QWindow: Cache the devicePixelRatio value

Make each QWindow instance cache the current DPR value.
This will make calling QWindow::devicePixelRatio() less
costly, since it now does not have to compute the DPR
value on each call.

The cache is invalidated when the DevicePixelRatioChange
event is sent. The common logic for handling this is
implemented in QWindowPrivate::updateDevicePixelRatio().

Change-Id: I97231a230347358d8e565d2fd62e8a398adaedfc
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/kernel/qguiapplication.cpp

@@ -2565,8 +2565,6 @@ void QGuiApplicationPrivate::processWindowScreenChangedEvent(QWindowSystemInterf
         if (window->screen() == wse->screen.data())
             return;
 
-        const qreal oldDevicePixelRatio = window->screen() ? window->screen()->devicePixelRatio() : 1.0;
-
         if (QWindow *topLevelWindow = window->d_func()->topLevelWindow(QWindow::ExcludeTransients)) {
             if (QScreen *screen = wse->screen.data())
                 topLevelWindow->d_func()->setTopLevelScreen(screen, false /* recreate */);
@@ -2583,12 +2581,7 @@ void QGuiApplicationPrivate::processWindowScreenChangedEvent(QWindowSystemInterf
             processGeometryChangeEvent(&gce);
         }
 #endif
-
-        const qreal newDevicePixelRatio = window->screen() ? window->screen()->devicePixelRatio() : 1.0;
-        if (!qFuzzyCompare(oldDevicePixelRatio, newDevicePixelRatio)) {
-            QEvent dprChangeEvent(QEvent::DevicePixelRatioChange);
-            QGuiApplication::sendSpontaneousEvent(window, &dprChangeEvent);
-        }
+        QWindowPrivate::get(window)->updateDevicePixelRatio();
     }
 }
 
@@ -2596,9 +2589,7 @@ void QGuiApplicationPrivate::processWindowDevicePixelRatioChangedEvent(QWindowSy
 {
     if (wde->window.isNull())
         return;
-
-    QEvent dprChangeEvent(QEvent::DevicePixelRatioChange);
-    QGuiApplication::sendSpontaneousEvent(wde->window, &dprChangeEvent);
+    QWindowPrivate::get(wde->window)->updateDevicePixelRatio();
 }
 
 void QGuiApplicationPrivate::processSafeAreaMarginsChangedEvent(QWindowSystemInterfacePrivate::SafeAreaMarginsChangedEvent *wse)

src/gui/kernel/qwindow.cpp

@@ -213,8 +213,10 @@ void QWindowPrivate::init(QScreen *targetScreen)
     isWindow = true;
     parentWindow = static_cast<QWindow *>(q->QObject::parent());
 
+    QScreen *connectScreen = targetScreen ? targetScreen : QGuiApplication::primaryScreen();
+
     if (!parentWindow)
-        connectToScreen(targetScreen ? targetScreen : QGuiApplication::primaryScreen());
+        connectToScreen(connectScreen);
 
     // If your application aborts here, you are probably creating a QWindow
     // before the screen list is populated.
@@ -224,6 +226,7 @@ void QWindowPrivate::init(QScreen *targetScreen)
     QGuiApplicationPrivate::window_list.prepend(q);
 
     requestedFormat = QSurfaceFormat::defaultFormat();
+    devicePixelRatio = connectScreen->devicePixelRatio();
 }
 
 /*!
@@ -505,8 +508,6 @@ void QWindowPrivate::create(bool recursive, WId nativeHandle)
     // the platformWindow, if there was one, is now gone, so make this flag reflect reality now
     updateRequestPending = false;
 
-    const qreal currentDevicePixelRatio = q->devicePixelRatio();
-
     if (q->parent())
         q->parent()->create();
 
@@ -552,10 +553,7 @@ void QWindowPrivate::create(bool recursive, WId nativeHandle)
     QPlatformSurfaceEvent e(QPlatformSurfaceEvent::SurfaceCreated);
     QGuiApplication::sendEvent(q, &e);
 
-    if (!qFuzzyCompare(currentDevicePixelRatio, q->devicePixelRatio())) {
-        QEvent dprChangeEvent(QEvent::DevicePixelRatioChange);
-        QGuiApplication::sendEvent(q, &dprChangeEvent);
-    }
+    updateDevicePixelRatio();
 
     if (needsUpdate)
         q->requestUpdate();
@@ -1333,14 +1331,29 @@ Qt::ScreenOrientation QWindow::contentOrientation() const
 qreal QWindow::devicePixelRatio() const
 {
     Q_D(const QWindow);
+    return d->devicePixelRatio;
+}
+
+/*
+    Updates the cached devicePixelRatio value by polling for a new value.
+    Sends QEvent::DevicePixelRatioChange to the window if the DPR has changed.
+*/
+void QWindowPrivate::updateDevicePixelRatio()
+{
+    Q_Q(QWindow);
 
     // If there is no platform window use the associated screen's devicePixelRatio,
     // which typically is the primary screen and will be correct for single-display
     // systems (a very common case).
-    if (!d->platformWindow)
-        return screen()->devicePixelRatio();
+    const qreal newDevicePixelRatio = platformWindow ?
+        platformWindow->devicePixelRatio() * QHighDpiScaling::factor(q) : q->screen()->devicePixelRatio();
 
-    return d->platformWindow->devicePixelRatio() * QHighDpiScaling::factor(this);
+    if (newDevicePixelRatio == devicePixelRatio)
+        return;
+
+    devicePixelRatio = newDevicePixelRatio;
+    QEvent dprChangeEvent(QEvent::DevicePixelRatioChange);
+    QGuiApplication::sendEvent(q, &dprChangeEvent);
 }
 
 Qt::WindowState QWindowPrivate::effectiveState(Qt::WindowStates state)

src/gui/kernel/qwindow_p.h

@@ -89,6 +89,8 @@ public:
     void setAutomaticPositionAndResizeEnabled(bool a)
     { positionAutomatic = resizeAutomatic = a; }
 
+    void updateDevicePixelRatio();
+
     static QWindowPrivate *get(QWindow *window) { return window->d_func(); }
 
     static Qt::WindowState effectiveState(Qt::WindowStates);
@@ -106,6 +108,7 @@ public:
     QString windowFilePath;
     QIcon windowIcon;
     QRect geometry;
+    qreal devicePixelRatio = 1.0;
     Qt::WindowStates windowState = Qt::WindowNoState;
     QWindow::Visibility visibility = QWindow::Hidden;
     bool resizeEventPending = true;