Create a simple WebSocket interface (#1128)

pkgs/web_socket/CHANGELOG.md

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+## 0.1.0-wip
+
+- Abstract interface definition.

pkgs/web_socket/README.md

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+TODO: Put a short description of the package here that helps potential users
+know whether this package might be useful for them.

pkgs/web_socket/example/web_socket_example.dart

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+void main() {
+  // TODO: add an example.
+}

pkgs/web_socket/lib/src/web_socket.dart

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+import 'dart:typed_data';
+
+/// An event received from the peer through the [WebSocket].
+sealed class WebSocketEvent {}
+
+/// Text data received from the peer through the [WebSocket].
+///
+/// See [WebSocket.events].
+final class TextDataReceived extends WebSocketEvent {
+  final String text;
+  TextDataReceived(this.text);
+
+  @override
+  bool operator ==(Object other) =>
+      other is TextDataReceived && other.text == text;
+
+  @override
+  int get hashCode => text.hashCode;
+}
+
+/// Binary data received from the peer through the [WebSocket].
+///
+/// See [WebSocket.events].
+final class BinaryDataReceived extends WebSocketEvent {
+  final Uint8List data;
+  BinaryDataReceived(this.data);
+
+  @override
+  bool operator ==(Object other) {
+    if (other is BinaryDataReceived && other.data.length == data.length) {
+      for (var i = 0; i < data.length; ++i) {
+        if (other.data[i] != data[i]) return false;
+      }
+      return true;
+    }
+    return false;
+  }
+
+  @override
+  int get hashCode => data.hashCode;
+
+  @override
+  String toString() => 'BinaryDataReceived($data)';
+}
+
+/// A close notification (Close frame) received from the peer through the
+/// [WebSocket] or a failure indication.
+///
+/// See [WebSocket.events].
+final class CloseReceived extends WebSocketEvent {
+  /// A numerical code indicating the reason why the WebSocket was closed.
+  ///
+  /// See [RFC-6455 7.4](https://www.rfc-editor.org/rfc/rfc6455.html#section-7.4)
+  /// for guidance on how to interpret these codes.
+  final int? code;
+
+  /// A textual explanation of the reason why the WebSocket was closed.
+  ///
+  /// Will be empty if the peer did not specify a reason.
+  final String reason;
+
+  CloseReceived([this.code, this.reason = '']);
+
+  @override
+  bool operator ==(Object other) =>
+      other is CloseReceived && other.code == code && other.reason == reason;
+
+  @override
+  int get hashCode => [code, reason].hashCode;
+
+  @override
+  String toString() => 'CloseReceived($code, $reason)';
+}
+
+class WebSocketException implements Exception {
+  final String message;
+  WebSocketException([this.message = '']);
+}
+
+/// Thrown if [WebSocket.sendText], [WebSocket.sendBytes], or
+/// [WebSocket.close] is called when the [WebSocket] is closed.
+class WebSocketConnectionClosed extends WebSocketException {
+  WebSocketConnectionClosed([super.message = 'Connection Closed']);
+}
+
+/// The interface for WebSocket connections.
+///
+/// TODO: insert a usage example.
+abstract interface class WebSocket {
+  /// Sends text data to the connected peer.
+  ///
+  /// Throws [WebSocketConnectionClosed] if the [WebSocket] is
+  /// closed (either through [close] or by the peer).
+  ///
+  /// Data sent through [sendText] will be silently discarded if the peer is
+  /// disconnected but the disconnect has not yet been detected.
+  void sendText(String s);
+
+  /// Sends binary data to the connected peer.
+  ///
+  /// Throws [WebSocketConnectionClosed] if the [WebSocket] is
+  /// closed (either through [close] or by the peer).
+  ///
+  /// Data sent through [sendBytes] will be silently discarded if the peer is
+  /// disconnected but the disconnect has not yet been detected.
+  void sendBytes(Uint8List b);
+
+  /// Closes the WebSocket connection and the [events] `Stream`.
+  ///
+  /// Sends a Close frame to the peer. If the optional [code] and [reason]
+  /// arguments are given, they will be included in the Close frame. If no
+  /// [code] is set then the peer will see a 1005 status code. If no [reason]
+  /// is set then the peer will not receive a reason string.
+  ///
+  /// Throws a [RangeError] if [code] is not in the range 3000-4999.
+  ///
+  /// Throws an [ArgumentError] if [reason] is longer than 123 bytes when
+  /// encoded as UTF-8
+  ///
+  /// Throws [WebSocketConnectionClosed] if the connection is already
+  /// closed (including by the peer).
+  Future<void> close([int? code, String? reason]);
+
+  /// A [Stream] of [WebSocketEvent] received from the peer.
+  ///
+  /// Data received by the peer will be delivered as a [TextDataReceived] or
+  /// [BinaryDataReceived].
+  ///
+  /// If a [CloseReceived] event is received then the [Stream] will be closed. A
+  /// [CloseReceived] event indicates either that:
+  ///
+  /// - A close frame was received from the peer. [CloseReceived.code] and
+  ///   [CloseReceived.reason] will be set by the peer.
+  /// - A failure occured (e.g. the peer disconnected). [CloseReceived.code] and
+  ///   [CloseReceived.reason] will be a failure code defined by
+  ///   (RFC-6455)[https://www.rfc-editor.org/rfc/rfc6455.html#section-7.4.1]
+  ///   (e.g. 1006).
+  ///
+  /// Errors will never appear in this [Stream].
+  Stream<WebSocketEvent> get events;
+}

pkgs/web_socket/lib/web_socket.dart

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+/// TODO: write this doc string.
+library;
+
+export 'src/web_socket.dart';