Elixir Mocking with Mox and Meck

Elixir Mocking with Mox and Meck

In Elixir, mocking is a powerful technique used to isolate and test individual units of code. It allows developers to simulate certain behaviors and responses during testing, providing better control and accuracy in test results. Two popular mocking libraries in Elixir are Mox and Meck.

Mox

Mox is an open-source Elixir library specifically designed for structured, contract-based mocking. It provides a simple and intuitive way to define and implement mocks in your test suite. With Mox, you can easily create mocks for Elixir protocols, behavior functions, and modules.

Creating Mocks

To create a mock using Mox, follow these steps:

1. Define the behavior of the mock using the `@callback` attribute in a dedicated behavior module. 2. Create a mock module that implements the defined behavior. 3. Use the `Mox.defmock/1` macro to define the module as a mock.

Here's an example:

```elixir defmodule MyBehavior do

 @callback my_function(arg1 :: any(), arg2 :: any()) :: any()

end

defmodule MyMock do

 use Mox

 defmock MyBehavior, for: MyMock

end ```

Implementing Mocks

Once the mock is defined, you can implement specific behavior for each function using the `defoverridable` macro provided by Mox. This macro lets you define how the mock should behave when each function is invoked.

```elixir defmodule MyMock do

 use Mox

 defmock MyBehavior, for: MyMock do
   defoverridable my_function(arg1, arg2) do
     # Define the behavior of the mock function here
     ...
   end
 end

end ```

Using Mocks in Tests

In your test cases, you can use Mox to dynamically replace dependencies with mocks and verify specific interactions. Mox provides helper functions like `expect` and `assert_called` to facilitate this process.

```elixir defmodule MyTest do

 use ExUnit.Case
 use Mox

 alias MyBehavior

 setup do
   Mox.stub(MyBehavior, :my_function, fn(_, _) -> :mocked_response end)
 end

 test "example test" do
   assert :mocked_response == MyModule.my_function(:arg1, :arg2)

   Mox.assert_called(MyBehavior, :my_function, 1)
 end

end ```

Meck

Meck is another popular mocking library in Elixir. It allows developers to replace module functions with custom implementations during tests. Meck provides a powerful set of functions and macros to create and utilize mocks.

Creating Mocks

To create a mock using Meck, follow these steps:

1. Use the `:meck.new` function to create a new mock module for the target module. 2. Use the `:meck.expect` function to define the expected function calls and their return values.

Here's an example:

```elixir

meck.new(MyModule)

meck.expect(MyModule, :my_function, fn(arg1, arg2) -> :mocked_response end)

```

Using Mocks in Tests

Once the mock is created, you can use it in your test cases by calling the target module's functions as usual. Meck will automatically substitute the mock implementation for the original module function.

```elixir defmodule MyTest do

 use ExUnit.Case
 alias MyModule

 setup do
   :meck.new(MyModule)
   :meck.expect(MyModule, :my_function, fn(_, _) -> :mocked_response end)
 end

 test "example test" do
   assert :mocked_response == MyModule.my_function(:arg1, :arg2)
 end

end ```

Conclusion

Mocking with Mox and Meck is a powerful technique that allows Elixir developers to better control and test their code. Mox provides a structured approach to mocking, while Meck enables easy replacement of module functions. By incorporating these libraries into your test suite, you can ensure accurate and reliable testing for your Elixir applications.

See Also

• Elixir Testing
• Elixir ExUnit
• Elixir Code Organization
• Elixir Documentation
• Elixir Project Structure
• Elixir Continuous Integration