How does @GenerateMocks handle custom mocks

Basic Usage

When you use `@GenerateMocks`, Mockito generates a mock class for each type you specify. For example, if you annotate your test file with `@GenerateMocks([Foo])`, Mockito will generate a `MockFoo` class that extends `Mock` and implements `Foo`. This generated mock class is placed in a file named after the original file but with a `.mocks.dart` extension.

Custom Mocks

To create custom mocks, you can use the `customMocks` named argument within the `@GenerateMocks` annotation. This allows you to specify custom mock classes using `MockSpec` objects. Here's an example:

dart
@GenerateMocks(
  [Foo],
  customMocks: [
    MockSpec(as: #CustomMockFoo),
  ],
)

In this case, Mockito will generate a mock class named `CustomMockFoo` instead of the default `MockFoo`. This is useful for avoiding name collisions or for giving more specific names to mocks, especially when dealing with generic classes.

Handling Generic Classes

If you need to mock a generic class, you can specify the type arguments in the `MockSpec`. For instance, if you have a class `Foo`, you can generate a mock like this:

dart
@GenerateMocks(
  [Foo],
  customMocks: [
    MockSpec>(as: #MockFooOfString),
  ],
)

This will generate a `MockFooOfString` class that extends `Mock` and implements `Foo`.

Missing Stub Behavior

By default, `@GenerateMocks` generates mocks that return a default value when a method is called without a stub. However, if you want to throw an exception instead (classic behavior), you can use `@GenerateNiceMocks` with its `returnValueForMissingStub` parameter or switch to using `@GenerateMocks` with a custom implementation.

Common Issues

One common issue is when Mockito complains that a class appears to already be mocked inline. This happens if you have manually defined a mock class with the same name that Mockito tries to generate. To resolve this, use the `customMocks` argument to specify a unique name for your custom mock.

Conclusion

`@GenerateMocks` provides a flexible way to generate mock classes in Mockito, allowing for both default and custom mock generation. By using `MockSpec` objects, you can customize the names and behaviors of your mocks, making your tests more robust and easier to maintain. However, for most use cases, `@GenerateNiceMocks` is recommended due to its ability to automatically stub all methods, reducing the need for explicit stubbing in your tests.