QNode: setParent create creation change only when needed

Two cases need to be handled by setParent:
1) Parent already has a backend node and receives a new child
   -> in which case we need to send the creation event to the backend
2) Parent was created in the frontend, but has no backend (delayed
   notification sending because a ctor can't call a virtual)
   -> in that case, when adding a child and setting its parent we shouldn't be
   sending the creation change. We rather let that be handled when the
   creation change for the parent is requested

Change-Id: I434c7d4e6af785c0314ac6538dc689992d90ed0c
Task-number: QTBUG-60612
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Oleg Evseev <ev.mipt@gmail.com>

1 files changed, 16 insertions, 2 deletions

diff --git a/src/core/nodes/qnode.cpp b/src/core/nodes/qnode.cpp
index 42c8887ce..ce5e76a55 100644
--- a/src/core/nodes/qnode.cpp
+++ b/src/core/nodes/qnode.cpp
@@ -114,7 +114,7 @@ void QNodePrivate::notifyCreationChange()
     Q_Q(QNode);
     // Do nothing if we already have already sent a node creation change
     // and not a subsequent node destroyed change.
-    if (m_hasBackendNode)
+    if (m_hasBackendNode || !m_scene)
         return;
     QNodeCreatedChangeGenerator generator(q);
     const auto creationChanges = generator.creationChanges();
@@ -305,7 +305,21 @@ void QNodePrivate::_q_setParentHelper(QNode *parent)
                 visitor.traverse(q, newParentNode->d_func(), &QNodePrivate::setSceneHelper);
             }
 
-            notifyCreationChange();
+            // We want to make sure that subTreeRoot is always created before
+            // child.
+            // Given a case such as below
+            // QEntity *subTreeRoot = new QEntity(someGlobalExisitingRoot)
+            // QEntity *child = new QEntity();
+            // child->setParent(subTreeRoot)
+            // We need to take into account that subTreeRoot needs to be
+            // created in the backend before the child.
+            // Therefore we only call notifyCreationChanges if the parent
+            // hasn't been created yet as we know that when the parent will be
+            // fully created, it will also send the changes for all of its
+            // children
+
+            if (QNodePrivate::get(newParentNode)->m_hasBackendNode)
+                notifyCreationChange();
         }
 
         // If we have a valid new parent, we let him know that we are its child