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+/****************************************************************************
+**
+** Copyright (C) 2015 Klaralvdalens Datakonsult AB (KDAB).
+** Copyright (C) 2017 The Qt Company Ltd.
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of Qt 3D Studio.
+**
+** $QT_BEGIN_LICENSE:GPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 or (at your option) any later version
+** approved by the KDE Free Qt Foundation. The licenses are as published by
+** the Free Software Foundation and appearing in the file LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef DRAGONNODETREE_P_H
+#define DRAGONNODETREE_P_H
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists for the convenience
+// of other Qt classes.  This header file may change from version to
+// version without notice, or even be removed.
+//
+// We mean it.
+//
+
+QT_BEGIN_NAMESPACE
+
+namespace Qt3DRender {
+namespace Dragon {
+
+/*!
+ * Qt3D passes very limited information to the backend about the child/parent hierarchy.
+ * Only parent information is passed on creation and only child information is passed in updates.
+ * The nodeTree tries its best to use information from the different
+ * NodeFunctors (a.k.a. QBackendNodeMappers) to construct a usable tree.
+ *
+ * TODO: Make Qt3D pass the parent id to the backend when parent changes and/or
+ * children on node creation.
+ */
+class NodeTree
+{
+public:
+    struct NodeInfo
+    {
+        Qt3DCore::QNodeId parentId;
+        QVector<Qt3DCore::QNodeId> childIds;
+    };
+    using Nodes = QHash<Qt3DCore::QNodeId, NodeInfo>;
+
+    void addNode(Qt3DCore::QNodeId id)
+    {
+        // node might already exist if setParent or addChild was called first
+        if (!m_nodes.contains(id))
+            m_nodes.insert(id, NodeInfo());
+    }
+
+    void removeNode(Qt3DCore::QNodeId id)
+    {
+        // this is mostly to clear the data in case the node appears again
+        NodeInfo node = m_nodes.take(id);
+        if (!node.parentId.isNull())
+            m_nodes[node.parentId].childIds.removeOne(id);
+    }
+
+    void setParent(Qt3DCore::QNodeId childId, Qt3DCore::QNodeId parentId)
+    {
+        // remove child from old parent
+        if (m_nodes.contains(childId)) {
+            Qt3DCore::QNodeId oldParentId = m_nodes[childId].parentId;
+            if (m_nodes.contains(oldParentId)) {
+                m_nodes[oldParentId].childIds.removeOne(childId);
+            }
+        }
+        // using operator[] here on purpose - we want to set it even if addNode has not been called
+        m_nodes[childId].parentId = parentId;
+        if (!parentId.isNull() && !m_nodes[parentId].childIds.contains(childId))
+            m_nodes[parentId].childIds.push_back(childId);
+    }
+
+    const QHash<Qt3DCore::QNodeId, NodeInfo> &nodes()
+    {
+        return m_nodes;
+    }
+
+    // Task interface
+    QHash<Qt3DCore::QNodeId, NodeInfo> result()
+    {
+        return m_nodes;
+    }
+
+    bool isFinished()
+    {
+        return true;
+    }
+
+private:
+    QHash<Qt3DCore::QNodeId, NodeInfo> m_nodes;
+};
+
+} // namespace Dragon
+} // namespace Qt3DRender
+
+QT_END_NAMESPACE
+
+#endif // DRAGONNODETREE_P_H