dependency_injection_in_elixir.ex

# Real implimentations:
# =====================
#
# This isn't really testing anything but the point is, rather than 
# reading the module to use from config and fixing the module for 
# all tests here we inject the module in the function call and supply
# a default for normal operation. This is akin to dependency injection
# in OO. Not sure if this is a good idea but seems more flexible as
# different tests can pass in a different fake (if needed).

defmodule GitHub do
  def fetch_repos(username) do
    IO.puts "Getting #{username}'s repos for real!..."

    # Do some HTTP to get a list of repos and return them
    []
  end
end

defmodule MyModule do
  def do_something(username, adapter \\ GitHub) do
    adapter.fetch_repos(username)
  end
end

# Test stuff
# ==========

defmodule FakeGithub do
  @moduledoc """
  A fake github module used only it tests to replace a real module

  Use pattern matching to specifiy a different result set
  """

  def fetch_repos("andypike") do
    IO.puts "Faking list of repos for andypike"

    ["rectify"]
  end

  def fetch_repos("cloud8421") do
    IO.puts "Faking list of repos for cloud8421"

    ["99-elixir-problems", "dragonfly-server"]
  end
end

defmodule MyModuleTest do
  use ExUnit.Case

  test "returns the repos list for andypike" do
    assert MyModule.do_something("andypike", FakeGithub) == ["rectify"]
  end

  test "returns the repos list for cloud8421" do
    assert MyModule.do_something("cloud8421", FakeGithub) == ["99-elixir-problems", "dragonfly-server"]
  end
end