Example of trying to use `SubjectSpek` and `itBehavesLike` if the subject requires a collaborator

ThinkSpec.kt

import org.jetbrains.spek.api.dsl.describe
import org.jetbrains.spek.api.dsl.given
import org.jetbrains.spek.api.dsl.it
import org.jetbrains.spek.api.dsl.on
import org.jetbrains.spek.subject.SubjectSpek
import org.jetbrains.spek.subject.itBehavesLike
import org.junit.Assert.assertFalse
import org.junit.Assert.assertTrue

open class Thing(protected val fn: () -> Unit) {
  fun doIfEven(n: Int) {
    if (n % 2 == 0) fn()
  }
}

object ThingSpec : SubjectSpek<Thing>({
  // mock collaborator is only visible inside this block
  val mockFn = object : () -> Unit {
    var invoked = false

    override fun invoke() {
      invoked = true
    }

    fun reset() {
      invoked = false
    }
  }

  subject {
    // constructor of subject depends on collaborator
    Thing(mockFn)
  }

  describe("doIfEven") {
    given("an even parameter") {
      afterGroup(mockFn::reset)

      on("calling doIfEven") {
        subject.doIfEven(2)
      }

      it("invokes the function") {
        assertTrue("mock was not invoked", mockFn.invoked)
      }
    }

    given("an odd parameter") {
      afterGroup(mockFn::reset)

      on("calling doIfEven") {
        subject.doIfEven(1)
      }

      it("does not invoke the function") {
        assertFalse("mock should not have been invoked", mockFn.invoked)
      }
    }
  }
})

class ExtendedThing(fn: () -> Unit) : Thing(fn)

object ExtendedThingSpec : SubjectSpek<ExtendedThing>({
  subject {
    // no way to access collaborator required to run tests in ThingSpec
    ExtendedThing(mockFn)
  }

  itBehavesLike(ThingSpec)
})