Add closure example

examples/closures/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "example-closures"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+axum = { path = "../../axum" }
+tokio = { version = "1", features = ["full"] }

examples/closures/src/main.rs

@@ -0,0 +1,92 @@
+//! Example for using associated methods as handlers and middleware.
+//!
+//! In situations where applications want to encapsulate state (e.g. auth secrets) it
+//! can be useful to create routes and middleware directly on specific structs (as opposed to
+//! handling everything in a central application state).
+//!
+//! This example demonstrates how do that using closures.  
+//!
+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-closures
+//! ```
+
+use axum::{
+    extract::Request,
+    http::StatusCode,
+    middleware::{from_fn, Next},
+    response::IntoResponse,
+    routing::get,
+    Router,
+};
+use std::{collections::HashMap, sync::Arc};
+
+#[derive(Clone)]
+struct Username(String);
+
+pub struct Auth {
+    database: HashMap<String, String>,
+}
+
+impl Auth {
+    /// read_username returns the username if it has been previously set by the middleware.
+    /// Otherwise it returns 401.
+    async fn read_username(self: Arc<Self>, req: Request) -> Result<impl IntoResponse, StatusCode> {
+        if let Some(username) = req.extensions().get::<Username>() {
+            Ok(format!("hello {}", username.0))
+        } else {
+            Err(StatusCode::UNAUTHORIZED)
+        }
+    }
+
+    /// middleware checks if the request has a valid secret and sets the username (if available
+    /// in the database) as a request extension).
+    async fn middleware(self: Arc<Self>, mut req: Request, next: Next) -> impl IntoResponse {
+        if let Some(secret) = req.headers().get("X-Auth-Secret") {
+            if let Some(username) = self.database.get(secret.to_str().unwrap()) {
+                req.extensions_mut().insert(Username(username.clone()));
+            }
+        }
+
+        next.run(req).await
+    }
+}
+
+#[tokio::main]
+async fn main() {
+    let auth = Auth {
+        database: {
+            let mut db = HashMap::new();
+            db.insert("open sesame".to_string(), "admin".to_string());
+            db
+        },
+    };
+
+    // wrap auth in an Arc smart pointer to share it between threads
+    let auth = Arc::new(auth);
+
+    let app = Router::new()
+        // route /username to the auth instance behind the smart pointer
+        .route(
+            "/username",
+            get({
+                let auth = Arc::clone(&auth);
+                |req| async { auth.read_username(req).await }
+            }),
+        )
+        // configure a middleware from the auth instance behind the smart pointer
+        .layer(from_fn({
+            let auth = Arc::clone(&auth);
+            move |req, next| {
+                let auth = Arc::clone(&auth);
+                async move { auth.middleware(req, next).await }
+            }
+        }));
+
+    let listener = tokio::net::TcpListener::bind("127.0.0.1:3000")
+        .await
+        .unwrap();
+
+    axum::serve(listener, app).await.unwrap();
+}