QQmlDelegateModel: add incubationStatus(), and use it to determine 'requestedIndex'

We used to assign the currently incubating item to 'requestedIndex' based
on requested incubation mode alone. This is not sufficient, as the item
can also be loaded async when the mode is AsyncIfNested. To check if the
item is really loading async (and that we're not getting nullptr because
of some other failure), we need to ask the incubator.

Task-number: QTBUG-61537
Change-Id: Id1f458db4a7584a6b58d5bad0e7832ce4fc341dc
Reviewed-by: J-P Nurmi <jpnurmi@qt.io>

1 files changed, 9 insertions, 3 deletions

diff --git a/src/quick/items/qquickitemview.cpp b/src/quick/items/qquickitemview.cpp
index 82dbf04090..10f6c63170 100644
--- a/src/quick/items/qquickitemview.cpp
+++ b/src/quick/items/qquickitemview.cpp
@@ -2340,14 +2340,20 @@ FxViewItem *QQuickItemViewPrivate::createItem(int modelIndex, QQmlIncubator::Inc
         }
     }
 
-    if (incubationMode == QQmlIncubator::Asynchronous)
-        requestedIndex = modelIndex;
     inRequest = true;
 
     QObject* object = model->object(modelIndex, incubationMode);
     QQuickItem *item = qmlobject_cast<QQuickItem*>(object);
+
     if (!item) {
-        if (object) {
+        if (!object) {
+            if (requestedIndex == -1 && model->incubationStatus(modelIndex) == QQmlIncubator::Loading) {
+                // The reason we didn't receive an item is because it's incubating async. We keep track
+                // of this by assigning the index we're waiting for to 'requestedIndex'. This will e.g. let
+                // the view avoid unnecessary layout calls until the item has been loaded.
+                requestedIndex = modelIndex;
+            }
+        } else {
             model->release(object);
             if (!delegateValidated) {
                 delegateValidated = true;