Allow using the kiosk on platforms without DBus

We previously added support for using the kiosk without connman
installed, but it still crashed when started on systems without DBus
(i.e. any non-Linux platform).

With this change we add a stub implementation of the core proxy
abstraction, which is going to provide safe no-op responses on non-Linux
platforms, allowing to use the kiosk, albeit without proxy support.

kiosk/Readme.md

@@ -28,3 +28,9 @@ Then, point a Chromium-based browser to `http://127.0.0.1:3355`.
 
 Additional documentation is available at:
 https://doc.qt.io/qt-6/qtwebengine-debugging.html
+
+## Supported platforms
+
+The kiosk is written with use within PlayOS in mind (implying connman and DBus as part of the system). To allow for testing web pages in the kiosk on developer machines, macOS is also supported, with the following limitations:
+
+- No proxy server support

kiosk/kiosk_browser/main_widget.py

@@ -14,7 +14,7 @@ def __init__(self, kiosk_url: str, settings_url: str, toggle_settings_key: str):
         super(MainWidget, self).__init__()
 
         # Proxy
-        proxy = proxy_module.Proxy()
+        proxy = proxy_module.init()
         proxy.start_monitoring_daemon()
 
         # Browser widget

kiosk/kiosk_browser/proxy.py

@@ -4,6 +4,7 @@
 import collections
 import dbus
 import logging
+import platform
 import threading
 import urllib
 from PyQt6.QtNetwork import QNetworkProxy
@@ -115,9 +116,47 @@ def set_no_proxy_in_qt_app():
     logging.info(f"Set no proxy in Qt application")
     QNetworkProxy.setApplicationProxy(QNetworkProxy())
 
-class Proxy():
+def init():
+    """Initialize a suitable Proxy instance for the current platform."""
+    if platform.system() in ['Linux']:
+        return DBusProxy()
+    else:
+        return Proxy()
+
+class Proxy:
+    """Base class for proxy querying.
+
+    The base class does not know how to query for proxy information and may be used as a fallback that always reports that no proxy is configured.
+    """
+    _proxy: ProxyConf | None
+
+    # For the base class, this is a pass, not knowing how to monitor in the general case.
+    def start_monitoring_daemon(self) -> None:
+        """Start a daemon monitoring for proxy changes.
+
+        In the base class, no monitoring method is known, and starting a daemon is skipped.
+        """
+        pass
+
+    def get_current(self) -> ProxyConf | None:
+        """Get the currently configured proxy.
+
+        This is always `None` in the base class.
+        """
+        return self._proxy
+
+class DBusProxy(Proxy):
+    """A Proxy class for DBus/Linux systems.
+
+    This class assumes that connman is the network manager and that it can be queried via DBus.
+    """
+
+    _proxy: ProxyConf | None
+    _bus: dbus.SystemBus
 
     def __init__(self):
+        super().__init__()
+
         DBusGMainLoop(set_as_default=True)
         self._bus = dbus.SystemBus()
         self._proxy = get_current_proxy(self._bus)
@@ -129,9 +168,6 @@ def start_monitoring_daemon(self):
         thread.daemon = True
         thread.start()
 
-    def get_current(self):
-        return self._proxy
-
     def _monitor(self):
         self._bus.add_signal_receiver(
             handler_function = self._on_property_changed,