mockito

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Mock library for Dart inspired by Mockito.

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Let's create mocks

import 'package:mockito/mockito.dart';

// Real class
class Cat {
  String sound() => "Meow";
  bool eatFood(String food, {bool hungry}) => true;
  Future<void> chew() async => print("Chewing...");
  int walk(List<String> places) => 7;
  void sleep() {}
  void hunt(String place, String prey) {}
  int lives = 9;
}

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

// Create mock object.
var cat = MockCat();

Let's verify some behaviour!

// Interact with the mock object.
cat.sound();
// Verify the 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);

// Stub a mock method before interacting.
when(cat.sound()).thenReturn("Purr");
expect(cat.sound(), "Purr");

// You can call it again.
expect(cat.sound(), "Purr");

// Let's change the stub.
when(cat.sound()).thenReturn("Meow");
expect(cat.sound(), "Meow");

// You can stub getters.
when(cat.lives).thenReturn(9);
expect(cat.lives, 9);

// You can stub a method to throw.
when(cat.lives).thenThrow(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 a 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. In other words: the order of stubbing matters, but it is meaningful rarely, e.g. when stubbing exactly the same method calls or sometimes when argument matchers are used, etc.

A quick word on async stubbing

Using thenReturn to return a Future or Stream will throw an ArgumentError. This is because it can lead to unexpected behaviors. For example:

• If the method is stubbed in a different zone than the zone that consumes the Future, unexpected behavior could occur.
• If the method is stubbed to return a failed Future or Stream and it doesn't get consumed in the same run loop, it might get consumed by the global exception handler instead of an exception handler the consumer applies.

Instead, use thenAnswer to stub methods that return a Future or Stream.

// BAD
when(mock.methodThatReturnsAFuture())
    .thenReturn(Future.value('Stub'));
when(mock.methodThatReturnsAStream())
    .thenReturn(Stream.fromIterable(['Stub']));

// GOOD
when(mock.methodThatReturnsAFuture())
    .thenAnswer((_) => Future.value('Stub'));
when(mock.methodThatReturnsAStream())
    .thenAnswer((_) => Stream.fromIterable(['Stub']));

If, for some reason, you desire the behavior of thenReturn, you can return a pre-defined instance.

// Use the above method unless you're sure you want to create the Future ahead
// of time.
final future = Future.value('Stub');
when(mock.methodThatReturnsAFuture()).thenAnswer((_) => future);

Argument matchers

Mockito provides the concept of the "argument matcher" (using the class ArgMatcher) to capture arguments and to track how named arguments are passed. In most cases, both plain arguments and argument matchers can be passed into mock methods:

// You can use plain arguments themselves
when(cat.eatFood("fish")).thenReturn(true);

// ... including collections
when(cat.walk(["roof","tree"])).thenReturn(2);

// ... or matchers
when(cat.eatFood(argThat(startsWith("dry")))).thenReturn(false);

// ... or mix aguments with matchers
when(cat.eatFood(argThat(startsWith("dry")), hungry: 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.
verify(cat.eatFood("fish"));
verify(cat.walk(["roof","tree"]));
verify(cat.eatFood(argThat(contains("food"))));

// You can verify setters.
cat.lives = 9;
verify(cat.lives=9);

If an argument other than an ArgMatcher (like any, anyNamed(), argThat, captureArg, etc.) is passed to a mock method, then the equals matcher is used for argument matching. If you need more strict matching consider use argThat(identical(arg)).

However, note that null cannot be used as an argument adjacent to ArgMatcher arguments, nor as an un-wrapped value passed as a named argument. For example:

verify(cat.hunt("backyard", null)); // OK: no arg matchers.
verify(cat.hunt(argThat(contains("yard")), null)); // BAD: adjacent null.
verify(cat.hunt(argThat(contains("yard")), argThat(isNull))); // OK: wrapped in an arg matcher.
verify(cat.eatFood("Milk", hungry: null)); // BAD: null as a named argument.
verify(cat.eatFood("Milk", hungry: argThat(isNull))); // BAD: null as a named argument.

Named arguments

Mockito currently has an awkward nuisance to its syntax: named arguments and argument matchers require more specification than you might think: you must declare the name of the argument in the argument matcher. This is because we can't rely on the position of a named argument, and the language doesn't provide a mechanism to answer "Is this element being used as a named element?"

// GOOD: argument matchers include their names.
when(cat.eatFood(any, hungry: anyNamed('hungry'))).thenReturn(true);
when(cat.eatFood(any, hungry: argThat(isNotNull, named: 'hungry'))).thenReturn(false);
when(cat.eatFood(any, hungry: captureAnyNamed('hungry'))).thenReturn(false);
when(cat.eatFood(any, hungry: captureThat(isNotNull, named: 'hungry'))).thenReturn(true);

// BAD: argument matchers do not include their names.
when(cat.eatFood(any, hungry: any)).thenReturn(true);
when(cat.eatFood(any, hungry: argThat(isNotNull))).thenReturn(false);
when(cat.eatFood(any, hungry: captureAny)).thenReturn(false);
when(cat.eatFood(any, hungry: captureThat(isNotNull))).thenReturn(true);

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(captureAny)).captured.single, "Fish");

// Capture multiple calls
cat.eatFood("Milk");
cat.eatFood("Fish");
expect(verify(cat.eatFood(captureAny)).captured, ["Milk", "Fish"]);

// Conditional capture
cat.eatFood("Milk");
cat.eatFood("Fish");
expect(verify(cat.eatFood(captureThat(startsWith("F")))).captured, ["Fish"]);

Waiting for an interaction

// Waiting for a call.
cat.eatFood("Fish");
await untilCalled(cat.chew()); // Completes when cat.chew() is called.

// Waiting for a call that has already happened.
cat.eatFood("Fish");
await untilCalled(cat.eatFood(any)); // Completes immediately.

Writing a fake

You can also write a simple fake class that implements a real class, by extending Fake. Fake allows your subclass to satisfy the implementation of your real class, without overriding the methods that aren't used in your test; the Fake class implements the default behavior of throwing UnimplementedError (which you can override in your fake class):

// Fake class
class FakeCat extends Fake implements Cat {
  @override
  bool eatFood(String food, {bool hungry}) {
    print('Fake eat $food');
    return true;
  }
}

void main() {
  // Create a new fake Cat at runtime.
  var cat = FakeCat();

  cat.eatFood("Milk"); // Prints 'Fake eat Milk'.
  cat.sleep(); // Throws.
}

Resetting mocks

// Clearing collected interactions:
cat.eatFood("Fish");
clearInteractions(cat);
cat.eatFood("Fish");
verify(cat.eatFood("Fish")).called(1);

// Resetting stubs and collected interactions:
when(cat.eatFood("Fish")).thenReturn(true);
cat.eatFood("Fish");
reset(cat);
when(cat.eatFood(any)).thenReturn(false);
expect(cat.eatFood("Fish"), false);

Debugging

// Print all collected invocations of any mock methods of a list of mock objects:
logInvocations([catOne, catTwo]);

// Throw every time that a mock method is called without a stub being matched:
throwOnMissingStub(cat);

Best Practices

Testing with real objects is preferred over testing with mocks - if you can construct a real instance for your tests, you should! If there are no calls to verify in your test, it is a strong signal that you may not need mocks at all, though it's also OK to use a Mock like a stub. When it's not possible to use the real object, a tested implementation of a fake is the next best thing - it's more likely to behave similarly to the real class than responses stubbed out in tests. Finally an object which extends Fake using manually overridden methods is preferred over an object which extends Mock used as either a stub or a mock.

A class which extends Mock should never stub out it's own responses with when in it's constructor or anywhere else. Stubbed responses should be defined in the tests where they are used. For responses controlled outside of the test use @override methods for either the entire interface, or with extends Fake to skip some parts of the interface.

Similarly, a class which extends Mock should never have any @override methods. These can't be stubbed by tests and can't be tracked and verified by Mockito. A mix of test defined stubbed responses and mock defined overrides will lead to confusion. It is OK to define static utilities on a class which extends Mock if it helps with code structure.

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:
when(cat.sound()).thenReturn("Purr");

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.

Argument matchers work by storing the wrapped arguments, one after another, until the when (or verify) call gathers everything that has been stored, and creates an InvocationMatcher with the arguments. This is a simple process for positional arguments: the order in which the arguments has been stored should be preserved for matching an invocation. Named arguments are trickier: their evaluation order is not specified, so if Mockito blindly stored them in the order of their evaluation, it wouldn't be able to match up each argument matcher with the correct name. This is why each named argument matcher must repeat its own name. foo: anyNamed('foo') tells Mockito to store an argument matcher for an invocation under the name 'foo'.

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 = MockUtils();
var mockStringUtils = MockStringUtils();

// Setting up mockUtils.stringUtils to return a mock StringUtils implementation
when(mockUtils.stringUtils).thenReturn(mockStringUtils);

// Some tests

// FAILS!
verify(mockUtils.stringUtils.uppercase()).called(1);
// Instead use this:
verify(mockStringUtils.uppercase()).called(1);

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.

NOTE: This is not an official Google product