Add example for parsing body based on Content-Type (#1320)

* Add example for parsing body based on `Content-Type`

* format

* Update examples/parse-body-based-on-content-type/src/main.rs

Co-authored-by: Jonas Platte <jplatte+git@posteo.de>

* fix copy/paste errors

* rename type params

Co-authored-by: Jonas Platte <jplatte+git@posteo.de>

examples/parse-body-based-on-content-type/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "example-parse-body-based-on-content-type"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+axum = { path = "../../axum" }
+serde = { version = "1.0", features = ["derive"] }
+tokio = { version = "1.0", features = ["full"] }
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }

examples/parse-body-based-on-content-type/src/main.rs

@@ -0,0 +1,89 @@
+//! Provides a RESTful web server managing some Todos.
+//!
+//! Run with
+//!
+//! ```not_rust
+//! cd examples && cargo run -p example-parse-body-based-on-content-type
+//! ```
+
+use axum::{
+    async_trait,
+    extract::FromRequest,
+    http::{header::CONTENT_TYPE, Request, StatusCode},
+    response::{IntoResponse, Response},
+    routing::post,
+    Form, Json, RequestExt, Router,
+};
+use serde::{Deserialize, Serialize};
+use std::net::SocketAddr;
+use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
+
+#[tokio::main]
+async fn main() {
+    tracing_subscriber::registry()
+        .with(tracing_subscriber::EnvFilter::new(
+            std::env::var("RUST_LOG").unwrap_or_else(|_| {
+                "example_parse_body_based_on_content_type=debug,tower_http=debug".into()
+            }),
+        ))
+        .with(tracing_subscriber::fmt::layer())
+        .init();
+
+    let app = Router::new().route("/", post(handler));
+
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
+    tracing::debug!("listening on {}", addr);
+    axum::Server::bind(&addr)
+        .serve(app.into_make_service())
+        .await
+        .unwrap();
+}
+
+#[derive(Serialize, Deserialize)]
+struct Payload {
+    foo: String,
+}
+
+async fn handler(payload: JsonOrForm<Payload>) -> Response {
+    match payload {
+        JsonOrForm::Json(payload) => Json(payload).into_response(),
+        JsonOrForm::Form(payload) => Form(payload).into_response(),
+    }
+}
+
+enum JsonOrForm<T, K = T> {
+    Json(T),
+    Form(K),
+}
+
+#[async_trait]
+impl<S, B, T, U> FromRequest<S, B> for JsonOrForm<T, U>
+where
+    B: Send + 'static,
+    S: Send + Sync,
+    Json<T>: FromRequest<(), B>,
+    Form<U>: FromRequest<(), B>,
+    T: 'static,
+    U: 'static,
+{
+    type Rejection = Response;
+
+    async fn from_request(req: Request<B>, _state: &S) -> Result<Self, Self::Rejection> {
+        let content_type_header = req.headers().get(CONTENT_TYPE);
+        let content_type = content_type_header.and_then(|value| value.to_str().ok());
+
+        if let Some(content_type) = content_type {
+            if content_type.starts_with("application/json") {
+                let Json(payload) = req.extract().await.map_err(IntoResponse::into_response)?;
+                return Ok(Self::Json(payload));
+            }
+
+            if content_type.starts_with("application/x-www-form-urlencoded") {
+                let Form(payload) = req.extract().await.map_err(IntoResponse::into_response)?;
+                return Ok(Self::Form(payload));
+            }
+        }
+
+        Err(StatusCode::UNSUPPORTED_MEDIA_TYPE.into_response())
+    }
+}