Linux: Implement dark mode detection

modules/juce_gui_basics/native/juce_linux_Windowing.cpp

@@ -534,16 +534,53 @@ bool Desktop::canUseSemiTransparentWindows() noexcept
     return XWindowSystem::getInstance()->canUseSemiTransparentWindows();
 }
 
-bool Desktop::isDarkModeActive() const
+class Desktop::NativeDarkModeChangeDetectorImpl  : private XWindowSystemUtilities::XSettings::Listener
 {
-    return false;
-}
+public:
+    NativeDarkModeChangeDetectorImpl()
+    {
+        const auto* windowSystem = XWindowSystem::getInstance();
+
+        if (auto* xSettings = windowSystem->getXSettings())
+            xSettings->addListener (this);
+
+        darkModeEnabled = windowSystem->isDarkModeActive();
+    }
+
+    ~NativeDarkModeChangeDetectorImpl() override
+    {
+        if (auto* windowSystem = XWindowSystem::getInstanceWithoutCreating())
+            if (auto* xSettings = windowSystem->getXSettings())
+                xSettings->removeListener (this);
+    }
+
+    bool isDarkModeEnabled() const noexcept  { return darkModeEnabled; }
+
+private:
+    void settingChanged (const XWindowSystemUtilities::XSetting& settingThatHasChanged) override
+    {
+        if (settingThatHasChanged.name == XWindowSystem::getThemeNameSettingName())
+        {
+            const auto wasDarkModeEnabled = std::exchange (darkModeEnabled, XWindowSystem::getInstance()->isDarkModeActive());
+
+            if (darkModeEnabled != wasDarkModeEnabled)
+                Desktop::getInstance().darkModeChanged();
+        }
+    }
 
-class Desktop::NativeDarkModeChangeDetectorImpl { public: NativeDarkModeChangeDetectorImpl() = default; };
+    bool darkModeEnabled = false;
+
+    JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (NativeDarkModeChangeDetectorImpl)
+};
 
 std::unique_ptr<Desktop::NativeDarkModeChangeDetectorImpl> Desktop::createNativeDarkModeChangeDetectorImpl()
 {
-    return nullptr;
+    return std::make_unique<NativeDarkModeChangeDetectorImpl>();
+}
+
+bool Desktop::isDarkModeActive() const
+{
+    return nativeDarkModeChangeDetectorImpl->isDarkModeEnabled();
 }
 
 static bool screenSaverAllowed = true;

modules/juce_gui_basics/native/x11/juce_linux_XWindowSystem.cpp

@@ -2018,6 +2018,37 @@ bool XWindowSystem::canUseARGBImages() const
     return canUseARGB;
 }
 
+bool XWindowSystem::isDarkModeActive() const
+{
+    const auto themeName = [this]() -> String
+    {
+        if (xSettings != nullptr)
+        {
+            const auto themeNameSetting = xSettings->getSetting (getThemeNameSettingName());
+
+            if (themeNameSetting.isValid()
+                && themeNameSetting.stringValue.isNotEmpty())
+            {
+                return themeNameSetting.stringValue;
+            }
+        }
+
+        ChildProcess gsettings;
+
+        if (File ("/usr/bin/gsettings").existsAsFile()
+            && gsettings.start ("/usr/bin/gsettings get org.gnome.desktop.interface gtk-theme", ChildProcess::wantStdOut))
+        {
+            if (gsettings.waitForProcessToFinish (200))
+                return gsettings.readAllProcessOutput();
+        }
+
+        return {};
+    }();
+
+    return (themeName.isNotEmpty()
+          && (themeName.containsIgnoreCase ("dark") || themeName.containsIgnoreCase ("black")));
+}
+
 Image XWindowSystem::createImage (bool isSemiTransparent, int width, int height, bool argb) const
 {
     auto visualAndDepth = displayVisuals->getBestVisualForWindow (isSemiTransparent);

modules/juce_gui_basics/native/x11/juce_linux_XWindowSystem.h

@@ -197,6 +197,7 @@ public:
 
     bool canUseSemiTransparentWindows() const;
     bool canUseARGBImages() const;
+    bool isDarkModeActive() const;
 
     int getNumPaintsPendingForWindow (::Window);
     void processPendingPaintsForWindow (::Window);
@@ -238,6 +239,7 @@ public:
     bool isX11Available() const noexcept  { return xIsAvailable; }
 
     static String getWindowScalingFactorSettingName()  { return "Gdk/WindowScalingFactor"; }
+    static String getThemeNameSettingName()            { return "Net/ThemeName"; }
 
     //==============================================================================
     void handleWindowMessage (LinuxComponentPeer*, XEvent&) const;