dart-mockito

Mock library for Dart inspired by Mockito.

Current mock libraries suffer from specifying method names as strings, which cause a lot of problems:

• Poor refactoring support: rename method and you need manually search/replace it's usage in when/verify clauses.
• Poor support from IDE: no code-completion, no hints on argument types, can't jump to definition

Dart-mockito fixes it - stubbing and verifying are first-class citizens.

Let's create mocks

  import 'package:mockito/mockito.dart';

  //Real class
  class Cat {
    String sound() => "Meow";
    bool eatFood(String food, {bool hungry}) => true;
    void sleep(){}
    int lives = 9;
  }

  //Mock class
  class MockCat extends Mock implements Cat{}

  //mock creation
  var cat = new MockCat();

Let's verify some behaviour!

  //using mock object
  cat.sound();
  //verify interaction
  verify(cat.sound());

Once created, mock will remember all interactions. Then you can selectively verify whatever interaction you are interested in.

How about some stubbing?

  //unstubbed methods return null
  expect(cat.sound(), nullValue);
  //stubbing - before execution
  when(cat.sound()).thenReturn("Purr");
  expect(cat.sound(), "Purr");
  //you can call it again
  expect(cat.sound(), "Purr");
  //let's change stub
  when(cat.sound()).thenReturn("Meow");
  expect(cat.sound(), "Meow");
  //you can stub getters
  when(cat.lives).thenReturn(9);
  expect(cat.lives, 9);

By default, for all methods that return value, mock returns null. Stubbing can be overridden: for example common stubbing can go to fixture setup but the test methods can override it. Please note that overridding stubbing is a potential code smell that points out too much stubbing. Once stubbed, the method will always return stubbed value regardless of how many times it is called. Last stubbing is more important - when you stubbed the same method with the same arguments many times. Other words: the order of stubbing matters but it is only meaningful rarely, e.g. when stubbing exactly the same method calls or sometimes when argument matchers are used, etc.

Argument matchers

  //you can use arguments itself...
  when(cat.eatFood("fish")).thenReturn(true);
  //..or matchers
  when(cat.eatFood(argThat(startsWith("dry"))).thenReturn(false);
  //..or mix aguments with matchers
  when(cat.eatFood(argThat(startsWith("dry")), true).thenReturn(true);
  expect(cat.eatFood("fish"), isTrue);
  expect(cat.eatFood("dry food"), isFalse);
  expect(cat.eatFood("dry food", hungry: true), isTrue);
  //you can also verify using an argument matcher
  verify(cat.eatFood("fish"));
  verify(cat.eatFood(argThat(contains("food"))));
  //you can verify setters
  cat.lives = 9;
  verify(cat.lives=9);

Argument matchers allow flexible verification or stubbing

Verifying exact number of invocations / at least x / never

  cat.sound();
  cat.sound();
  //exact number of invocations
  verify(cat.sound()).called(2);
  //or using matcher
  verify(cat.sound()).called(greaterThan(1));
  //or never called
  verifyNever(cat.eatFood(any));

Verification in order

  cat.eatFood("Milk");
  cat.sound();
  cat.eatFood("Fish");
  verifyInOrder([
    cat.eatFood("Milk"),
    cat.sound(),
    cat.eatFood("Fish")
  ]);

Verification in order is flexible - you don't have to verify all interactions one-by-one but only those that you are interested in testing in order.

Making sure interaction(s) never happened on mock

  verifyZeroInteractions(cat);

Finding redundant invocations

  cat.sound();
  verify(cat.sound());
  verifyNoMoreInteractions(cat);

Capturing arguments for further assertions

  //simple capture
  cat.eatFood("Fish");
  expect(verify(cat.eatFood(capture)).captured.single, "Fish");
  //capture multiple calls
  cat.eatFood("Milk");
  cat.eatFood("Fish");
  expect(verify(cat.eatFood(capture)).captured, ["Milk", "Fish"]);
  //conditional capture
  cat.eatFood("Milk");
  cat.eatFood("Fish");
  expect(verify(cat.eatFood(captureThat(startsWith("F")).captured, ["Fish"]);

Spy

  //spy creation
  var cat = spy(new MockCat(), new Cat());
  //stubbing - before execution
  when(cat.sound()).thenReturn("Purr");
  //using mocked interaction
  expect(cat.sound(), "Purr");  
  //using real object
  expect(cat.lives, 9);