Rerender natively rendered text items when DPI scaling changes.

The whole purpose of native rendering is to optimize precisely for a
given pixel grid.

DPI scaling can change when either settings of the current screen are
changed, or when the window is moved to a different screen.

Details:
- add an ItemDevicePixelRatioHasChanged item change event
- detect DPI scaling changes by watching screen (identity) and screen
  config changes
- when DPI scaling changes, recursively send an
  ItemDevicePixelRatioHasChanged signal to all items with content
- when a natively renderet TextItem catches such an event, call
  updateLayout() which automatically picks up the new logical DPI

Task-number: QTBUG-49019
Change-Id: I9f4f8d1a7f2c172ed26c276294ab143161c4a48b
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>

1 files changed, 38 insertions, 1 deletions

diff --git a/src/quick/items/qquickwindow.cpp b/src/quick/items/qquickwindow.cpp
index b93aa06336..24bac168ea 100644
--- a/src/quick/items/qquickwindow.cpp
+++ b/src/quick/items/qquickwindow.cpp
@@ -285,10 +285,43 @@ void QQuickWindow::update()
         QQuickRenderControlPrivate::get(d->renderControl)->update();
 }
 
+static void updatePixelRatioHelper(QQuickItem *item, float pixelRatio)
+{
+    if (item->flags() & QQuickItem::ItemHasContents) {
+        QQuickItemPrivate *itemPrivate = QQuickItemPrivate::get(item);
+        itemPrivate->itemChange(QQuickItem::ItemDevicePixelRatioHasChanged, pixelRatio);
+    }
+
+    QList <QQuickItem *> items = item->childItems();
+    for (int i = 0; i < items.size(); ++i)
+        updatePixelRatioHelper(items.at(i), pixelRatio);
+}
+
+void QQuickWindow::physicalDpiChanged()
+{
+    Q_D(QQuickWindow);
+    const qreal newPixelRatio = screen()->devicePixelRatio();
+    if (qFuzzyCompare(newPixelRatio, d->devicePixelRatio))
+        return;
+    d->devicePixelRatio = newPixelRatio;
+    if (d->contentItem)
+        updatePixelRatioHelper(d->contentItem, newPixelRatio);
+}
+
 void QQuickWindow::handleScreenChanged(QScreen *screen)
 {
     Q_D(QQuickWindow);
-    Q_UNUSED(screen)
+    if (screen) {
+        physicalDpiChanged();
+        // When physical DPI changes on the same screen, either the resolution or the device pixel
+        // ratio changed. We must check what it is. Device pixel ratio does not have its own
+        // ...Changed() signal.
+        d->physicalDpiChangedConnection = connect(screen, SIGNAL(physicalDotsPerInchChanged(qreal)),
+                                                  this, SLOT(physicalDpiChanged()));
+    } else {
+        disconnect(d->physicalDpiChangedConnection);
+    }
+
     d->forcePolish();
 }
 
@@ -407,6 +440,7 @@ QQuickWindowPrivate::QQuickWindowPrivate()
     , touchMouseId(-1)
     , touchMousePressTimestamp(0)
     , dirtyItemList(0)
+    , devicePixelRatio(0)
     , context(0)
     , renderer(0)
     , windowManager(0)
@@ -459,6 +493,9 @@ void QQuickWindowPrivate::init(QQuickWindow *c, QQuickRenderControl *control)
 
     Q_ASSERT(windowManager || renderControl);
 
+    if (QScreen *screen = q->screen())
+       devicePixelRatio = screen->devicePixelRatio();
+
     QSGContext *sg;
     if (renderControl) {
         QQuickRenderControlPrivate *renderControlPriv = QQuickRenderControlPrivate::get(renderControl);