Cocoa Helpers: Add qt_objc_cast() function

This generalizes what qnsview_cast() does to any Objective-C class.

Notice that this is a loose cast, using isKindOfClass: instead
of isMemberOfClass:.

Change-Id: I745f548e474f2f9eef89f370d7d45496f0c51434
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoahelpers.h

@@ -78,6 +78,16 @@ NSDragOperation qt_mac_mapDropActions(Qt::DropActions actions);
 Qt::DropAction qt_mac_mapNSDragOperation(NSDragOperation nsActions);
 Qt::DropActions qt_mac_mapNSDragOperations(NSDragOperation nsActions);
 
+template <typename T>
+typename std::enable_if<std::is_pointer<T>::value, T>::type
+qt_objc_cast(id object)
+{
+    if ([object isKindOfClass:[typename std::remove_pointer<T>::type class]])
+        return static_cast<T>(object);
+
+    return nil;
+}
+
 QT_MANGLE_NAMESPACE(QNSView) *qnsview_cast(NSView *view);
 
 // Misc

src/plugins/platforms/cocoa/qcocoahelpers.mm

@@ -160,10 +160,7 @@ Qt::DropActions qt_mac_mapNSDragOperations(NSDragOperation nsActions)
 */
 QNSView *qnsview_cast(NSView *view)
 {
-    if (![view isKindOfClass:[QNSView class]])
-        return nil;
-
-    return static_cast<QNSView *>(view);
+    return qt_objc_cast<QNSView *>(view);
 }
 
 //