QTreeView: fetch as many nested entries as fit into the view

QAbstractItemModel::canFetchMore and fetchMore implementations might only
add a few rows to the model each time they are called. The item views don't
generally expect that, and don't call fetchMore repeatedly, even if there
would be room in the itemview for more data. This problem cannot be
generally solved for all item views, as it would require in repeated expensive
laying out of items.

For nested indexes in a treeview however, we can try to fetch enough child
rows to populate the screen when the item is laid out by repeatedly
calling canFetchMore and fetchMore. To calculate how many items have space,
apply the same heuristics as in the scrollContentsBy implementation to
guess the number of items that can fit into the viewport.

Created test case for the fix.

Done-with: Doris Verria <doris.verria@qt.io>
Fixes: QTBUG-85366
Pick-to: 5.15
Change-Id: I54f95552993873dd4cba80b0f70f4db9d98ddc1d
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

1 files changed, 10 insertions, 2 deletions

diff --git a/src/widgets/itemviews/qtreeview.cpp b/src/widgets/itemviews/qtreeview.cpp
index dedc27501f..3fd0da37cf 100644
--- a/src/widgets/itemviews/qtreeview.cpp
+++ b/src/widgets/itemviews/qtreeview.cpp
@@ -3342,9 +3342,17 @@ void QTreeViewPrivate::layout(int i, bool recursiveExpanding, bool afterIsUninit
 
     int count = 0;
     if (model->hasChildren(parent)) {
-        if (model->canFetchMore(parent))
+        if (model->canFetchMore(parent)) {
+            // fetchMore first, otherwise we might not yet have any data for sizeHintForRow
             model->fetchMore(parent);
-        count = model->rowCount(parent);
+            // guestimate the number of items in the viewport, and fetch as many as might fit
+            const int itemHeight = defaultItemHeight <= 0 ? q->sizeHintForRow(0) : defaultItemHeight;
+            const int viewCount = viewport->height() / itemHeight;
+            while ((count = model->rowCount(parent)) < viewCount && model->canFetchMore(parent))
+                model->fetchMore(parent);
+        } else {
+            count = model->rowCount(parent);
+        }
     }
 
     bool expanding = true;