QPA: Fix semantics of GUI event dispatcher ownership in platform plugins

The QPlatformIntegration::guiThreadEventDispatcher() function acted as an
accessor to event dispatchers created in the constructor of each platform
plugin, but the logic and semantics of event-dispatcher handling in Qt
itself (QCoreApplication/QGuiApplication) still assumed both ownership
and control over the event dispatcher, such as when to create one, which
one to create, and when to delete it. This conflicted with the explicit
calls in the platform plugins to QGuiApplication::setEventDispatcher(),
as well as left a possibility that the event-dispatcher created by
the platform plugin would never be deleted, as none of the platform
plugins actually took full ownership of the dispatcher and deleted it
in its destructor.

The integration function has now been renamed back to its old name,
createEventDispatcher(), and acts as a factory function, leaving
the logic and lifetime of event dispatcher to QtCoreApplication.

The only platform left with creating the event-dispatcher in the
constructor is QNX, where other parts of the platform relies on
having an event-dispatcher before their initialization. We then
need to manually take care of the ownership transfer, so that the
event-dispatcher is still destroyed at some point.

Change-Id: I113db97d2545ebda39ebdefa865e488d2ce9368b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

4 files changed, 27 insertions, 26 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index b242a3cc68..8e5c290cc6 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -1029,27 +1029,23 @@ void QGuiApplicationPrivate::createPlatformIntegration()
 
 }
 
+/*!
+    Called from QCoreApplication::init()
+
+    Responsible for creating an event dispatcher when QCoreApplication
+    decides that it needs one (because a custom one has not been set).
+*/
 void QGuiApplicationPrivate::createEventDispatcher()
 {
+    Q_ASSERT(!eventDispatcher);
+
     if (platform_integration == 0)
         createPlatformIntegration();
 
-    if (!eventDispatcher) {
-        QAbstractEventDispatcher *eventDispatcher = platform_integration->guiThreadEventDispatcher();
-        setEventDispatcher(eventDispatcher);
-    }
-}
-
-void QGuiApplicationPrivate::setEventDispatcher(QAbstractEventDispatcher *eventDispatcher)
-{
-    Q_Q(QGuiApplication);
-
-    if (!QCoreApplicationPrivate::eventDispatcher) {
-        QCoreApplicationPrivate::eventDispatcher = eventDispatcher;
-        QCoreApplicationPrivate::eventDispatcher->setParent(q);
-        threadData->eventDispatcher = eventDispatcher;
-    }
+    // The platform integration should not mess with the event dispatcher
+    Q_ASSERT(!eventDispatcher);
 
+    eventDispatcher = platform_integration->createEventDispatcher();
 }
 
 #if defined(QT_DEBUG) && defined(Q_OS_LINUX)
diff --git a/src/gui/kernel/qguiapplication_p.h b/src/gui/kernel/qguiapplication_p.h
index 7a4a161476..91c63e54c5 100644
--- a/src/gui/kernel/qguiapplication_p.h
+++ b/src/gui/kernel/qguiapplication_p.h
@@ -83,8 +83,7 @@ public:
     ~QGuiApplicationPrivate();
 
     void createPlatformIntegration();
-    void createEventDispatcher();
-    void setEventDispatcher(QAbstractEventDispatcher *eventDispatcher);
+    void createEventDispatcher() Q_DECL_OVERRIDE;
 
     virtual void notifyLayoutDirectionChange();
     virtual void notifyActiveWindowChange(QWindow *previous);
diff --git a/src/gui/kernel/qplatformintegration.cpp b/src/gui/kernel/qplatformintegration.cpp
index dc775bcb61..3f93856349 100644
--- a/src/gui/kernel/qplatformintegration.cpp
+++ b/src/gui/kernel/qplatformintegration.cpp
@@ -231,17 +231,23 @@ QPlatformServices *QPlatformIntegration::services() const
     are never repositioned by the window manager. The default implementation returns true.
  */
 
-
 /*!
 
-    \fn QAbstractEventDispatcher *QPlatformIntegration::guiThreadEventDispatcher() const = 0
+    \fn QAbstractEventDispatcher *QPlatformIntegration::createEventDispatcher() const = 0
+
+    Factory function for the GUI event dispatcher. The platform plugin should create
+    and return a QAbstractEventDispatcher subclass when this function is called.
+
+    If the platform plugin for some reason creates the event dispatcher outside of
+    this function (for example in the constructor), it needs to handle the case
+    where this function is never called, ensuring that the event dispatcher is
+    still deleted at some point (typically in the destructor).
+
+    Note that the platform plugin should never explicitly set the event dispatcher
+    itself, using QCoreApplication::setEventDispatcher(), but let QCoreApplication
+    decide when and which event dispatcher to create.
 
-    Accessor function for the event dispatcher. The platform plugin should create
-    an instance of the QAbstractEventDispatcher in its constructor and set it
-    on the application using QGuiApplicationPrivate::instance()->setEventDispatcher().
-    The event dispatcher is owned by QGuiApplication, the accessor should return
-    a flat pointer.
-    \sa QGuiApplicationPrivate
+    \since 5.2
 */
 
 bool QPlatformIntegration::hasCapability(Capability cap) const
diff --git a/src/gui/kernel/qplatformintegration.h b/src/gui/kernel/qplatformintegration.h
index 0204181ca6..0af74370b5 100644
--- a/src/gui/kernel/qplatformintegration.h
+++ b/src/gui/kernel/qplatformintegration.h
@@ -111,7 +111,7 @@ public:
     virtual QPaintEngine *createImagePaintEngine(QPaintDevice *paintDevice) const;
 
 // Event dispatcher:
-    virtual QAbstractEventDispatcher *guiThreadEventDispatcher() const = 0;
+    virtual QAbstractEventDispatcher *createEventDispatcher() const = 0;
 
 //Deeper window system integrations
     virtual QPlatformFontDatabase *fontDatabase() const;