shelf_exception_response 0.1.1

Shelf middleware that provides the abillity to simply throw an HttpException that gets converted to an appropriate response code and a formatted response body dependent on the clients accept header.

Shelf Exception Response

Shelf middleware for easily creating shelf.Responses by simply throwing an HttpException.

What problem does it solve

When creating web service you have to handle a lot of http responses simply informing the client about a server-side error. Not all of those situations occure withing your request handlers but rather in some service classes or elsewhere in your application. In your controller you would write something like:

// given a simple function that throws in certain conditions
String doJob() {
	if(!myCondition) {
		throw new MyError();
	}
	return "Successfully done Job";
}

// code in the request handler
try {
	String res = doJob();
	return new shelf.Response.ok(res);
} on MyError {
	return new shelf.Response.forbidden("You have to be logged in");
}

When using HttpExceptions and the exception response middleware your controller could be simplified to the following:

// given a simple function that throws HttpException in certain conditions
String doJob() {
	if(!myCondition) {
		throw new ForbiddenException();
	}
	return "Successfully done Job";
}

// code in request handler
String res = doJob();
return new shelf.Response.ok(res);

The thrown HttpException will be handled and an appropriate shelf.Response will be generated. The response will have a correct Http status (eg.: 403), and the response body will be encoded in an acceptable format (Json, Xml, Text) using Tentacle Response Formatter.

Example

import 'package:shelf/shelf.dart' as shelf;
import 'package:shelf/shelf_io.dart' as io;
import '../lib/exception_response.dart';

// a simple shelf server with exception response enabled.
void main() {

  var handler = const shelf.Pipeline()
    // add exception response middleware to the pipeline
    .addMiddleware(exceptionResponse())
    .addHandler((shelf.Request request){
      // handler can now throw HttpException that is handled a formatted.
      throw new NotFoundException({"something":["additional", 4, true]}, "This will show in the message");
    });

  io.serve(handler, 'localhost', 8080).then((server) {
    print('Serving at http://${server.address.host}:${server.port}');
  });
}

Adding additional response data for the body

Every HttpException can take a data parameter that accepts a Map<String, dynamic> data in the constructor that provides you with the ability to add custom data to the generated response. The fields status and message are always added to the response data but can be overridden by data. The message field can now receive additional text which will be appended to the HTTP Status Name (e.g.: Not Acceptable: Your text here) via the Exceptions details param.

Please keep in mind that responses of type "text/plain" do currently not output any data other than status and message (how should I format them anyhow ?). To add support for your own output format or change an existing one look at Tentacle Response Formatter under the section "Add your own response formatter".

// adding custom data and error details to response
throw new NotAcceptableException({
	"validation_errors": {
		"username": "to_long",
		"password": "not_contains_special_characters"
	},
	"Validation failed"
});

Add your own exception types

There are already predefined exceptions for the most common Http error code available but you may find that there is a specific error code missing. In that case you can simply define your own HttpException type by extending or implementing HttpException.

// creating custom exception
class IamATeapotException extends HttpException {
  final int status = 418;
  final String message = "I'm a teapot";
  final Map<String, dynamic> data;
  const IamATeapotException([this.data]);
}

License

Apache 2.0