Extension Examples

This page contains some simple examples of how to write extensions.

System Out Listener

A real example of an extension, is the NoSystemOutListener which comes with Kotest. This extension throws an error if any output is written to standard out.

class MyTestSpec : DescribeSpec({

  extensions(NoSystemOutListener)

  describe("All these tests should not write to standard out") {
    it("silence in the court") {
      println("boom") // failure
    }
  }
})

Timer Listener

Another example would be if we wanted to log the time taken for each test case. We can do this by using the beforeTest and afterTest functions as follows:

object TimerListener : BeforeTestListener, AfterTestListener {

  var started = 0L

  override fun beforeTest(testCase: TestCase): Unit {
    started = System.currentTimeMillis()
  }

  override fun afterTest(testCase: TestCase, result: TestResult): Unit {
    println("Duration of ${testCase.descriptor} = " + (System.currentTimeMillis() - started))
  }
}

Then we can register it in a single spec like so:

class MyTestClass : FunSpec({
  extensions(TimerListener)
  // tests here
})

Or we could register it project wide:

object MyConfig : AbstractProjectConfig() {
    override fun extensions(): List<Extension> = listOf(TimerListener)
}