Archive for October, 2012

ScalaMock3 step-by-step

This post describes ScalaMock 3, which supports Scala 2.10 only. For ScalaMock 2, which supports earlier Scala versions, go here.

This post describes how to setup a project that uses ScalaMock in conjunction with ScalaTest and sbt. The sample code described in this article is available on GitHub.

The example assumes that we’re writing code to control a mechanical turtle, similar to that used by Logo programs. Mocking is useful in this kind of situation because we might want to create tests that function even if we don’t have the hardware to hand, which run more quickly than would be the case if we ran on real hardware, and where we can use mocks to simulate errors or other situations difficult to reproduce on demand.

  1. Create a root directory for your project:
    $ mkdir myproject[/soucecode]
    	<li>Create <code>build.sbt</code> containing:
    organization := "com.example"
    version := "1.0"
    scalaVersion := "2.10.0"
    scalacOptions ++= Seq("-deprecation", "-unchecked")
    libraryDependencies +=
      "org.scalamock" %% "scalamock-scalatest-support" % "3.0" % "test"
  2. Now we’ve got a project, we need some code to test. Let’s start with a simple trait representing a turtle. Create src/main/scala/Turtle.scala containing:
    package com.example
    trait Turtle {
      def penDown()
      def penUp()
      def forward(distance: Double)
      def turn(angle: Double)
      def getPosition: (Double, Double)
      def getAngle: Double
  3. The turtle API is not very convenient, we have no way to move to a specific position, instead we need to work out how to get from where we are now to where we want to get by calculating angles and distances. Here’s some code that draws a line from a specific point to another by doing exactly that.Create src/main/scala/Controller.scala containing:
    package com.example
    import scala.math.{atan2, sqrt}
    class Controller(turtle: Turtle) {
      def drawLine(start: (Double, Double), end: (Double, Double)) {
        val initialAngle = turtle.getAngle
        val deltaPos = delta(start, end)
        turtle.turn(angle(deltaPos) - initialAngle)
      def delta(pos1: (Double, Double), pos2: (Double, Double)) =
        (pos2._1 - pos1._1, pos2._2 - pos1._2)
      def distance(delta: (Double, Double)) =
        sqrt(delta._1 * delta._1 + delta._2 * delta._2)
      def angle(delta: (Double, Double)) =
        atan2(delta._2, delta._1)
      def moveTo(pos: (Double, Double)) {
        val initialPos = turtle.getPosition
        val initialAngle = turtle.getAngle
        val deltaPos = delta(initialPos, pos)
        turtle.turn(angle(deltaPos) - initialAngle)
  4. We can now write a test. We’ll create a mock turtle that pretends to start at the origin (0, 0) and verifies that if we draw a line from (1, 1) to (2, 1) it performs the correct sequence of turns and movements.

    Turtle diagram

    Create src/test/scala/ControllerTest.scala containing:

    package com.example
    import org.scalatest.FunSuite
    import org.scalamock.scalatest.MockFactory
    import scala.math.{Pi, sqrt}
    class ControllerTest extends FunSuite with MockFactory {
      test("draw line") {
        val mockTurtle = mock[Turtle]
        val controller = new Controller(mockTurtle)
        inSequence {
          inAnyOrder {
            (mockTurtle.penUp _).expects()
            (mockTurtle.getPosition _).expects().returning(0.0, 0.0)
            (mockTurtle.getAngle _).expects().returning(0.0)
          (mockTurtle.turn _).expects(~(Pi / 4))
          (mockTurtle.forward _).expects(~sqrt(2.0))
          (mockTurtle.getAngle _).expects().returning(Pi / 4)
          (mockTurtle.turn _).expects(~(-Pi / 4))
          (mockTurtle.penDown _).expects()
          (mockTurtle.forward _).expects(1.0)
        controller.drawLine((1.0, 1.0), (2.0, 1.0))

    This should (hopefully!) be self-explanatory, with one possible exception. The tilde (~) operator represents an epsilon match, useful for taking account of rounding errors when dealing with floating-point values.

  5. Run the tests with sbt test:
    $ sbt
    > test
    [info] ControllerTest:
    [info] - draw line
    [info] Passed: : Total 1, Failed 0, Errors 0, Passed 1, Skipped 0

ScalaMock 3.0-M4 for Scala 2.10.0-RC1

ScalaMock 3.0-M4 (scaladoc) for Scala 2.10.0-RC1 is now released. It supports:

  • Mock functions, traits and classes
  • Both expectation-first and record-then-verify (Mockito-style) mocking
  • ScalaTest and Specs2

To use with sbt and ScalaTest:

libraryDependencies +=
  "org.scalamock" % "scalamock-scalatest-support_2.10.0-RC1" % "3.0-M4"

or for Specs2:

libraryDependencies +=
  "org.scalamock" % "scalamock-specs2-support_2.10.0-RC1" % "3.0-M4"

For background information, see ScalaMock 3.0 Preview Release.

Known limitations (these should all be fixed when Scala adds support for mock types):

  • No support for mocking object creation (constructors)
  • No support for mocking singleton/companion objects
  • No support for mocking final classes or classes with private constructors
  • No support for mocking concrete vars
  • Limited support for overloaded methods
  • No support for mocking Java methods with repeated parameters