Trial patch for laggy scrolling in Qt4.8 blackberry plugin

Note: this is an attempt at a 4.8 version of https://codereview.qt-project.org/#change,29001,patchset=1 intended for very temporary use only, until that patch is upstreamed and they do their own 4.8 version.
-------------------------------

diff --git a/src/plugins/platforms/blackberry/blackberry.pro b/src/plugins/platforms/blackberry/blackberry.pro
index 6fcef85..9d7924c 100644
--- a/src/plugins/platforms/blackberry/blackberry.pro
+++ b/src/plugins/platforms/blackberry/blackberry.pro
@@ -49,6 +49,8 @@ blackberry {

     HEADERS += qbbbpseventfilter.h \
                qbbvirtualkeyboardbps.h
+
+    DEFINES += QBB_SCREENEVENTTHREAD
 }

 QMAKE_CXXFLAGS += -I./private
diff --git a/src/plugins/platforms/blackberry/qbbbpseventfilter.cpp b/src/plugins/platforms/blackberry/qbbbpseventfilter.cpp
index a9e8523..42307f7 100644
--- a/src/plugins/platforms/blackberry/qbbbpseventfilter.cpp
+++ b/src/plugins/platforms/blackberry/qbbbpseventfilter.cpp
@@ -96,12 +96,20 @@ void QBBBpsEventFilter::installOnEventDispatcher(QAbstractEventDispatcher *dispa

 void QBBBpsEventFilter::registerForScreenEvents(QBBScreen *screen)
 {
+    if (!mScreenEventHandler) {
+        qWarning("QBB: trying to register for screen events, but no handler provided.");
+        return;
+    }
     if (screen_request_events(screen->nativeContext()) != BPS_SUCCESS)
         qWarning("QBB: failed to register for screen events on screen %p", screen->nativeContext());
 }

 void QBBBpsEventFilter::unregisterForScreenEvents(QBBScreen *screen)
 {
+    if (!mScreenEventHandler) {
+        qWarning("QBB: trying to unregister for screen events, but no handler provided.");
+        return;
+    }
     if (screen_stop_events(screen->nativeContext()) != BPS_SUCCESS)
         qWarning("QBB: failed to unregister for screen events on screen %p", screen->nativeContext());
 }
@@ -127,6 +135,10 @@ bool QBBBpsEventFilter::bpsEventFilter(bps_event_t *event)
 #endif

     if (eventDomain == screen_get_domain()) {
+        if (!mScreenEventHandler) {
+            qWarning("QBB: registered for screen events, but no handler provided.");
+            return false;
+        }
         screen_event_t screenEvent = screen_event_get_event(event);
         return mScreenEventHandler->handleEvent(screenEvent);
     }
diff --git a/src/plugins/platforms/blackberry/qbbintegration.cpp b/src/plugins/platforms/blackberry/qbbintegration.cpp
index bfd1eb8..b84015c 100644
--- a/src/plugins/platforms/blackberry/qbbintegration.cpp
+++ b/src/plugins/platforms/blackberry/qbbintegration.cpp
@@ -121,8 +121,7 @@ QBBIntegration::QBBIntegration() :
     createDisplays();

     // create/start event thread
-    // Not on BlackBerry, it has specialised event dispatcher which also handles screen events
-#if !defined(Q_OS_BLACKBERRY)
+#if defined(QBB_SCREENEVENTTHREAD)
     mScreenEventThread = new QBBScreenEventThread(mContext, mScreenEventHandler);
     mScreenEventThread->start();
 #endif
@@ -138,11 +137,14 @@ QBBIntegration::QBBIntegration() :

     QBBVirtualKeyboardBps *virtualKeyboardBps = new QBBVirtualKeyboardBps;

-    mBpsEventFilter = new QBBBpsEventFilter(mNavigatorEventHandler, mScreenEventHandler, virtualKeyboardBps);
+    mBpsEventFilter = new QBBBpsEventFilter(mNavigatorEventHandler,
+        (mScreenEventThread ? 0 : mScreenEventHandler), virtualKeyboardBps);
+#if !defined(QBB_SCREENEVENTTHREAD)
     Q_FOREACH (QPlatformScreen *platformScreen, mScreens) {
         QBBScreen *screen = static_cast<QBBScreen*>(platformScreen);
         mBpsEventFilter->registerForScreenEvents(screen);
     }
+#endif

     mBpsEventFilter->installOnEventDispatcher(QAbstractEventDispatcher::instance());

@@ -178,20 +180,20 @@ QBBIntegration::~QBBIntegration()
     delete mLocaleThread;
 #endif

-#if !defined(Q_OS_BLACKBERRY)
-    // stop/destroy event thread
-    delete mScreenEventThread;
-#endif
-
-    // stop/destroy navigator event handling classes
+   // stop/destroy navigator event handling classes
     delete mNavigatorEventNotifier;

-#if defined(Q_OS_BLACKBERRY)
+#if defined(QBB_SCREENEVENTTHREAD)
+    // stop/destroy event thread
+    delete mScreenEventThread;
+#elif defined(Q_OS_BLACKBERRY)
     Q_FOREACH (QPlatformScreen *platformScreen, mScreens) {
         QBBScreen *screen = static_cast<QBBScreen*>(platformScreen);
         mBpsEventFilter->unregisterForScreenEvents(screen);
     }
+#endif

+#if defined(Q_OS_BLACKBERRY)
     delete mBpsEventFilter;
 #endif