From 226051be588f14787d08bdd58f82fdba66ca9aee Mon Sep 17 00:00:00 2001
From: Maciej Szlosarczyk <maciej@sosek.net>
Date: Sat, 12 Mar 2022 10:36:00 +0200
Subject: [PATCH] Add Client.update_middleware/2 function

A typical use-case is the following:

A third-party library (i.e GitHub) exposes a function that returns a
Tesla client, but I want to append a header or add logging/telemetry
to it. I need to write a wrapper for Tesla.Client.middleware/1 and
Tesla.client/2.
---
 lib/tesla/client.ex        | 19 +++++++++++++++++++
 test/tesla/client_test.exs | 17 +++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/lib/tesla/client.ex b/lib/tesla/client.ex
index a120075c..74afcd44 100644
--- a/lib/tesla/client.ex
+++ b/lib/tesla/client.ex
@@ -43,6 +43,25 @@ defmodule Tesla.Client do
     unruntime(client.pre)
   end
 
+  @doc ~S"""
+  Given an update function, create a new client by transforming the
+  list of middlewares for an existing one.
+
+  ## Examples
+
+      iex> middleware = [{Tesla.Middleware.BaseUrl, "https://api.github.com"}]
+      iex> client = Tesla.client(middleware)
+      iex> new_client = Tesla.Client.update_middleware(client, &([Tesla.Middleware.JSON] ++ &1))
+      iex> Tesla.Client.middleware(new_client)
+      [Tesla.Middleware.JSON, {Tesla.Middleware.BaseUrl, "https://api.github.com"}]
+  """
+  @spec update_middleware(t(), ([middleware()] -> [middleware()])) :: t()
+  def update_middleware(client, update_fun) do
+    existing_middleware = middleware(client)
+    new_middleware = update_fun.(existing_middleware)
+    Tesla.client(new_middleware, adapter(client))
+  end
+
   defp unruntime(list) when is_list(list), do: Enum.map(list, &unruntime/1)
   defp unruntime({module, :call, [[]]}) when is_atom(module), do: module
   defp unruntime({module, :call, [opts]}) when is_atom(module), do: {module, opts}
diff --git a/test/tesla/client_test.exs b/test/tesla/client_test.exs
index 75ae161f..7fb31696 100644
--- a/test/tesla/client_test.exs
+++ b/test/tesla/client_test.exs
@@ -49,4 +49,21 @@ defmodule Tesla.ClientTest do
       assert middlewares == Tesla.Client.middleware(client)
     end
   end
+
+  describe "Tesla.Client.update_middleware/2" do
+    test "updates middleware" do
+      existing_middleware = [FirstMiddleware]
+      client = Tesla.client(existing_middleware)
+
+      updated_client =
+        Tesla.Client.update_middleware(
+          client,
+          &([{SecondMiddleware, options: :are, fun: 1}] ++ &1)
+        )
+
+      expected_middlewares = [{SecondMiddleware, options: :are, fun: 1}, FirstMiddleware]
+
+      assert expected_middlewares == Tesla.Client.middleware(updated_client)
+    end
+  end
 end