QQuickListView/GridView: load items that results from model changes synchronously

The current implementation created new items with default incubation mode, which
is AsynchronousIfNested.  But from reading the code, it seems like changes to the
model were really expected to be handled synchronously, since there aren't any
sections in the code that will recover from situations were requested items ends
up incubating async. This is also backed by the fact that the second argument
used to be a bool set to 'synchronous'. The fact that this was translated to
AsynchronousIfNested later down the chain didn't seems to be taken into account.

A bug with this is found in ListView when it's embedded inside an async Loader.
In that case, all list items will be incubating async at startup, which is normally
expected and fine. But if the model assigned to the ListView is modified before the
loader has finished, async loading will also happen to the items created because of
the change. And then the situation described above will occur.

This patch will force any items loaded because of a model change to be synchronous.
This seems to be in line with the synchronous logic that already exists. And its
also quite acceptable, since changing the model before everything is completed is
a corner case, and can naturally lead to some stuttering in the list view anyway.

A potential for improvement on this logic is to try to recover whenever creating an
item fails. But this takes a lot of work because of the way model changes are
structured and stored, and can easily cause regressions. Since this is a corner
case, it is probably not worth it.

[ChangeLog][QtQml][ListView] Fixed a bug in ListView that sometimes make items
end up with wrong geometry when changes to its model happens while the ListView
is being loaded async (e.g if wrapped inside an async Loader).

Task-number: QTBUG-61537
Change-Id: I8d6857beaf8ef98b02bb46282a1312749b0fb801
Reviewed-by: J-P Nurmi <jpnurmi@qt.io>

2 files changed, 4 insertions, 4 deletions

diff --git a/src/quick/items/qquickgridview.cpp b/src/quick/items/qquickgridview.cpp
index e59c579790..c56cfd3005 100644
--- a/src/quick/items/qquickgridview.cpp
+++ b/src/quick/items/qquickgridview.cpp
@@ -2417,7 +2417,7 @@ bool QQuickGridViewPrivate::applyInsertionChange(const QQmlChangeSet::Change &ch
                 if (change.isMove() && (item = currentChanges.removedItems.take(change.moveKey(modelIndex + i))))
                     item->index = modelIndex + i;
                 if (!item)
-                    item = createItem(modelIndex + i);
+                    item = createItem(modelIndex + i, QQmlIncubator::Synchronous);
                 if (!item)
                     return false;
 
@@ -2470,7 +2470,7 @@ bool QQuickGridViewPrivate::applyInsertionChange(const QQmlChangeSet::Change &ch
                 item->index = modelIndex + i;
             bool newItem = !item;
             if (!item)
-                item = createItem(modelIndex + i);
+                item = createItem(modelIndex + i, QQmlIncubator::Synchronous);
             if (!item)
                 return false;
 
diff --git a/src/quick/items/qquicklistview.cpp b/src/quick/items/qquicklistview.cpp
index d924bc4460..11eaf393ea 100644
--- a/src/quick/items/qquicklistview.cpp
+++ b/src/quick/items/qquicklistview.cpp
@@ -3273,7 +3273,7 @@ bool QQuickListViewPrivate::applyInsertionChange(const QQmlChangeSet::Change &ch
                 if (change.isMove() && (item = currentChanges.removedItems.take(change.moveKey(modelIndex + i))))
                     item->index = modelIndex + i;
                 if (!item)
-                    item = createItem(modelIndex + i);
+                    item = createItem(modelIndex + i, QQmlIncubator::Synchronous);
                 if (!item)
                     return false;
 
@@ -3315,7 +3315,7 @@ bool QQuickListViewPrivate::applyInsertionChange(const QQmlChangeSet::Change &ch
                 item->index = modelIndex + i;
             bool newItem = !item;
             if (!item)
-                item = createItem(modelIndex + i);
+                item = createItem(modelIndex + i, QQmlIncubator::Synchronous);
             if (!item)
                 return false;