Shelf makes it easy to create and compose web servers and parts of web servers. How?
See example/example_server.dart
import 'package:shelf/shelf.dart' as shelf; import 'package:shelf/shelf_io.dart' as io; void main() { var handler = const shelf.Pipeline().addMiddleware(shelf.logRequests()) .addHandler(_echoRequest); io.serve(handler, 'localhost', 8080).then((server) { print('Serving at http://${server.address.host}:${server.port}'); }); } shelf.Response _echoRequest(shelf.Request request) { return new shelf.Response.ok('Request for "${request.url}"'); }
A handler is any function that handles a shelf.Request and returns a shelf.Response. It can either handle the request itself--for example, a static file server that looks up the requested URI on the filesystem--or it can do some processing and forward it to another handler--for example, a logger that prints information about requests and responses to the command line.
The latter kind of handler is called “middleware”, since it sits in the middle of the server stack. Middleware can be thought of as a function that takes a handler and wraps it in another handler to provide additional functionality. A Shelf application is usually composed of many layers of middleware with one or more handlers at the very center; the shelf.Pipeline class makes this sort of application easy to construct.
Some middleware can also take multiple handlers and call one or more of them for each request. For example, a routing middleware might choose which handler to call based on the request's URI or HTTP method, while a cascading middleware might call each one in sequence until one returns a successful response.
An adapter is any code that creates shelf.Request objects, passes them to a handler, and deals with the resulting shelf.Response. For the most part, adapters forward requests from and responses to an underlying HTTP server; shelf_io.serve is this sort of adapter. An adapter might also synthesize HTTP requests within the browser using window.location
and window.history
, or it might pipe requests directly from an HTTP client to a Shelf handler.
When implementing an adapter, some rules must be followed. The adapter must not pass the url
or scriptName
parameters to new shelf.Request; it should only pass requestedUri
. If it passes the context
parameter, all keys must begin with the adapter's package name followed by a period. If multiple headers with the same name are received, the adapter must collapse them into a single header separated by commas as per RFC 2616 section 4.2.
An adapter must handle all errors from the handler, including the handler returning a null
response. It should print each error to the console if possible, then act as though the handler returned a 500 response. The adapter may include body data for the 500 response, but this body data must not include information about the error that occurred. This ensures that unexpected errors don't result in exposing internal information in production by default; if the user wants to return detailed error descriptions, they should explicitly include middleware to do so.
An adapter should include information about itself in the Server header of the response by default. If the handler returns a response with the Server header set, that must take precedence over the adapter's default header.
An adapter should ensure that asynchronous errors thrown by the handler don‘t cause the application to crash, even if they aren’t reported by the future chain. Specifically, these errors shouldn‘t be passed to the root zone’s error handler; however, if the adapter is run within another error zone, it should allow these errors to be passed to that zone. The following function can be used to capture only errors that would otherwise be top-leveled:
/// Run [callback] and capture any errors that would otherwise be top-leveled. /// /// If [this] is called in a non-root error zone, it will just run [callback] /// and return the result. Otherwise, it will capture any errors using /// [runZoned] and pass them to [onError]. catchTopLevelErrors(callback(), void onError(error, StackTrace stackTrace)) { if (Zone.current.inSameErrorZone(Zone.ROOT)) { return runZoned(callback, onError: onError); } else { return callback(); } }