mox

1) Add behaviour, defining the contract

# lib/weather_behaviour.ex
defmodule WeatherBehaviour do
  @callback get_weather(binary()) :: {:ok, map()} | {:error, binary()}
end

2) Add implementation for the behaviour

# lib/weather_impl.ex
defmodule WeatherImpl do
  @moduledoc """
  An implementation of a WeatherBehaviour
  """

  @behaviour WeatherBehaviour

  @impl WeatherBehaviour
  def get_weather(city) when is_binary(city) do
    # Here you could call an external api directly with an HTTP client or use a third
    # party library that does that work for you. In this example we send a
    # request using a `httpc` to get back some html, which we can process later.

    :inets.start()
    :ssl.start()

    case :httpc.request(:get, {"https://www.google.com/search?q=weather+#{city}", []}, [], []) do
      {:ok, {_, _, html_content}} -> {:ok, %{body: html_content}}
      error -> {:error, "Error getting weather: #{inspect(error)}"}
    end
  end
end

3) Add a switch

This can pull from your config/config.exs, config/test.exs, or, you can have no config as shown below and rely on a default. We also add a function to a higher level abstraction that will call the correct implementation:

# bound.ex, the main context we chose to call this function from
defmodule Bound do
  def get_weather(city) do
    weather_impl().get_weather(city)
  end

  defp weather_impl() do
    Application.get_env(:bound, :weather, WeatherImpl)
  end
end

4) Define the mock so it is used during tests

# In your test/test_helper.exs
Mox.defmock(WeatherBehaviourMock, for: WeatherBehaviour) # <- Add this
Application.put_env(:bound, :weather, WeatherBehaviourMock) # <- Add this

ExUnit.start()

5) Create a test and use expect to assert on the mock arguments

# test/bound_test.exs
defmodule BoundTest do
  use ExUnit.Case

  import Mox

  setup :verify_on_exit!

  describe "get_weather/1" do
    test "fetches weather based on a location" do
      expect(WeatherBehaviourMock, :get_weather, fn args ->
        # here we can assert on the arguments that get passed to the function
        assert args == "Chicago"

        # here we decide what the mock returns
        {:ok, %{body: "Some html with weather data"}}
      end)

      assert {:ok, _} = Bound.get_weather("Chicago")
    end
  end
end

Enforcing consistency with behaviour typespecs

Hammox is an enhanced version of Mox which automatically makes sure that calls to mocks match the typespecs defined in the behaviour. If you find this useful, see the project homepage.