Unit Testing and External Resources

Question

Hi all, I'm kind of new to unit testing, but I've recently seen how it can be quite useful. I've seen that most unit tests are self running. In fact most unit testing frameworks provide a way of running several tests at once (such as unit testing a whole system).

I wonder though; How do you deal with external resources in self running unit tests? I like the idea of testing a whole system and seeing which classes failed, but a class may, for example, create thumbnails from an uploaded image. How would that test be self running when it relies on an upload image? Would I keep a directory of images, and "pretend" to upload one of them in the test?

Any thoughts on the matter would be greatly appreciated.

Answer 1

If you need to test what happens with user input, you indeed need some sample data at hand. A directory with images, text files, PDFs or whatever else is needed, should be there along your unit tests. Or you can generate random data programmatically in your tests.

Answer 2

If you are planning on testing external resources then it would be integration testing. In pure unit testing -> to test external resources, you would have to mock the external resource. So in this case you create a IDirectory interface and then use say a FakeDirectory class and then use FakeDirectory to "Upload" the image. And when you are actually using the application you would pass an actual directory.

In integration testing, you could have a setup class which would do all the work to set up and then you would test.

Answer 3

Yes, ideally a class that creates a thumbnail can use a placeholder image that you provide as a resource in your unit test directory. You should be able to test the class in isolation, with as little dependency on the rest of your application as possible. That's kind of what people mean when they recommend to design your code to be "testable."

Answer 4

Mock external dependencies. I have no real experience of mocking in php but I have seen enough resources online just googling for mock and php that it's being done

Answer 5

I have come across this same situation while unit testing my PHP classes. There are functions which can be tested without using any other resources (unit testing), but many functions perform file read/write operations or require database access (integration testing). In order to test these functions, I've combined unit testing with integration testing. In my setUp and tearDown testing classes, it may load a database schema or fetch test data from a local test_data/ directory required by the class functions.