WebSockets: implement a JavaScript object-oriented client API

Replace previous callback based basic client with an easier
to use object-oriented API that further abstracts the low level
details of the WebSockets Server surface messaging protocol.

All built-in web surface demos were updated to use the new API.

share/web_surfaces/shared/base/channel.js

@@ -0,0 +1,81 @@
+/*
+ * Copyright © 2020 Luciano Iam <lucianito@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+import { Message } from './protocol.js';
+
+export class MessageChannel {
+
+	constructor (host) {
+		// https://developer.mozilla.org/en-US/docs/Web/API/URL/host
+		this._host = host;
+		this._pending = null;
+	}
+
+	async open () {
+		return new Promise((resolve, reject) => {
+			this._socket = new WebSocket(`ws://${this._host}`);
+
+			this._socket.onclose = () => this.onClose();
+
+			this._socket.onerror = (error) => this.onError(error);
+
+			this._socket.onmessage = (event) => {
+				const msg = Message.fromJsonText(event.data);
+
+				if (this._pending && (this._pending.nodeAddrId == msg.nodeAddrId)) {
+					this._pending.resolve(msg);
+					this._pending = null;
+				} else {
+					this.onMessage(msg, true);
+				}
+			};
+
+			this._socket.onopen = resolve;
+		});
+	}
+
+	close () {
+		this._socket.close();
+
+		if (this._pending) {
+			this._pending.reject(Error('MessageChannel: socket closed awaiting response'));
+			this._pending = null;
+		}
+	}
+
+	send (msg) {
+		if (this._socket) {
+			this._socket.send(msg.toJsonText());
+			this.onMessage(msg, false);
+		} else {
+			this.onError(Error('MessageChannel: cannot call send() before open()'));
+		}
+	}
+
+	async sendAndReceive (msg) {
+		return new Promise((resolve, reject) => {
+			this._pending = {resolve: resolve, reject: reject, nodeAddrId: msg.nodeAddrId};
+			this.send(msg);
+		});
+	}
+
+	onClose () {}
+	onError (error) {}
+	onMessage (msg, inbound) {}
+
+}