Handler

Learn request handling with a shelf Handler.

The Handler type

shelf defines a handler as typedef Handler = FutureOr<Response> Function(Request). So a handler can be synchronous (Response) or asynchronous (Future<Response>).

A synchronous handler suits simple work: Response okHandler(Request r) => Response.ok('ok');.

An asynchronous handler is needed for I/O: Future<Response> dbHandler(Request r) async { var data = await fetch(); return Response.ok(data); }.

Reading from a Request

A handler reads request info via the Request parameter: req.method (GET, POST...), req.requestedUri.path, req.headers.

Reading the body is asynchronous: var body = await req.readAsString();. This is used to parse JSON or form data.

Example: Response echoHandler(Request r) => Response.ok('path: ${r.requestedUri.path}');. Writes the request path into the response.

Exercise Echo the request path

Add import 'package:shelf/shelf.dart';. Define Response echoHandler(Request r) => Response.ok('yol: ${r.requestedUri.path}');. In main add var resp = echoHandler(Request('GET', Uri.parse('http://localhost/merhaba'))); and print(await resp.readAsString()); (make main async).

Show example solution

import 'package:shelf/shelf.dart';

Response echoHandler(Request r) => Response.ok('yol: ${r.requestedUri.path}');

void main() async {
  var resp = echoHandler(Request('GET', Uri.parse('http://localhost/merhaba')));
  print(await resp.readAsString());
}

Test yourself

Async handler return type

Future<____> dbHandler(Request r) async { ... }

Show answer

Future<Response> dbHandler(Request r) async { ... }

Can a handler be synchronous?
- No, always async
- Yes, it can return Response
- Only Future<Response>
- Only Stream<Response>
Show answer

Yes, it can return Response — a handler can return a synchronous Response (thanks to FutureOr).
Match the Request parts

req.method

GET/POST
req.requestedUri

URL
req.headers

Headers
req.readAsString

Body

Next lesson: Middleware

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