How to make an axum router handle return different content-type responses?

For example, when user access http://127.0.0.1:8080/hello, if query parameter id is 1, a plain text response return. If id is 2, give a JSON structure.

Summary:

struct HelloParam {
    id: u16,
}

struct HelloResponse {
    name: String,
}

async fn hello_get(Query(params): Query<HelloParam>) -> Response {
    // how to implement it? 
}

let router = Router::new().route("/hello", get(hello_get));

Check out the examples at the beginning of the response module. Axum gives you a bevvy of different ways to return data so that the Content-type header is automatically set appropriately.

(This seems like a homework question to me, so I'm not going to write your function for you exactly.)

For example, if you return a String as a body, the Content-type will automatically be set to "text/plain":

use axum::response::{IntoResponse, Response};

async fn returns_string() -> Response {
    String::from("Hello, world!").into_response()
}

There is also a custom Json struct for returning as JSON (with the Content-type header set appropriately) any type that implements serde::Serialize.

use axum::response::{Json, IntoResponse, Response};
use serde::Serialize;

#[derive(Serialize)]
struct Hello {
    name: String,
}

async fn returns_json() -> Response {
    let hello = Hello {
        name: String::from("world"),
    };

    Json(hello).into_response()
}

So we can write a function that could return either type of response based on some property of the request. Let's choose based on the value of the "Accept" header:

use axum::{
    http::{
        header::{ACCEPT, HeaderMap},
        status::StatusCode,
    },
    response::{Json, IntoResponse, Response},
};
use serde::Serialize;

#[derive(Serialize)]
struct Hello {
    name: String,
}

async fn heterogeneous_handle(headers: HeaderMap) -> Response {
    match headers.get(ACCEPT).map(|x| x.as_bytes()) {
        Some(b"text/plain") =>
            String::from("Hello, world!").into_response(),
        Some(b"application/json") => {
            let hello = Hello {
                name: String::from("world"),
            };
            Json(hello).into_response()
        },
        _ => StatusCode::BAD_REQUEST.into_response(),
    }
}