wasm: add echo_client_mainthread example

This example connects an echo server running behind
WebSockify, on localhost.

For example, start websockify with

    websockify 1515 localhost:1516

to accept a webscoket connection on 1515 and forward
to echo_server at 1516.

Pick-to: 6.4
Change-Id: Id71364e4ab8c46d3482b515fcd1b991b61d7404b
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

tests/manual/wasm/network/echo_client_mainthread/CMakeLists.txt

@@ -0,0 +1,8 @@
+qt_internal_add_manual_test(echo_client_mainthread
+    GUI
+    SOURCES
+        main.cpp
+    PUBLIC_LIBRARIES
+        Qt::Core
+        Qt::Network
+)

tests/manual/wasm/network/echo_client_mainthread/main.cpp

@@ -0,0 +1,99 @@
+/****************************************************************************
+**
+** Copyright (C) 2021 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the examples of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** BSD License Usage
+** Alternatively, you may use this file under the terms of the BSD license
+** as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
+**   * Redistributions in binary form must reproduce the above copyright
+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
+**   * Neither the name of The Qt Company Ltd nor the names of its
+**     contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QtCore>
+#include <QtNetwork>
+
+int main(int argc, char **argv) {
+
+    QCoreApplication app(argc, argv);
+
+    // This example connects to localhost, but note that the host can
+    // be any host reachable from the client using webscokets, at any port.
+    QString hostName = "localhost";
+    int port = 1515;
+    qDebug() << "This example connects to a server at" << hostName << "port" << port << ","
+             << "where it expects to find a WebSockify server, which forwards to the fortune server.";
+
+    auto echo = [hostName, port]() {
+        QTcpSocket *socket = new QTcpSocket();
+
+        QObject::connect(socket, &QAbstractSocket::connected, [socket]() {
+            qDebug() << "Connected";
+            socket->write("Hello, echo server!");
+            socket->flush();
+        });
+
+        QObject::connect(socket, &QIODevice::readyRead, [socket]() {
+            QByteArray data = socket->readAll();
+            qDebug() << "Ready Read, got echo:" << data;
+            socket->disconnectFromHost();
+            socket->deleteLater();
+        });
+
+        QObject::connect(socket, &QAbstractSocket::errorOccurred, [socket]() {
+            qDebug() << "Error Occurred" << socket->error();
+        });
+
+        QObject::connect(socket, &QAbstractSocket::disconnected, [socket]() {
+            qDebug() << "Disconnected";
+            socket->deleteLater();
+        });
+
+        qDebug() << "Connect to host" << hostName << port;
+        socket->connectToHost(hostName, port);
+    };
+
+    QTimer::singleShot(500, [echo](){
+        echo();
+    });
+
+    return app.exec();
+}