Implement virtual void QPlatformWindow::setBackingStore()

QWidget re-uses an existing backing store. Platform windows depend on
being associated to a backing store, in case they become toplevel
windows. If a platform window gets deleted and re-created each time it
is shown or hidden, it has to be manually associated to the re-used
backing store.

This patch partly reverts fbf0aeea7d3b38ced7a16fcd5c3e2e9b45536292.
It removes Android specific code from QWidgetPrivate::create(), which
has been added to suppress re-using backing stores in the absence of
the new API.

Fixes: QTBUG-97482
Change-Id: Iaa1b7652efa120ec1955914c0383e8ccd8a41429
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>
(cherry picked from commit a4ca9e80658bca7dad1529f03c1b59173a6ecf62)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/kernel/qplatformwindow.h

@@ -32,6 +32,7 @@ class QScreen;
 class QWindow;
 class QIcon;
 class QRegion;
+class QPlatformBackingStore;
 
 class Q_GUI_EXPORT QPlatformWindow : public QPlatformSurface
 {
@@ -117,6 +118,7 @@ public:
     virtual void requestUpdate();
     bool hasPendingUpdateRequest() const;
     virtual void deliverUpdateRequest();
+    virtual void setBackingStore(QPlatformBackingStore *) {}
 
     // Window property accessors. Platform plugins should use these
     // instead of accessing QWindow directly.

src/platformsupport/fbconvenience/qfbscreen.cpp

@@ -47,6 +47,7 @@ bool QFbScreen::event(QEvent *event)
 
 void QFbScreen::addWindow(QFbWindow *window)
 {
+    Q_ASSERT(window->backingStore());
     mWindowStack.prepend(window);
     if (!mPendingBackingStores.isEmpty()) {
         //check if we have a backing store for this window

src/platformsupport/fbconvenience/qfbwindow.cpp

@@ -3,9 +3,11 @@
 
 #include "qfbwindow_p.h"
 #include "qfbscreen_p.h"
+#include "qfbbackingstore_p.h"
 
 #include <QtGui/QScreen>
 #include <qpa/qwindowsysteminterface.h>
+#include <qpa/qplatformbackingstore.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -111,5 +113,11 @@ void QFbWindow::repaint(const QRegion &region)
     for (auto rect : region)
         platformScreen()->setDirty(rect.translated(topLeft));
 }
-
+void QFbWindow::setBackingStore(QPlatformBackingStore *store)
+{
+    Q_ASSERT(store);
+    Q_ASSERT_X(dynamic_cast<QFbBackingStore *>(store), __FUNCTION__,
+               "Argument is not a QFbBackingStore.");
+    mBackingStore = static_cast<QFbBackingStore *>(store);
+}
 QT_END_NAMESPACE

src/platformsupport/fbconvenience/qfbwindow_p.h

@@ -41,7 +41,7 @@ public:
 
     WId winId() const override { return mWindowId; }
 
-    void setBackingStore(QFbBackingStore *store) { mBackingStore = store; }
+    void setBackingStore(QPlatformBackingStore *store) override;
     QFbBackingStore *backingStore() const { return mBackingStore; }
 
     QFbScreen *platformScreen() const;

src/plugins/platforms/android/qandroidplatformwindow.cpp

@@ -5,9 +5,9 @@
 #include "qandroidplatformwindow.h"
 #include "qandroidplatformopenglcontext.h"
 #include "qandroidplatformscreen.h"
-
+#include "qandroidplatformbackingstore.h"
 #include "androidjnimain.h"
-
+#include "qpa/qplatformbackingstore.h"
 #include <qguiapplication.h>
 #include <qpa/qwindowsysteminterface.h>
 #include <private/qhighdpiscaling_p.h>
@@ -169,4 +169,12 @@ void QAndroidPlatformWindow::applicationStateChanged(Qt::ApplicationState)
     QWindowSystemInterface::flushWindowSystemEvents();
 }
 
+void QAndroidPlatformWindow::setBackingStore(QPlatformBackingStore *store)
+{
+    Q_ASSERT(store);
+    Q_ASSERT_X(dynamic_cast<QAndroidPlatformBackingStore *>(store), __FUNCTION__,
+               "Argument is not a QAndroidPlatformBackingStore.");
+    m_backingStore = static_cast<QAndroidPlatformBackingStore *>(store);
+}
+
 QT_END_NAMESPACE

src/plugins/platforms/android/qandroidplatformwindow.h

@@ -50,7 +50,7 @@ public:
 
     virtual void applicationStateChanged(Qt::ApplicationState);
 
-    void setBackingStore(QAndroidPlatformBackingStore *store) { m_backingStore = store; }
+    void setBackingStore(QPlatformBackingStore *store) override;
     QAndroidPlatformBackingStore *backingStore() const { return m_backingStore; }
 
     virtual void repaint(const QRegion &) { }

src/widgets/kernel/qwidget.cpp

@@ -1332,6 +1332,11 @@ void QWidgetPrivate::create()
     QBackingStore *store = q->backingStore();
     usesRhiFlush = false;
 
+    // Re-use backing store, in case a new platform window was created and doesn't know about it.
+    // (e.g. QAndroidPlatformWindow)
+    if (store && q->windowHandle())
+        q->windowHandle()->handle()->setBackingStore(store->handle());
+
     if (!store) {
         if (q->windowType() != Qt::Desktop) {
             if (q->isWindow()) {