add header example

examples/src/main/scala/sttp/client4/examples/wrapper/addHeaderBackend.scala

@@ -0,0 +1,36 @@
+// {cat=Backend wrapper; effects=Synchronous; backend=HttpClient}: A backend which adds a header to all outgoing requests
+
+//> using dep com.softwaremill.sttp.client4::core:4.0.0-M22
+
+package sttp.client4.examples.wrapper
+
+import sttp.capabilities.Effect
+import sttp.client4.*
+import sttp.client4.wrappers.DelegateBackend
+import sttp.model.Header
+
+// If some headers should be added to each request, the best approach is to create a request description which includes
+// these headers. Then, specific requests can use such a request template, just as `basicRequest` is used to create
+// requests normally (it already includes some a default `Accept-Encoding` header). However, if the request description
+// is for any reason outside the control of the user, a backend wrapper can be used to add headers to each request.
+
+class AddHeaderBackendWrapper[F[_], P](delegate: GenericBackend[F, P], headers: List[Header])
+    extends DelegateBackend(delegate):
+  override def send[T](request: GenericRequest[T, P with Effect[F]]): F[Response[T]] =
+    delegate.send(headers.foldLeft(request) { case (r, h) => r.header(h) })
+
+object AddHeaderBackendWrapper:
+  def apply(backend: WebSocketSyncBackend, headers: List[Header]): WebSocketSyncBackend =
+    new AddHeaderBackendWrapper(backend, headers) with WebSocketSyncBackend {}
+
+  // depending on the backend & effect type used, other "apply" variants might be needed here
+
+@main def addHeaderBackend(): Unit =
+  val backend: WebSocketSyncBackend = AddHeaderBackendWrapper(
+    DefaultSyncBackend(),
+    List(Header("X-My-Header1", "value1"), Header("X-My-Header2", "value2"))
+  )
+  val response: Response[Either[String, String]] = basicRequest.get(uri"https://httpbin.org/get").send(backend)
+
+  println("Response:")
+  println(response.body)