Allow proxy models access to the parent-child hierarchy of source

As discussed in the related bug report, the current way proxy models
access the parent-child hierarchy of the source model requires modifying
Qt sources. This changes adds a method to allow easy implementation
of this common feature to proxy model subclasses.

Fixes: QTBUG-83911
Change-Id: Ic88d40c13c2be7b6b44fcc58118bac471a11da95
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/itemmodels/qabstractitemmodel.h

@@ -272,8 +272,7 @@ class Q_CORE_EXPORT QAbstractItemModel : public QObject
 
     friend class QPersistentModelIndexData;
     friend class QAbstractItemViewPrivate;
-    friend class QIdentityProxyModel;
-    friend class QTransposeProxyModelPrivate;
+    friend class QAbstractProxyModel;
 public:
 
     explicit QAbstractItemModel(QObject *parent = nullptr);

src/corelib/itemmodels/qabstractproxymodel.cpp

@@ -474,6 +474,25 @@ QHash<int,QByteArray> QAbstractProxyModel::roleNames() const
   return d->model->roleNames();
 }
 
+/*!
+    Equivalent to calling createIndex on the source model.
+
+    This method is useful if your proxy model wants to maintain the
+    parent-child relationship of items in the source model.
+    When reimplementing mapToSource(), you can call this method to
+    create an index of the source model.
+    A typical use would be to save the internal pointer coming from the source model
+    in the proxy index when reimplementing mapFromSource() and use the same internal
+    pointer as \a internalPtr to recover the original source index when
+    reimplementing mapToSource().
+    \since 6.2
+ */
+QModelIndex QAbstractProxyModel::createSourceIndex(int row, int col, void *internalPtr) const
+{
+    if (sourceModel())
+        return sourceModel()->createIndex(row, col, internalPtr);
+    return QModelIndex();
+}
 
 QT_END_NAMESPACE
 

src/corelib/itemmodels/qabstractproxymodel.h

@@ -102,6 +102,7 @@ Q_SIGNALS:
     void sourceModelChanged(QPrivateSignal);
 
 protected:
+    QModelIndex createSourceIndex(int row, int col, void *internalPtr) const;
     QAbstractProxyModel(QAbstractProxyModelPrivate &, QObject *parent);
 
 private:

src/corelib/itemmodels/qidentityproxymodel.cpp

@@ -257,7 +257,7 @@ QModelIndex QIdentityProxyModel::mapToSource(const QModelIndex& proxyIndex) cons
     if (!d->model || !proxyIndex.isValid())
         return QModelIndex();
     Q_ASSERT(proxyIndex.model() == this);
-    return d->model->createIndex(proxyIndex.row(), proxyIndex.column(), proxyIndex.internalPointer());
+    return createSourceIndex(proxyIndex.row(), proxyIndex.column(), proxyIndex.internalPointer());
 }
 
 /*!

src/corelib/itemmodels/qtransposeproxymodel.cpp

@@ -49,9 +49,8 @@ QModelIndex QTransposeProxyModelPrivate::uncheckedMapToSource(const QModelIndex
 {
     if (!model || !proxyIndex.isValid())
         return QModelIndex();
-    if (proxyIndex.internalPointer())
-        return model->createIndex(proxyIndex.column(), proxyIndex.row(), proxyIndex.internalPointer());
-    return model->index(proxyIndex.column(), proxyIndex.row());
+    Q_Q(const QTransposeProxyModel);
+    return q->createSourceIndex(proxyIndex.column(), proxyIndex.row(), proxyIndex.internalPointer());
 }
 
 QModelIndex QTransposeProxyModelPrivate::uncheckedMapFromSource(const QModelIndex &sourceIndex) const