Unify application palette handling between QGuiApplication and QApplication

The logic is now mostly handled in QGuiApplication, with QApplication
only dealing with the widget-specific palettes and interaction between
the style and the palette.

The application now picks up changes to the platform theme and will
re-resolve the current application palette appropriately. This also
works even if an explicit application palette has been set, in which
case any missing roles are filled in by the theme.

The palette can now also be reset back to the default application
palette that's fully based on the theme, by passing in the default
constructed palette (or any palette that doesn't have any roles set).
This is also correctly reflected in the Qt::AA_SetPalette attribute.

Conceptually this means QGuiApplication and QApplication follow the
same behavior as QWidget, where the palette falls back to a base or
inherited palette for roles that are not set, in this case the theme.

Behavior-wise this means that the default application palette of the
application does not have any roles set, but clients should not have
relied on this, nor does QWidget rely on that internally.

It also means that setting a palette on the application and then
getting it back again will not produce the same palette as set,
since the palette was resolved against the theme in the meantime.
This is the same behavior as for QWidget, and although it's a
behavior change it's one towards a more sane behavior, so we
accept it.

[ChangeLog] Application palettes are now resolved against the platform's
theme palette, the same way widget palettes are resolved against their
parents, and the application palette. This means the application palette
reflected through QGuiApplication::palette() may not be exactly the same
palette as set via QGuiApplication::setPalette().

Change-Id: I76b99fcd27285e564899548349aa2a5713e5965d
Reviewed-by: Vitaly Fanaskov <vitaly.fanaskov@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

2 files changed, 76 insertions, 52 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index ffd5b04276..445ad6b835 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -236,21 +236,6 @@ static bool qt_detectRTLLanguage()
                          " and Arabic) to get proper widget layout.") == QLatin1String("RTL"));
 }
 
-static void initPalette()
-{
-    if (!QGuiApplicationPrivate::app_pal)
-        if (const QPalette *themePalette = QGuiApplicationPrivate::platformTheme()->palette())
-            QGuiApplicationPrivate::app_pal = new QPalette(*themePalette);
-    if (!QGuiApplicationPrivate::app_pal)
-        QGuiApplicationPrivate::app_pal = new QPalette(Qt::gray);
-}
-
-static inline void clearPalette()
-{
-    delete QGuiApplicationPrivate::app_pal;
-    QGuiApplicationPrivate::app_pal = nullptr;
-}
-
 static void initFontUnlocked()
 {
     if (!QGuiApplicationPrivate::app_font) {
@@ -691,7 +676,7 @@ QGuiApplication::~QGuiApplication()
     d->session_manager = nullptr;
 #endif //QT_NO_SESSIONMANAGER
 
-    clearPalette();
+    QGuiApplicationPrivate::clearPalette();
     QFontDatabase::removeAllApplicationFonts();
 
 #ifndef QT_NO_CURSOR
@@ -1611,7 +1596,7 @@ void QGuiApplicationPrivate::init()
     if (platform_integration == nullptr)
         createPlatformIntegration();
 
-    initPalette();
+    updatePalette();
     QFont::initialize();
     initThemeHints();
 
@@ -3289,46 +3274,97 @@ QClipboard * QGuiApplication::clipboard()
 */
 
 /*!
-    Returns the default application palette.
+    Returns the current application palette.
+
+    Roles that have not been explicitly set will reflect the system's platform theme.
 
     \sa setPalette()
 */
 
 QPalette QGuiApplication::palette()
 {
-    initPalette();
+    if (!QGuiApplicationPrivate::app_pal)
+        QGuiApplicationPrivate::updatePalette();
+
     return *QGuiApplicationPrivate::app_pal;
 }
 
+void QGuiApplicationPrivate::updatePalette()
+{
+    if (app_pal) {
+        if (setPalette(*app_pal) && qGuiApp)
+            qGuiApp->d_func()->handlePaletteChanged();
+    } else {
+        setPalette(QPalette());
+    }
+}
+
+void QGuiApplicationPrivate::clearPalette()
+{
+    delete app_pal;
+    app_pal = nullptr;
+}
+
 /*!
-    Changes the default application palette to \a pal.
+    Changes the application palette to \a pal.
+
+    The color roles from this palette are combined with the system's platform
+    theme to form the application's final palette.
 
     \sa palette()
 */
 void QGuiApplication::setPalette(const QPalette &pal)
 {
-    if (!QGuiApplicationPrivate::setPalette(pal))
-        return;
-
-    QCoreApplication::setAttribute(Qt::AA_SetPalette);
-
-    if (qGuiApp)
-        qGuiApp->d_func()->sendApplicationPaletteChange();
+    if (QGuiApplicationPrivate::setPalette(pal) && qGuiApp)
+        qGuiApp->d_func()->handlePaletteChanged();
 }
 
 bool QGuiApplicationPrivate::setPalette(const QPalette &palette)
 {
-    if (app_pal && palette.isCopyOf(*app_pal))
+    // Resolve the palette against the theme palette, filling in
+    // any missing roles, while keeping the original resolve mask.
+    QPalette basePalette = qGuiApp ? qGuiApp->d_func()->basePalette() : Qt::gray;
+    basePalette.resolve(0); // The base palette only contributes missing colors roles
+    QPalette resolvedPalette = palette.resolve(basePalette);
+
+    if (app_pal && resolvedPalette == *app_pal && resolvedPalette.resolve() == app_pal->resolve())
         return false;
 
     if (!app_pal)
-        app_pal = new QPalette(palette);
+        app_pal = new QPalette(resolvedPalette);
     else
-        *app_pal = palette;
+        *app_pal = resolvedPalette;
+
+    QCoreApplication::setAttribute(Qt::AA_SetPalette, app_pal->resolve() != 0);
 
     return true;
 }
 
+/*
+    Returns the base palette used to fill in missing roles in
+    the current application palette.
+
+    Normally this is the theme palette, but QApplication
+    overrides this for compatibility reasons.
+*/
+QPalette QGuiApplicationPrivate::basePalette() const
+{
+    return platformTheme() ? *platformTheme()->palette() : Qt::gray;
+}
+
+void QGuiApplicationPrivate::handlePaletteChanged(const char *className)
+{
+    if (!className) {
+        Q_ASSERT(app_pal);
+        emit qGuiApp->paletteChanged(*QGuiApplicationPrivate::app_pal);
+    }
+
+    if (is_app_running && !is_app_closing) {
+        QEvent event(QEvent::ApplicationPaletteChange);
+        QGuiApplication::sendEvent(qGuiApp, &event);
+    }
+}
+
 void QGuiApplicationPrivate::applyWindowGeometrySpecificationTo(QWindow *window)
 {
     windowGeometrySpecification.applyTo(window);
@@ -4127,11 +4163,8 @@ QPixmap QGuiApplicationPrivate::getPixmapCursor(Qt::CursorShape cshape)
 
 void QGuiApplicationPrivate::notifyThemeChanged()
 {
-    if (!testAttribute(Qt::AA_SetPalette)) {
-        clearPalette();
-        initPalette();
-        sendApplicationPaletteChange();
-    }
+    updatePalette();
+
     if (!(applicationResourceFlags & ApplicationFontExplicitlySet)) {
         const auto locker = qt_scoped_lock(applicationFontMutex);
         clearFontUnlocked();
@@ -4140,20 +4173,6 @@ void QGuiApplicationPrivate::notifyThemeChanged()
     initThemeHints();
 }
 
-void QGuiApplicationPrivate::sendApplicationPaletteChange(bool toAllWidgets, const char *className)
-{
-    Q_UNUSED(toAllWidgets)
-
-    if (!className)
-        emit qGuiApp->paletteChanged(*QGuiApplicationPrivate::app_pal);
-
-    if (!is_app_running || is_app_closing)
-        return;
-
-    QEvent event(QEvent::ApplicationPaletteChange);
-    QGuiApplication::sendEvent(QGuiApplication::instance(), &event);
-}
-
 #if QT_CONFIG(draganddrop)
 void QGuiApplicationPrivate::notifyDragStarted(const QDrag *drag)
 {
diff --git a/src/gui/kernel/qguiapplication_p.h b/src/gui/kernel/qguiapplication_p.h
index 2fe4ae02cf..5239b58c97 100644
--- a/src/gui/kernel/qguiapplication_p.h
+++ b/src/gui/kernel/qguiapplication_p.h
@@ -323,17 +323,22 @@ public:
 
     static void resetCachedDevicePixelRatio();
 
-    static bool setPalette(const QPalette &palette);
-
 protected:
     virtual void notifyThemeChanged();
-    virtual void sendApplicationPaletteChange(bool toAllWidgets = false, const char *className = nullptr);
+
+    static bool setPalette(const QPalette &palette);
+    virtual QPalette basePalette() const;
+    virtual void handlePaletteChanged(const char *className = nullptr);
+
     bool tryCloseRemainingWindows(QWindowList processedWindows);
 #if QT_CONFIG(draganddrop)
     virtual void notifyDragStarted(const QDrag *);
 #endif // QT_CONFIG(draganddrop)
 
 private:
+    static void clearPalette();
+    static void updatePalette();
+
     friend class QDragManager;
 
     static QGuiApplicationPrivate *self;