virtualmonitor: implemented capabilities check

BUG: 485829

## Summary

Currently, the plugin just fails silently if the local device is missing the `krfb` package or if the remote device misses an `vnc://` protocol/scheme handler. You click the button and nothing happens.

One issue is, that the plugin is considered `virtualmonitor.available` in the `DeviceDelegate.qml`, even if the check for `krfb-virtualmonitor` fails and no dbus-path is provided. I investigated the behavior a bit, but ignored it in the end as this MR benefits from being shown for device constellations that _could_ provide this feature.

A warning is shown with brief instructions, how to get the plugin working correctly.

- Check if krfb-virtualmonitor is available locally
- Check default scheme handler for vnc:// on device (Linux)
- Show warnings / reasons, if no connection could be established


## Test Plan

Regarding if the devices have mentioned packages installed, we should see different behaviors.

If the remote device has no VNC client, it can not connect to out server. _A warning should be shown._

If the local device hasn't the `krfb-virtualmonitor` available, the remote device couldn't connect. _A warning should be shown._

If both problems are present, _both warnings should be shown._


If none of these are present, no warning should be shown and we should try to establish a connection.


The connection attempts failed? _A warning should be shown._

declarativeplugin/qml/PluginChecker.qml

@@ -36,7 +36,6 @@ QtObject {
     function pluginsChanged() {
         availableResponse.setPendingCall(device.hasPlugin("kdeconnect_" + pluginName))
         iconResponse.setPendingCall(device.pluginIconName("kdeconnect_" + pluginName))
-
     }
 
     readonly property var vv: DBusAsyncResponse {

plasmoid/package/contents/ui/DeviceDelegate.qml

@@ -23,6 +23,14 @@ PlasmaComponents.ItemDelegate
     hoverEnabled: false
     down: false
 
+    Kirigami.PromptDialog {
+        id: prompt
+        visible: false
+        showCloseButton: true
+        standardButtons: Kirigami.Dialog.NoButton
+        title: i18n("Virtual Monitor is not available")
+    }
+
     DropArea {
         id: fileDropArea
         anchors.fill: parent
@@ -76,6 +84,11 @@ PlasmaComponents.ItemDelegate
                 device: root.device
             }
 
+            VirtualMonitor {
+                id: virtualmonitor
+                device: root.device
+            }
+
             PlasmaComponents.Label {
                 id: deviceName
                 elide: Text.ElideRight
@@ -85,16 +98,25 @@ PlasmaComponents.ItemDelegate
             }
 
             PlasmaComponents.ToolButton {
-                VirtualMonitor {
-                    id: vd
-                    device: root.device
-                }
                 icon.name: "video-monitor"
+                visible: virtualmonitor.available
                 text: i18n("Virtual Display")
-                visible: vd.available
                 onClicked: {
-                    if (!vd.plugin.requestVirtualMonitor()) {
+                    let err = "";
-                        console.warn("Failed to create the virtual monitor")
+                    if (virtualmonitor?.plugin?.hasRemoteVncClient === false) {
+                        err = i18n("Remote device does not have a VNC client (eg. krdc) installed.");
+                    }
+                    if (virtualmonitor?.plugin?.isVirtualMonitorAvailable === false) {
+                        err = (err ? err + "\n\n" : "")
+                            + i18n("The krfb package is required on the local device.");
+                    }
+
+                    if (err) {
+                        prompt.subtitle = err;
+                        prompt.visible = true;
+                    } else if (!virtualmonitor.plugin.requestVirtualMonitor()) {
+                        prompt.subtitle = i18n("Failed to create the virtual monitor.");
+                        prompt.visible = true;
                     }
                 }
             }

plugins/virtualmonitor/kdeconnect_virtualmonitor.json

@@ -46,6 +46,10 @@
                 "Name[x-test]": "xxAleix Pol i Gonzalezxx",
                 "Name[zh_CN]": "Aleix Pol i Gonzalez",
                 "Name[zh_TW]": "Aleix Pol i Gonzalez"
+            },
+            {
+                "Email": "fabian.arndt@root-core.net",
+                "Name": "Fabian Arndt"
             }
         ],
         "Description": "Use your devices as virtual monitors",

plugins/virtualmonitor/virtualmonitorplugin.cpp

@@ -1,5 +1,6 @@
 /**
  * SPDX-FileCopyrightText: 2021 Aleix Pol i Gonzalez <aleixpol@kde.org>
+ * SPDX-FileCopyrightText: 2024 Fabian Arndt <fabian.arndt@root-core.net>
  *
  * SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
  */
@@ -16,6 +17,12 @@
 #include <QScreen>
 #include <QStandardPaths>
 
+#if defined(Q_OS_WIN)
+#include <QSettings>
+#else
+#include <KApplicationTrader>
+#endif
+
 K_PLUGIN_CLASS_WITH_JSON(VirtualMonitorPlugin, "kdeconnect_virtualmonitor.json")
 #define QS QLatin1String
 static const int DEFAULT_PORT = 5901;
@@ -41,6 +48,13 @@ void VirtualMonitorPlugin::stop()
 
 void VirtualMonitorPlugin::connected()
 {
+    // Get local capabilities
+    // We test again, since the user may have installed the necessary packages in the meantime
+    m_capabilitiesLocal.vncClient = checkVncClient();
+    m_capabilitiesLocal.virtualMonitor = !QStandardPaths::findExecutable(QS("krfb-virtualmonitor")).isEmpty();
+    qCDebug(KDECONNECT_PLUGIN_VIRTUALMONITOR) << "Local device supports VNC:" << m_capabilitiesLocal.vncClient;
+    qCDebug(KDECONNECT_PLUGIN_VIRTUALMONITOR) << "Local device supports Virtual Monitor:" << m_capabilitiesLocal.virtualMonitor;
+
     auto screen = QGuiApplication::primaryScreen();
     auto resolution = screen->size();
     QString resolutionString = QString::number(resolution.width()) + QLatin1Char('x') + QString::number(resolution.height());
@@ -51,6 +65,8 @@ void VirtualMonitorPlugin::connected()
                               {QS("resolution"), resolutionString},
                               {QS("scale"), screen->devicePixelRatio()},
                           }}},
+                         {QS("supports_vnc"), m_capabilitiesLocal.vncClient},
+                         {QS("supports_virt_mon"), m_capabilitiesLocal.virtualMonitor},
                      });
     sendPacket(np);
 }
@@ -92,13 +108,19 @@ void VirtualMonitorPlugin::receivePacket(const NetworkPacket &received)
         if (received.has(QS("failed"))) {
             stop();
         }
+
+        // Get device's capabilities
+        m_capabilitiesRemote.vncClient = received.get<bool>(QS("supports_vnc"), false);
+        m_capabilitiesRemote.virtualMonitor = received.get<bool>(QS("supports_virt_mon"), false);
+        qCDebug(KDECONNECT_PLUGIN_VIRTUALMONITOR) << "Remote device supports VNC:" << m_capabilitiesRemote.vncClient;
+        qCDebug(KDECONNECT_PLUGIN_VIRTUALMONITOR) << "Remote device supports Virtual Monitor:" << m_capabilitiesRemote.virtualMonitor;
     }
 }
 
 QString VirtualMonitorPlugin::dbusPath() const
 {
     // Don't offer the feature if krfb-virtualmonitor is not around
-    if (QStandardPaths::findExecutable(QS("krfb-virtualmonitor")).isEmpty())
+    if (!m_capabilitiesLocal.virtualMonitor)
         return {};
 
     return QLatin1String("/modules/kdeconnect/devices/%1/virtualmonitor").arg(device()->id());
@@ -159,5 +181,43 @@ bool VirtualMonitorPlugin::requestVirtualMonitor()
     return true;
 }
 
+bool VirtualMonitorPlugin::checkVncClient() const
+{
+#if defined(Q_OS_MAC)
+    // TODO: macOS has some VNC client preinstalled.. does it work for us?
+    return true;
+#else
+    // krdc is default on KDE
+    if (!QStandardPaths::findExecutable(QS("krdc")).isEmpty())
+        return true;
+
+    // Check if another client is available
+    return checkDefaultSchemeHandler(QS("vnc"));
+#endif
+}
+
+bool VirtualMonitorPlugin::checkDefaultSchemeHandler(const QString &scheme) const
+{
+    // TODO: macOS built-in tool: defaults read com.apple.LaunchServices/com.apple.launchservices.secure
+    // A function to use such a program was implemented and later removed in MR !670 (Commit e72f688ae1fad8846c006a3f87d57e507f6dbcb6)
+#if defined(Q_OS_WIN)
+    // Scheme handlers are stored in the registry on Windows
+    // https://learn.microsoft.com/en-us/previous-versions/windows/internet-explorer/ie-developer/platform-apis/aa767914(v=vs.85)
+
+    // Check if url handler is present
+    QSettings handler(QS("HKEY_CLASSES_ROOT\\") + scheme, QSettings::Registry64Format);
+    if (handler.value("URL Protocol").isNull())
+        return false;
+
+    // Check if a command is actually present
+    QSettings command(QS("HKEY_CLASSES_ROOT\\") + scheme + QS("\\shell\\open\\command"), QSettings::Registry64Format);
+    return !command.value("Default").isNull();
+#else
+    KService::Ptr service = KApplicationTrader::preferredService(QS("x-scheme-handler/") + scheme);
+    const QString defaultApp = service ? service->desktopEntryName() : QString();
+    return !defaultApp.isEmpty();
+#endif
+}
+
 #include "moc_virtualmonitorplugin.cpp"
 #include "virtualmonitorplugin.moc"

plugins/virtualmonitor/virtualmonitorplugin.h

@@ -1,5 +1,6 @@
 /**
  * SPDX-FileCopyrightText: 2021 Aleix Pol i Gonzalez <aleixpol@kde.org>
+ * SPDX-FileCopyrightText: 2024 Fabian Arndt <fabian.arndt@root-core.net>
  *
  * SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
  */
@@ -19,6 +20,8 @@ class VirtualMonitorPlugin : public KdeConnectPlugin
 {
     Q_OBJECT
     Q_CLASSINFO("D-Bus Interface", "org.kde.kdeconnect.device.virtualmonitor")
+    Q_PROPERTY(bool hasRemoteVncClient READ hasRemoteVncClient CONSTANT)
+    Q_PROPERTY(bool isVirtualMonitorAvailable READ isVirtualMonitorAvailable CONSTANT)
 
 public:
     using KdeConnectPlugin::KdeConnectPlugin;
@@ -30,8 +33,30 @@ public:
     QString dbusPath() const override;
     void receivePacket(const NetworkPacket &np) override;
 
+    // The remote device has a VNC client installed
+    bool hasRemoteVncClient() const
+    {
+        return m_capabilitiesRemote.vncClient;
+    }
+
+    // krfb is installed on local device, virtual monitors is supported
+    bool isVirtualMonitorAvailable() const
+    {
+        return m_capabilitiesLocal.virtualMonitor;
+    }
+
 private:
     void stop();
+    bool checkVncClient() const;
+    bool checkDefaultSchemeHandler(const QString &scheme) const;
+
+    struct Capabilities {
+        bool virtualMonitor = false;
+        bool vncClient = false;
+    };
+
+    Capabilities m_capabilitiesLocal;
+    Capabilities m_capabilitiesRemote;
 
     QProcess *m_process = nullptr;
     QJsonObject m_remoteResolution;