mockito 1.0.1

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Mock library for Dart inspired by Mockito.

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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;
  int walk(List<String> places);
  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
//verify interaction

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
expect(cat.sound(), "Purr");
//you can call it again
expect(cat.sound(), "Purr");
//let's change stub
expect(cat.sound(), "Meow");
//you can stub getters
expect(cat.lives, 9);
//you can stub a method to throw
when(cat.lives).thenThrow(new RangeError('Boo'));
expect(() => cat.lives, throwsRangeError);
//we can calculate a response at call time:
var responses = ["Purr", "Meow"];
when(cat.sound()).thenAnswer(() => responses.removeAt(0));
expect(cat.sound(), "Purr");
expect(cat.sound(), "Meow");

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...
//..or collections
//..or matchers
//..or mix aguments with matchers
when(cat.eatFood(argThat(startsWith("dry")), true).thenReturn(true);
expect(cat.eatFood("fish"), isTrue);
expect(cat.walk(["roof","tree"]), equals(2));
expect(cat.eatFood("dry food"), isFalse);
expect(cat.eatFood("dry food", hungry: true), isTrue);
//you can also verify using an argument matcher
//you can verify setters
cat.lives = 9;

By default equals matcher is used to argument matching (since 0.11.0). It simplifies matching for collections as arguments. If you need more strict matching consider use argThat(identical(arg)). Argument matchers allow flexible verification or stubbing

Verifying exact number of invocations / at least x / never #

//exact number of invocations
//or using matcher
//or never called

Verification in order #


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 #


Finding redundant invocations #


Capturing arguments for further assertions #

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

Spy #

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

Strong mode compliance #

Unfortunately, the use of the arg matchers in mock method calls (like cat.eatFood(any)) violates the Strong mode type system. Specifically, if the method signature of a mocked method has a parameter with a parameterized type (like List<int>), then passing any or argThat will result in a Strong mode warning:

[warning] Unsound implicit cast from dynamic to List<int>

In order to write Strong mode-compliant tests with Mockito, you might need to use typed, annotating it with a type parameter comment. Let's use a slightly different Cat class to show some examples:

class Cat {
  bool eatFood(List<String> foods, [List<String> mixins]) => true;
  int walk(List<String> places, {Map<String, String> gaits}) => 0;

class MockCat extends Mock implements Cat {}

var cat = new MockCat();

OK, what if we try to stub using any:


Let's analyze this code:

$ dartanalyzer --strong test/cat_test.dart
Analyzing [lib/cat_test.dart]...
[warning] Unsound implicit cast from dynamic to List<String> (test/cat_test.dart, line 12, col 20)
1 warning found.

This code is not Strong mode-compliant. Let's change it to use typed:

$ dartanalyzer --strong test/cat_test.dart
Analyzing [lib/cat_test.dart]...
No issues found

Great! A little ugly, but it works. Here are some more examples:

when(cat.eatFood(typed(any), typed(any))).thenReturn(true);

Named args require one more component: typed needs to know what named argument it is being passed into:

when(cat.walk(typed(any), gaits: typed(any, named: 'gaits')))

Note the named argument. Mockito should fail gracefully if you forget to name a typed call passed in as a named argument, or name the argument incorrectly.

One more note about the typed API: you cannot mix typed arguments with null arguments:

when(cat.eatFood(null, typed(any))).thenReturn(true); // Throws!
    typed(any))).thenReturn(true); // Works.

How it works #

The basics of the Mock class are nothing special: It uses noSuchMethod to catch all method invocations, and returns the value that you have configured beforehand with when() calls.

The implementation of when() is a bit more tricky. Take this example:

//unstubbed methods return null
expect(cat.sound(), nullValue);
//stubbing - before execution

Since cat.sound() returns null, how can the when() call configure it?

It works, because when is not a function, but a top level getter that returns a function. Before returning the function, it sets a flag (_whenInProgress), so that all Mock objects know to return a "matcher" (internally _WhenCall) instead of the expected value. As soon as the function has been invoked _whenInProgress is set back to false and Mock objects behave as normal.

Be careful never to write when; (without the function call) anywhere. This would set _whenInProgress to true, and the next mock invocation will return an unexpected value.

The same goes for "chaining" mock objects in a test call. This will fail:

var mockUtils = new MockUtils();
var mockStringUtils = new MockStringUtils();
// Setting up mockUtils.stringUtils to return a mock StringUtils implementation

// Some tests

// Instead use this:

This fails, because verify sets an internal flag, so mock objects don't return their mocked values anymore but their matchers. So mockUtils.stringUtils will not return the mocked stringUtils object you put inside.

You can look at the when and Mock.noSuchMethod implementations to see how it's done. It's very straightforward.

1.0.1 #

  • Add a new thenThrow method to the API.
  • Document thenAnswer in the README.
  • Add more dartdoc.

1.0.0 #

  • Add a new typed API that is compatible with Dart Dev Compiler; documented in

0.11.1 #

  • Move the reflection-based spy code into a private source file. Now package:mockito/mockito.dart includes this reflection-based API, and a new package:mockito/mockito_no_mirrors.dart doesn't require mirrors.

0.11.0 #

  • Equality matcher used by default to simplify matching collections as arguments. Should be non-breaking change in most cases, otherwise consider using argThat(identical(arg)).

0.10.0 #

  • Added support for spy.

0.9.0 #

  • Migrate from the unittest package to use the new test package.
  • Format code using dartformat

Use this package as a library

1. Depend on it

Add this to your package's pubspec.yaml file:

  mockito: ^1.0.1

2. Install it

You can install packages from the command line:

with pub:

$ pub get

Alternatively, your editor might support pub get. Check the docs for your editor to learn more.

3. Import it

Now in your Dart code, you can use:

import 'package:mockito/mockito.dart';
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The package version is not analyzed, because it does not support Dart 2. Until this is resolved, the package will receive a health and maintenance score of 0.

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Support Dart 2 in pubspec.yaml.

The SDK constraint in pubspec.yaml doesn't allow the Dart 2.0.0 release. For information about upgrading it to be Dart 2 compatible, please see


Package Constraint Resolved Available
Direct dependencies
Dart SDK >=1.0.0 <2.0.0