their corresponding side effects actually happened or not, which could be painful.
In this case, a unit test can make an attempt to receive and analyze that network traffic. Or, if the hardware components are connected with a wire, the unit test can check whether the voltage was applied to the appropriate electrical circuit. Or, after all, it can check that the light actually turned on or off using an additional light sensor. No one is asserting here that they were adopting agile. Hi Arialdo, What do you exactly mean by rough?
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Director, Global Channel. Account Manager, Enterprise. The rest of us are checking our work. Writing unit tests after you have written the production code may be a more traditional way of doing it, but it is no less useful.
It's also something you're familiar with if you have been in a math class any time in the past ten years. After your work is checked and it is clear that the code is doing what you think it is doing, the value of the unit tests change a little bit. Tests that can be easily run with every build of your product act as change detection notifying you when code changes in unexpected ways.
Code documentation is a drag, and it shows, mostly by how little code documentation gets written. Unit testing can make the documentation burden a little easier by encouraging better coding practices and also leaving behind pieces of code that describe what your product is doing. Rather than having to constantly feed the documentation beast with code change, you'll be updating a system of checks that is working for you.
There are a few uses of unit testing that you will want to avoid when possible. Creating integration tests that cross system borders and touch databases or 3rd party systems can be done, but this quickly results in a test suite that takes longer and longer to run with each added test. There are plenty of test frameworks out there that specialize in higher level testing.
If you want to test larger pieces of your product at a time, you might want to investigate those other frameworks. One other risky area is end to end tests. These usually require careful ordering, dependencies on other tests, and careful set up to get your system in a special 'test ready' state.
Much like integration testing, there are plenty of tools to choose from made just for this purpose. You definitely can do these things with unit frameworks, but it might quickly become more work than it is worth.
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