Add an example returning HttpResponse from middleware.

middleware/middleware-return-httpresponse/Cargo.toml

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+[package]
+name = "middleware-return-httpresponse"
+version = "1.0.0"
+edition = "2021"
+
+[dependencies]
+actix-web = "4"
+env_logger = "0.9"
+futures-util = { version = "0.3.17", default-features = false, features = ["std"] }
+log = "0.4"
+pin-project = "1"
+serde = { version = "*", features = ["derive"] }

middleware/middleware-return-httpresponse/README.md

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+## Middleware : Return HttpResponse from Middleware
+
+```rs
+cd middleware-return-httpresponse
+cargo run
+# Started http server: 127.0.0.1:8080
+```
+
+## What is this?
+
+A Middleware example which returning HttpResponse.
+
+## How to test
+
+### success case
+```sh
+curl http://127.0.0.1:8080/ -H 'Authorization:ok' | json_pp -json_opt pretty,canonical
+ % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
+                                 Dload  Upload   Total   Spent    Left  Speed
+100    42  100    42    0     0  42000      0 --:--:-- --:--:-- --:--:-- 42000
+{
+   "data" : "Hello this is success response!"
+}
+```
+
+### failed case
+```sh
+curl http://127.0.0.1:8080/ | json_pp -json_opt pretty,canonical
+ % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
+                                 Dload  Upload   Total   Spent    Left  Speed
+100   102  100   102    0     0    99k      0 --:--:-- --:--:-- --:--:--   99k
+{
+   "data" : "Hello this is default error message! you need to set Authorization header to get thru this."
+}
+```

middleware/middleware-return-httpresponse/src/main.rs

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+use actix_web::{web, App, HttpServer, HttpResponse};
+
+mod simple;
+
+// You can move this struct to a separate file.
+// this struct below just for example.
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct HttpData {
+    pub data: String,
+}
+// this implementation is optional
+impl Default for HttpData {
+    fn default() -> Self {
+        Self {
+            data: "Hello this is success response!".to_string(),
+        }
+    }
+}
+
+#[actix_web::main]
+async fn main() -> std::io::Result<()> {
+    env_logger::init_from_env(env_logger::Env::new().default_filter_or("info"));
+
+    log::info!("starting HTTP server at http://localhost:8080");
+
+    HttpServer::new(|| {
+        App::new()
+            .wrap(simple::ReturnHttpResponse)
+            .service(
+                web::resource("/").to(|| async {
+                    HttpResponse::Ok().json(HttpData::default())
+                }),
+            )
+    })
+    .bind(("127.0.0.1", 8080))?
+    .run()
+    .await
+}

middleware/middleware-return-httpresponse/src/simple.rs

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+use std::{
+    future::{ready, Ready},
+    rc::Rc
+};
+
+use actix_web::{
+    dev::{self, Service, ServiceRequest, ServiceResponse, Transform},
+    Error,
+    http::{header, StatusCode}, HttpResponseBuilder
+};
+use futures_util::future::LocalBoxFuture;
+
+
+// You can move this struct to a separate file.
+// this struct below just for example.
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct HttpData {
+    pub data: String,
+}
+// this implementation is optional
+impl Default for HttpData {
+    fn default() -> Self {
+        Self {
+            data: "Hello this is default error message! you need to set Authorization header to get thru this.".to_string(),
+        }
+    }
+}
+
+
+pub struct ReturnHttpResponse;
+
+impl<S: 'static> Transform<S, ServiceRequest> for ReturnHttpResponse
+where
+    S: Service<ServiceRequest, Response = ServiceResponse, Error = Error>,
+    S::Future: 'static
+{
+    type Response = ServiceResponse;
+    type Error = Error;
+    type InitError = ();
+    type Transform = AuthMiddleware<S>;
+    type Future = Ready<Result<Self::Transform, Self::InitError>>;
+
+    fn new_transform(&self, service: S) -> Self::Future {
+        ready(Ok(AuthMiddleware {
+            service: Rc::new(service),
+        }))
+    }
+}
+
+pub struct AuthMiddleware<S> {
+    // This is special: We need this to avoid lifetime issues.
+    service: Rc<S>,
+}
+
+impl<S> Service<ServiceRequest> for AuthMiddleware<S>
+where
+    S: Service<ServiceRequest, Response = ServiceResponse, Error = Error> + 'static,
+    S::Future: 'static
+{
+    type Response = ServiceResponse;
+    type Error = Error;
+    type Future = LocalBoxFuture<'static, Result<Self::Response, Self::Error>>;
+
+    dev::forward_ready!(service);
+
+    fn call(&self, req: ServiceRequest) -> Self::Future {
+        let svc = self.service.clone();
+
+        Box::pin(async move {
+
+            let headers =  req.headers();
+            let _ = match headers.get("Authorization") {
+                Some(e) => e,
+                None => {
+                    let new_response = HttpResponseBuilder::new(StatusCode::BAD_REQUEST)
+                            .insert_header((header::CONTENT_TYPE, "application/json"))
+                            .json(HttpData::default());
+                    return Ok(ServiceResponse::new(
+                        req.request().to_owned(), /* or req.request().clone() */ 
+                        new_response
+                    ))
+                }
+            };
+
+            let res = svc.call(req).await?;
+            Ok(res)
+        })
+    }
+}