Introduce events for Window device pixel ratio changes

There is a mix between screen device pixel ratio. Currently we store the
property on a per-window basis, but the change notifications are still
on a per screen basis which can fall apart on edge cases.

On wayland we are getting per window DPR changes without as useful
screen change events so it's important to fix. It also has potential to
clean up the Windows backend in the future where the backend is
currently papering over the two concepts.

This patch introduces two new events:

A QWindowSystemInterface to trigger a window DPR change
independently of a screen change.

An event to notify windows the new DPR rather than needing to track
signals on the screen. This happens either when the window dpr changes
or implicitly through a screen change. This can deprecate an existing
event ScreenChangeInternal so the value is reused and renamed for
clarity.

Change-Id: I637a07fd4520ba3184ccc2c987c29d8d23a65ad3
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

6 files changed, 49 insertions, 1 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index f37095053e..ca2d215541 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -2052,6 +2052,9 @@ void Q_TRACE_INSTRUMENT(qtgui) QGuiApplicationPrivate::processWindowSystemEvent(
     case QWindowSystemInterfacePrivate::WindowScreenChanged:
         QGuiApplicationPrivate::processWindowScreenChangedEvent(static_cast<QWindowSystemInterfacePrivate::WindowScreenChangedEvent *>(e));
         break;
+    case QWindowSystemInterfacePrivate::WindowDevicePixelRatioChanged:
+        QGuiApplicationPrivate::processWindowDevicePixelRatioChangedEvent(static_cast<QWindowSystemInterfacePrivate::WindowDevicePixelRatioChangedEvent *>(e));
+        break;
     case QWindowSystemInterfacePrivate::SafeAreaMarginsChanged:
         QGuiApplicationPrivate::processSafeAreaMarginsChangedEvent(static_cast<QWindowSystemInterfacePrivate::SafeAreaMarginsChangedEvent *>(e));
         break;
@@ -2555,6 +2558,9 @@ void QGuiApplicationPrivate::processWindowScreenChangedEvent(QWindowSystemInterf
     if (QWindow *window  = wse->window.data()) {
         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 */);
@@ -2571,9 +2577,24 @@ 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);
+        }
     }
 }
 
+void QGuiApplicationPrivate::processWindowDevicePixelRatioChangedEvent(QWindowSystemInterfacePrivate::WindowDevicePixelRatioChangedEvent *wde)
+{
+    if (wde->window.isNull())
+        return;
+
+    QEvent dprChangeEvent(QEvent::DevicePixelRatioChange);
+    QGuiApplication::sendSpontaneousEvent(wde->window, &dprChangeEvent);
+}
+
 void QGuiApplicationPrivate::processSafeAreaMarginsChangedEvent(QWindowSystemInterfacePrivate::SafeAreaMarginsChangedEvent *wse)
 {
     if (wse->window.isNull())
diff --git a/src/gui/kernel/qguiapplication_p.h b/src/gui/kernel/qguiapplication_p.h
index 47228a0738..433cf1a088 100644
--- a/src/gui/kernel/qguiapplication_p.h
+++ b/src/gui/kernel/qguiapplication_p.h
@@ -116,6 +116,7 @@ public:
     static void processActivatedEvent(QWindowSystemInterfacePrivate::ActivatedWindowEvent *e);
     static void processWindowStateChangedEvent(QWindowSystemInterfacePrivate::WindowStateChangedEvent *e);
     static void processWindowScreenChangedEvent(QWindowSystemInterfacePrivate::WindowScreenChangedEvent *e);
+    static void processWindowDevicePixelRatioChangedEvent(QWindowSystemInterfacePrivate::WindowDevicePixelRatioChangedEvent *e);
 
     static void processSafeAreaMarginsChangedEvent(QWindowSystemInterfacePrivate::SafeAreaMarginsChangedEvent *e);
 
diff --git a/src/gui/kernel/qwindow.cpp b/src/gui/kernel/qwindow.cpp
index 1e368914f3..5a0fc79f30 100644
--- a/src/gui/kernel/qwindow.cpp
+++ b/src/gui/kernel/qwindow.cpp
@@ -505,6 +505,8 @@ 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();
 
@@ -550,6 +552,11 @@ 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);
+    }
+
     if (needsUpdate)
         q->requestUpdate();
 }
diff --git a/src/gui/kernel/qwindowsysteminterface.cpp b/src/gui/kernel/qwindowsysteminterface.cpp
index ab07002191..00343fef66 100644
--- a/src/gui/kernel/qwindowsysteminterface.cpp
+++ b/src/gui/kernel/qwindowsysteminterface.cpp
@@ -259,6 +259,12 @@ QT_DEFINE_QPA_EVENT_HANDLER(void, handleWindowScreenChanged, QWindow *window, QS
     handleWindowSystemEvent<QWindowSystemInterfacePrivate::WindowScreenChangedEvent, Delivery>(window, screen);
 }
 
+QT_DEFINE_QPA_EVENT_HANDLER(void, handleWindowDevicePixelRatioChanged, QWindow *window)
+{
+    handleWindowSystemEvent<QWindowSystemInterfacePrivate::WindowDevicePixelRatioChangedEvent, Delivery>(window);
+}
+
+
 QT_DEFINE_QPA_EVENT_HANDLER(void, handleSafeAreaMarginsChanged, QWindow *window)
 {
     handleWindowSystemEvent<QWindowSystemInterfacePrivate::SafeAreaMarginsChangedEvent, Delivery>(window);
diff --git a/src/gui/kernel/qwindowsysteminterface.h b/src/gui/kernel/qwindowsysteminterface.h
index 004019ae2d..37b06b0b04 100644
--- a/src/gui/kernel/qwindowsysteminterface.h
+++ b/src/gui/kernel/qwindowsysteminterface.h
@@ -191,6 +191,9 @@ public:
     static void handleWindowScreenChanged(QWindow *window, QScreen *newScreen);
 
     template<typename Delivery = QWindowSystemInterface::DefaultDelivery>
+    static void handleWindowDevicePixelRatioChanged(QWindow *window);
+
+    template<typename Delivery = QWindowSystemInterface::DefaultDelivery>
     static void handleSafeAreaMarginsChanged(QWindow *window);
 
     template<typename Delivery = QWindowSystemInterface::DefaultDelivery>
diff --git a/src/gui/kernel/qwindowsysteminterface_p.h b/src/gui/kernel/qwindowsysteminterface_p.h
index 27aa7dbc57..3569667325 100644
--- a/src/gui/kernel/qwindowsysteminterface_p.h
+++ b/src/gui/kernel/qwindowsysteminterface_p.h
@@ -68,7 +68,8 @@ public:
         WindowScreenChanged = 0x21,
         SafeAreaMarginsChanged = 0x22,
         ApplicationTermination = 0x23,
-        Paint = 0x24
+        Paint = 0x24,
+        WindowDevicePixelRatioChanged = 0x25,
     };
 
     class WindowSystemEvent {
@@ -154,6 +155,15 @@ public:
         QPointer<QScreen> screen;
     };
 
+    class WindowDevicePixelRatioChangedEvent : public WindowSystemEvent {
+    public:
+        WindowDevicePixelRatioChangedEvent(QWindow *w)
+            : WindowSystemEvent(WindowDevicePixelRatioChanged), window(w)
+        { }
+
+        QPointer<QWindow> window;
+    };
+
     class SafeAreaMarginsChangedEvent : public WindowSystemEvent {
     public:
         SafeAreaMarginsChangedEvent(QWindow *w)