Minimize the number of objects created per item in VisualDataModel.

Derive from QDeclarativeContext and reference count the context object
instead of parenting it to the context.  Combined with a weak
persistent v8 handle the allows the context object to be returned by
javascript accessors instead of creating a new instance for every use.

In addition to the efficiency advantages of creating fewer objects,
routing all data access through a single object means that object can
also persist data instead of just acting as a proxy.

Change-Id: I107dc8c901f16f2a4b420ff1cbffa7a6be27de89
Reviewed-by: Martin Jones <martin.jones@nokia.com>

examples/declarative/modelviews/visualdatamodel/dragselection.qml

@@ -83,7 +83,7 @@ Item {
                 }
                 DropArea {
                     anchors.fill: parent
-                    onEntered: visualModel.items.move(selectedItems, 0, packageRoot.VisualDataModel.itemsIndex, selectedItems.count)
+                    onEntered: selectedItems.move(0, visualModel.items.get(packageRoot.VisualDataModel.itemsIndex), selectedItems.count)
                 }
             }
             Item {

src/quick/items/items.pri

@@ -39,6 +39,7 @@ HEADERS += \
     $$PWD/qquicklistview_p.h \
     $$PWD/qquickvisualadaptormodel_p.h \
     $$PWD/qquickvisualdatamodel_p.h \
+    $$PWD/qquickvisualdatamodel_p_p.h \
     $$PWD/qquickvisualitemmodel_p.h \
     $$PWD/qquickrepeater_p.h \
     $$PWD/qquickrepeater_p_p.h \

src/quick/items/qquickvisualadaptormodel_p.h

@@ -45,14 +45,15 @@
 #include <QtCore/qobject.h>
 #include <QtCore/qabstractitemmodel.h>
 
-#include <private/qdeclarativerefcount_p.h>
-
 QT_BEGIN_HEADER
 
 QT_BEGIN_NAMESPACE
 
 class QDeclarativeEngine;
 
+class QQuickVisualDataModelItem;
+class QQuickVisualDataModelItemMetaType;
+
 class QQuickVisualAdaptorModelPrivate;
 class QQuickVisualAdaptorModel : public QObject
 {
@@ -80,10 +81,9 @@ class QQuickVisualAdaptorModel : public QObject
     QVariant parentModelIndex() const;
 
     int count() const;
-    QObject *data(int index);
+    QQuickVisualDataModelItem *createItem(QQuickVisualDataModelItemMetaType *metaType, int index);
     QString stringValue(int index, const QString &role);
     void replaceWatchedRoles(const QList<QByteArray> &oldRoles, const QList<QByteArray> &newRoles);
-    int indexOf(QObject *object) const;
 
     bool canFetchMore() const;
     void fetchMore();