Create a middleware from an async function.

from_fn requires the function given to

1. Be an async fn.
2. Take zero or more FromRequestParts extractors.
3. Take exactly one FromRequest extractor as the second to last argument.
4. Take Next as the last argument.
5. Return something that implements IntoResponse.

Note that this function doesn't support extracting State. For that, use from_fn_with_state.

Example

use axum::{
    Router,
    http,
    routing::get,
    response::Response,
    middleware::{self, Next},
    extract::Request,
};

async fn my_middleware(
    request: Request,
    next: Next,
) -> Response {
    // do something with `request`...

    let response = next.run(request).await;

    // do something with `response`...

    response
}

let app = Router::new()
    .route("/", get(|| async { /* ... */ }))
    .layer(middleware::from_fn(my_middleware));
# let app: Router = app;

Running extractors

use axum::{
    Router,
    extract::Request,
    http::{StatusCode, HeaderMap},
    middleware::{self, Next},
    response::Response,
    routing::get,
};

async fn auth(
    // run the `HeaderMap` extractor
    headers: HeaderMap,
    // you can also add more extractors here but the last
    // extractor must implement `FromRequest` which
    // `Request` does
    request: Request,
    next: Next,
) -> Result<Response, StatusCode> {
    match get_token(&headers) {
        Some(token) if token_is_valid(token) => {
            let response = next.run(request).await;
            Ok(response)
        }
        _ => {
            Err(StatusCode::UNAUTHORIZED)
        }
    }
}

fn get_token(headers: &HeaderMap) -> Option<&str> {
    // ...
    # None
}

fn token_is_valid(token: &str) -> bool {
    // ...
    # false
}

let app = Router::new()
    .route("/", get(|| async { /* ... */ }))
    .route_layer(middleware::from_fn(auth));
# let app: Router = app;