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Testing GenServers in Elixir
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=== Unit Testing === Unit testing at the GenServer level involves testing individual functions and their behavior in isolation. ==== Using ExUnit ==== ExUnit is the built-in testing framework in Elixir. We can use it to create test cases for our GenServers. ```elixir defmodule MyGenServerTest do use ExUnit.Case alias MyApp.MyGenServer setup do {:ok, pid} = MyGenServer.start_link() {:ok, pid: pid} end test "handle_message/2 returns expected response" do pid = Map.get(:pid) assert {:response, "Hello"} = MyGenServer.handle_message(pid, :request) end test "handle_message/2 updates server state correctly" do pid = Map.get(:pid) assert {:ok, _} = MyGenServer.handle_message(pid, :update_state) assert :updated == GenServer.call(pid, :get_state) end end ``` ==== Using Mocks ==== Mocks can be used to replace external dependencies or simulate certain scenarios during testing. ```elixir defmodule MyGenServerTest do use ExUnit.Case import Mox alias MyApp.ExternalDependency alias MyApp.MyGenServer defmodule MockedExternalDependency do @behaviour ExternalDependency def some_function(_pid), do: "Mocked response" end setup do {:ok, pid} = GenServer.start_link(MyGenServer, nil) {:ok, pid: pid} end test "handle_message/2 with mocked dependency" do pid = Map.get(:pid) stub_with(MockedExternalDependency, fn _pid -> "Mocked response" end) assert "Mocked response" == MyGenServer.handle_message(pid, :request) assert_receive "Expected message" end test "handle_message/2 behaves correctly without mocked dependency" do pid = Map.get(:pid) assert "Actual response" == MyGenServer.handle_message(pid, :request) refute_receive "Unexpected message" end end ```
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