QBackingStore: Make QPlatformBackingStore creation lazy

Some platform backing stores may require that the window has been
created, so let's defer the platform backingstore creation until
absolutely necessary.

Change-Id: Ib93151c6473e3bbe77d994782d84289c2f63bcf2
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 13 insertions, 10 deletions

diff --git a/src/gui/painting/qbackingstore.cpp b/src/gui/painting/qbackingstore.cpp
index 0dfb52e7c3..d935deb4d6 100644
--- a/src/gui/painting/qbackingstore.cpp
+++ b/src/gui/painting/qbackingstore.cpp
@@ -62,7 +62,7 @@ public:
     }
 
     QWindow *window;
-    QPlatformBackingStore *platformBackingStore;
+    QPlatformBackingStore *platformBackingStore = nullptr;
     QScopedPointer<QImage> highDpiBackingstore;
     QRegion staticContents;
     QSize size;
@@ -95,8 +95,6 @@ public:
 QBackingStore::QBackingStore(QWindow *window)
     : d_ptr(new QBackingStorePrivate(window))
 {
-    d_ptr->platformBackingStore = QGuiApplicationPrivate::platformIntegration()->createPlatformBackingStore(window);
-    d_ptr->platformBackingStore->setBackingStore(this);
 }
 
 /*!
@@ -131,7 +129,8 @@ void QBackingStore::beginPaint(const QRegion &region)
         d_ptr->highDpiBackingstore->devicePixelRatio() != d_ptr->window->devicePixelRatio())
         resize(size());
 
-    d_ptr->platformBackingStore->beginPaint(QHighDpi::toNativeLocalRegion(region, d_ptr->window));
+    QPlatformBackingStore *platformBackingStore = handle();
+    platformBackingStore->beginPaint(QHighDpi::toNativeLocalRegion(region, d_ptr->window));
 
     // When QtGui is applying a high-dpi scale factor the backing store
     // creates a "large" backing store image. This image needs to be
@@ -139,7 +138,7 @@ void QBackingStore::beginPaint(const QRegion &region)
     // devicePixelRatio. Do this on a separate image instance that shares
     // the image data to avoid having the new devicePixelRatio be propagated
     // back to the platform plugin.
-    QPaintDevice *device = d_ptr->platformBackingStore->paintDevice();
+    QPaintDevice *device = platformBackingStore->paintDevice();
     if (QHighDpiScaling::isActive() && device->devType() == QInternal::Image) {
         QImage *source = static_cast<QImage *>(device);
         const bool needsNewImage = d_ptr->highDpiBackingstore.isNull()
@@ -168,7 +167,7 @@ void QBackingStore::beginPaint(const QRegion &region)
 */
 QPaintDevice *QBackingStore::paintDevice()
 {
-    QPaintDevice *device = d_ptr->platformBackingStore->paintDevice();
+    QPaintDevice *device = handle()->paintDevice();
 
     if (QHighDpiScaling::isActive() && device->devType() == QInternal::Image)
         return d_ptr->highDpiBackingstore.data();
@@ -189,7 +188,7 @@ void QBackingStore::endPaint()
     if (paintDevice()->paintingActive())
         qWarning() << "QBackingStore::endPaint() called with active painter on backingstore paint device";
 
-    d_ptr->platformBackingStore->endPaint();
+    handle()->endPaint();
 }
 
 static bool isRasterSurface(QWindow *window)
@@ -247,7 +246,7 @@ void QBackingStore::flush(const QRegion &region, QWindow *window, const QPoint &
 
     Q_ASSERT(window == topLevelWindow || topLevelWindow->isAncestorOf(window, QWindow::ExcludeTransients));
 
-    d_ptr->platformBackingStore->flush(window, QHighDpi::toNativeLocalRegion(region, window),
+    handle()->flush(window, QHighDpi::toNativeLocalRegion(region, window),
                                             QHighDpi::toNativeLocalPosition(offset, window));
 }
 
@@ -259,7 +258,7 @@ void QBackingStore::flush(const QRegion &region, QWindow *window, const QPoint &
 void QBackingStore::resize(const QSize &size)
 {
     d_ptr->size = size;
-    d_ptr->platformBackingStore->resize(QHighDpi::toNativePixels(size, d_ptr->window), d_ptr->staticContents);
+    handle()->resize(QHighDpi::toNativePixels(size, d_ptr->window), d_ptr->staticContents);
 }
 
 /*!
@@ -286,7 +285,7 @@ bool QBackingStore::scroll(const QRegion &area, int dx, int dy)
     if (qFloor(nativeDx) != nativeDx || qFloor(nativeDy) != nativeDy)
         return false;
 
-    return d_ptr->platformBackingStore->scroll(QHighDpi::toNativeLocalRegion(area, d_ptr->window),
+    return handle()->scroll(QHighDpi::toNativeLocalRegion(area, d_ptr->window),
                                                nativeDx, nativeDy);
 }
 
@@ -367,6 +366,10 @@ void Q_GUI_EXPORT qt_scrollRectInImage(QImage &img, const QRect &rect, const QPo
 */
 QPlatformBackingStore *QBackingStore::handle() const
 {
+    if (!d_ptr->platformBackingStore) {
+        d_ptr->platformBackingStore = QGuiApplicationPrivate::platformIntegration()->createPlatformBackingStore(d_ptr->window);
+        d_ptr->platformBackingStore->setBackingStore(const_cast<QBackingStore*>(this));
+    }
     return d_ptr->platformBackingStore;
 }