Call canFetchMore/fetchMore when scrolled to the bottom of the view

When scrolling to the bottom of the view, if there are expanded items
then it should call canFetchMore/fetchMore up the chain until it
finds one with more items that can possibly be retrieved.

This brings it in line with the QAbstractItemView implementation
which would call canFetchMore on the root index, therefore we
go up the chain to the root to see if anything before that can be fetched.

[ChangeLog][QtWidgets][QTreeView] QTreeView now calls canFetchMore and
fetchMore when the bottom of the QTreeView is scrolled to.

Task-number: QTBUG-48725
Change-Id: I2b2145684bb34c8c317bfce4a0ef14f243a64719
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

src/widgets/itemviews/qtreeview.cpp

@@ -3992,6 +3992,27 @@ int QTreeView::visualIndex(const QModelIndex &index) const
     return d->viewIndex(index);
 }
 
+/*!
+   \reimp
+*/
+
+void QTreeView::verticalScrollbarValueChanged(int value)
+{
+    Q_D(QTreeView);
+    if (!d->viewItems.isEmpty() && value == verticalScrollBar()->maximum()) {
+        QModelIndex ret = d->viewItems.last().index;
+        // Root index will be handled by base class implementation
+        while (ret.isValid()) {
+            if (isExpanded(ret) && d->model->canFetchMore(ret)) {
+                d->model->fetchMore(ret);
+                break;
+            }
+            ret = ret.parent();
+        }
+    }
+    QAbstractItemView::verticalScrollbarValueChanged(value);
+}
+
 QT_END_NAMESPACE
 
 #include "moc_qtreeview.cpp"

src/widgets/itemviews/qtreeview.h

@@ -169,6 +169,7 @@ protected Q_SLOTS:
     void columnMoved();
     void reexpand();
     void rowsRemoved(const QModelIndex &parent, int first, int last);
+    void verticalScrollbarValueChanged(int value) Q_DECL_OVERRIDE;
 
 protected:
     QTreeView(QTreeViewPrivate &dd, QWidget *parent = Q_NULLPTR);

tests/auto/widgets/itemviews/qtreeview/tst_qtreeview.cpp

@@ -171,6 +171,7 @@ private slots:
 
     void statusTip_data();
     void statusTip();
+    void fetchMoreOnScroll();
 
     // task-specific tests:
     void task174627_moveLeftToRoot();
@@ -4470,5 +4471,49 @@ void tst_QTreeView::statusTip()
     QTest::mouseMove(mw.windowHandle(), centerPoint);
     QTRY_COMPARE(mw.statusBar()->currentMessage(), QLatin1String("Header 0 -- Status"));
 }
+
+class FetchMoreModel : public QStandardItemModel
+{
+public:
+    FetchMoreModel() : QStandardItemModel(), canFetchReady(false)
+    {
+        for (int i = 0; i < 20; ++i) {
+            QStandardItem *item = new QStandardItem("Row");
+            item->appendRow(new QStandardItem("Child"));
+            appendRow(item);
+        }
+    }
+    bool canFetchMore(const QModelIndex &parent) const override
+    {
+        if (!canFetchReady || !parent.isValid())
+            return false;
+        if (!parent.parent().isValid())
+            return rowCount(parent) < 20;
+        return false;
+    }
+    void fetchMore(const QModelIndex &parent) override
+    {
+        QStandardItem *item = itemFromIndex(parent);
+        for (int i = 0; i < 19; ++i)
+            item->appendRow(new QStandardItem(QString("New Child %1").arg(i)));
+    }
+    bool canFetchReady;
+};
+
+void tst_QTreeView::fetchMoreOnScroll()
+{
+    QTreeView tw;
+    FetchMoreModel im;
+    tw.setModel(&im);
+    tw.show();
+    tw.expandAll();
+    QTest::qWaitForWindowActive(&tw);
+    // Now we can allow the fetch to happen
+    im.canFetchReady = true;
+    tw.verticalScrollBar()->setValue(tw.verticalScrollBar()->maximum());
+    // The item should have now fetched the other children, thus bringing the count to 20
+    QCOMPARE(im.item(19)->rowCount(), 20);
+}
+
 QTEST_MAIN(tst_QTreeView)
 #include "tst_qtreeview.moc"