testing.md

Testing and Debugging

Press uses fakes for writing tests and avoids mocks because they’re terrible. Assertions are written using AssertK. Tests that involve storage are run on the JVM so that an in-memory SQL database can be used (example).

The best way to run the tests right now would be from the command line using ./gradlew shared:testDebug. They can be run from the Android Studio or IntelliJ IDEA as well, but testing support for multiplatform code from the IDE is a bit flaky right now (related issue). An alternative is to manually create run configurations for running custom gradle tasks:

Debugging with breakpoints

Debugging of Android code should work as expected. For native code, IntelliJ offers a plugin that can be used for debugging with breakpoints. Unfortunately it's only available for IntelliJ Ultimate so it won't work with either Android Studio or the free/community version of IDEA.

On mocks vs fakes

Mocks are terrible because they strongly couple tests with their implementation. When a test is written to check if certain functions on a mocked object were called, the test will be need to be updated every time those functions are refactored. Here’s an example:

class Adder {
  fun add(a: Int, b: Int): Int = a + b
}

class Calculator(val adder: Adder) {
  fun multiply(number: Int, factor: Int): Int {
    var sum = 0
    repeat(factor) {
      sum = adder.add(sum, number)
    }
    return sum
  }
}

@Test `test multiplication`() {
  val adder = mock(Adder::class)
  val calculator = Calculator(adder)
  calculator.multiply(2, 2)

  verify(adder).add(0, 2)
  verify(adder).add(2, 4)
}

Let’s make this worse. What happens if Adder#add() is changed to return a Single<Int> because we now want to offload additions to a server?

The test will continue to pass, but the implementation won’t work anymore. Mocks become worse when we start passing around a “promise” of result instead of the actual result.

If you’re still unconvinced, Artur Dryomov’s blog post is a good read this subject: Superior Testing: Make Fakes not Mocks