Add private QObjCWeakPointer for tracking Objective-C object lifetime

Uses __weak if possible, and otherwise falls back to associated objects.

Replaces manual tracking of QNSView lifetime for QCocoaDrag.

Task-number: QTBUG-116554
Change-Id: I56f2707bbf5aa14a9efd0ec29e37b157e97cfc3e
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit fa9449bbe8fb20cbdfc5b1e81181a30e4a7c358a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qcore_mac.mm

@@ -774,5 +774,19 @@ QMacVersion::VersionTuple QMacVersion::libraryVersion()
 
 // -------------------------------------------------------------------------
 
+#if !(__has_feature(objc_arc_weak) && __has_feature(objc_arc_fields))
+QT_END_NAMESPACE
+@implementation QT_MANGLE_NAMESPACE(WeakPointerLifetimeTracker)
+- (void)dealloc
+{
+    *self.pointer = {};
+    [super dealloc];
+}
+@end
+QT_BEGIN_NAMESPACE
+#endif
+
+// -------------------------------------------------------------------------
+
 QT_END_NAMESPACE
 

src/corelib/kernel/qcore_mac_p.h

@@ -462,6 +462,89 @@ qt_objc_cast(id object)
 
 // -------------------------------------------------------------------------
 
+#if defined( __OBJC__)
+
+template <typename T = NSObject>
+class QObjCWeakPointer;
+
+#if __has_feature(objc_arc_weak) && __has_feature(objc_arc_fields)
+#  define USE_OBJC_WEAK 1
+#endif
+
+#if !USE_OBJC_WEAK
+QT_END_NAMESPACE
+#include <objc/runtime.h>
+Q_CORE_EXPORT
+QT_DECLARE_NAMESPACED_OBJC_INTERFACE(WeakPointerLifetimeTracker, NSObject
+@property (atomic, assign) QT_PREPEND_NAMESPACE(QObjCWeakPointer)<NSObject> *pointer;
+)
+QT_BEGIN_NAMESPACE
+#endif
+
+template <typename T>
+class QObjCWeakPointer
+{
+public:
+    QObjCWeakPointer(T *object = nil) : m_object(object)
+    {
+#if !USE_OBJC_WEAK
+        trackObjectLifetime();
+#endif
+    }
+
+    QObjCWeakPointer(const QObjCWeakPointer &other)
+    {
+        QMacAutoReleasePool pool;
+        m_object = other.m_object;
+#if !USE_OBJC_WEAK
+        trackObjectLifetime();
+#endif
+    }
+
+    QObjCWeakPointer &operator=(const QObjCWeakPointer &other)
+    {
+        QMacAutoReleasePool pool;
+        m_object = other.m_object;
+#if !USE_OBJC_WEAK
+        trackObjectLifetime();
+#endif
+        return *this;
+    }
+
+    ~QObjCWeakPointer()
+    {
+#if !USE_OBJC_WEAK
+        if (m_object)
+            objc_setAssociatedObject(m_object, this, nil, OBJC_ASSOCIATION_RETAIN);
+#endif
+    }
+
+    operator T*() const { return static_cast<T*>([[m_object retain] autorelease]); }
+
+private:
+#if USE_OBJC_WEAK
+    __weak
+#else
+    void trackObjectLifetime()
+    {
+        if (!m_object)
+            return;
+
+        auto *lifetimeTracker = [WeakPointerLifetimeTracker new];
+        lifetimeTracker.pointer = reinterpret_cast<QObjCWeakPointer<NSObject>*>(this);
+        objc_setAssociatedObject(m_object, this, lifetimeTracker, OBJC_ASSOCIATION_RETAIN);
+        [lifetimeTracker release];
+    }
+#endif
+    NSObject *m_object = nil;
+};
+
+#undef USE_OBJC_WEAK
+
+#endif //  __OBJC__
+
+// -------------------------------------------------------------------------
+
 QT_END_NAMESPACE
 
 #endif // QCORE_MAC_P_H

src/plugins/platforms/cocoa/qcocoadrag.h

@@ -8,6 +8,8 @@
 #include <qpa/qplatformdrag.h>
 #include <private/qsimpledrag_p.h>
 
+#include <QtCore/private/qcore_mac_p.h>
+
 #include <QtGui/private/qdnd_p.h>
 #include <QtGui/private/qinternalmimedata_p.h>
 
@@ -36,14 +38,13 @@ public:
     * event and view when handling an event in QNSView
     */
     void setLastInputEvent(NSEvent *event, NSView *view);
-    void viewDestroyed(NSView *view);
 
     void setAcceptedAction(Qt::DropAction act);
     void exitDragLoop();
 private:
     QDrag *m_drag;
     NSEvent *m_lastEvent;
-    NSView *m_lastView;
+    QObjCWeakPointer<NSView> m_lastView;
     Qt::DropAction m_executed_drop_action;
     QEventLoop *m_internalDragLoop = nullptr;
 

src/plugins/platforms/cocoa/qcocoadrag.mm

@@ -38,17 +38,6 @@ void QCocoaDrag::setLastInputEvent(NSEvent *event, NSView *view)
     m_lastView = view;
 }
 
-void QCocoaDrag::viewDestroyed(NSView *view)
-{
-    if (view == m_lastView) {
-        if (m_lastEvent.window.contentView == view) {
-            [m_lastEvent release];
-            m_lastEvent = nil;
-        }
-        m_lastView = nil;
-    }
-}
-
 QMimeData *QCocoaDrag::dragMimeData()
 {
     if (m_drag)
@@ -107,8 +96,11 @@ Qt::DropAction QCocoaDrag::defaultAction(Qt::DropActions possibleActions,
 Qt::DropAction QCocoaDrag::drag(QDrag *o)
 {
     m_executed_drop_action = Qt::IgnoreAction;
-    if (!m_lastEvent)
+    if (!m_lastView) {
+        [m_lastEvent release];
+        m_lastEvent = nil;
         return m_executed_drop_action;
+    }
 
     m_drag = o;
     QMacPasteboard dragBoard(CFStringRef(NSPasteboardNameDrag), QUtiMimeConverter::HandlerScopeFlag::DnD);
@@ -151,7 +143,7 @@ bool QCocoaDrag::maybeDragMultipleItems()
 
     const QMacAutoReleasePool pool;
 
-    NSView *view = m_lastView ? m_lastView : m_lastEvent.window.contentView;
+    NSView *view = m_lastView ? static_cast<NSView*>(m_lastView) : m_lastEvent.window.contentView;
     if (![view respondsToSelector:@selector(draggingSession:sourceOperationMaskForDraggingContext:)])
         return false;
 

src/plugins/platforms/cocoa/qnsview.mm

@@ -173,10 +173,6 @@ QT_NAMESPACE_ALIAS_OBJC_CLASS(QNSViewMenuHelper);
     [[NSNotificationCenter defaultCenter] removeObserver:self];
     [m_mouseMoveHelper release];
 
-    // FIXME: Replace with __weak or someting equivalent
-    QCocoaDrag* nativeDrag = QCocoaIntegration::instance()->drag();
-    nativeDrag->viewDestroyed(self);
-
     [super dealloc];
 }