Companion Code to - Writing Awesome Tests - https://medium.com/p/b271c7838344

TestDelegate.kt

import org.assertj.core.api.Assertions
import org.junit.Before
import org.junit.Test

// function to test
fun Int.add(valueToAdd: Int): Int = this + valueToAdd

class AddTest {
    private lateinit var test: TestDelegate

    @Before
    fun setUp() {
        test = TestDelegate()
    }

    @Test
    fun `2 plus 5 is 7`() {
        test.given(initialValue = 2)
            .whenValueIsAdded(5)
            .thenSumIs(7)
    }

    @Test
    fun `-1 plus 3 is 2`() {
        test.given(initialValue = -1)
            .whenValueIsAdded(3)
            .thenSumIs(2)
    }

    @Test
    fun `2 plus 5 is 7 - structured`() = addTest(
        givenInitialValue = 2,
        whenValueIsAdded = 5,
        thenSumIs = 7
    )

    @Test
    fun `-1 plus 3 is 2 - structured`() = addTest(
        givenInitialValue = -1,
        whenValueIsAdded = 3,
        thenSumIs = 2
    )

    private fun addTest(
        givenInitialValue: Int,
        whenValueIsAdded: Int,
        thenSumIs: Int
    ) {
        test.given(givenInitialValue)
            .whenValueIsAdded(whenValueIsAdded)
            .thenSumIs(thenSumIs)
    }

    private class TestDelegate(scope: CoroutineScope): CoroutineScope by scope {

        private var initialValue: Int? = null
        private var valueToAdd: Int? = null
        private var actualSum: Int? = null

        fun given(initialValue: Int) = apply {
            this.initialValue = initialValue
        }

        fun whenValueIsAdded(valueToAdd: Int) = apply {
            this.valueToAdd = valueToAdd

            // this is the function we are testing
            actualSum = initialValue!!.add(this.valueToAdd!!)
        }

        fun thenSumIs(expected: Int) = apply {
            Assertions.assertThat(actualSum)
                .describedAs("Adding $initialValue and $valueToAdd should produce $expected, but it produced $actualSum")
                .isEqualTo(expected)
        }
    }
}