ListView: fix countChanged not being emitted in certain cases

This broke after 477c15def834bd49553c00b90f3a2006456ea931.
countChanged would have normally been emitted in the call to
applyModelChanges that we skipped after adding the q->size().isNull()
check.

We can check hasPendingChanges() to know if a count change
is pending (which is also the first thing applyModelChanges checks
before doing its work), and if so, emit countChanged so that
any bindings that are relying on it can function as expected.

Fixes: QTBUG-129165
Pick-to: 6.5 6.7 6.8 dev
Change-Id: Ic045f1870b39d192f6880e23daab03fd73a16d58
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

4 files changed, 78 insertions, 6 deletions

diff --git a/src/quick/items/qquickitemview.cpp b/src/quick/items/qquickitemview.cpp
index 0a6a3e949b..19e5b7f402 100644
--- a/src/quick/items/qquickitemview.cpp
+++ b/src/quick/items/qquickitemview.cpp
@@ -10,6 +10,7 @@
 QT_BEGIN_NAMESPACE
 
 Q_LOGGING_CATEGORY(lcItemViewDelegateLifecycle, "qt.quick.itemview.lifecycle")
+Q_LOGGING_CATEGORY(lcCount, "qt.quick.itemview.count")
 
 // Default cacheBuffer for all views.
 #ifndef QML_VIEW_DEFAULTCACHEBUFFER
@@ -223,7 +224,7 @@ void QQuickItemView::setModel(const QVariant &m)
                 this, SLOT(modelUpdated(QQmlChangeSet,bool)));
         if (QQmlDelegateModel *dataModel = qobject_cast<QQmlDelegateModel*>(d->model))
             QObjectPrivate::connect(dataModel, &QQmlDelegateModel::delegateChanged, d, &QQuickItemViewPrivate::applyDelegateChange);
-        emit countChanged();
+        d->emitCountChanged();
     }
     emit modelChanged();
     d->moveReason = QQuickItemViewPrivate::Other;
@@ -255,7 +256,7 @@ void QQuickItemView::setDelegate(QQmlComponent *delegate)
         int oldCount = dataModel->count();
         dataModel->setDelegate(delegate);
         if (oldCount != dataModel->count())
-            emit countChanged();
+            d->emitCountChanged();
     }
     emit delegateChanged();
     d->delegateValidated = false;
@@ -1125,6 +1126,14 @@ void QQuickItemViewPrivate::showVisibleItems() const
     }
 }
 
+// Simplifies debugging of count.
+void QQuickItemViewPrivate::emitCountChanged()
+{
+    Q_Q(QQuickItemView);
+    qCDebug(lcCount).nospace() << "about to emit countChanged for " << q << "; count changed to " << q->count();
+    emit q->countChanged();
+}
+
 void QQuickItemViewPrivate::itemGeometryChanged(QQuickItem *item, QQuickGeometryChange change,
                                                 const QRectF &oldGeometry)
 {
@@ -1224,7 +1233,7 @@ void QQuickItemView::modelUpdated(const QQmlChangeSet &changeSet, bool reset)
             d->updateTrackedItem();
         }
         d->moveReason = QQuickItemViewPrivate::Other;
-        emit countChanged();
+        d->emitCountChanged();
 #if QT_CONFIG(quick_viewtransitions)
         if (d->transitioner && d->transitioner->populateTransition)
             d->forceLayoutPolish();
@@ -1487,7 +1496,7 @@ void QQuickItemView::componentComplete()
         d->fixupPosition();
     }
     if (d->model && d->model->count())
-        emit countChanged();
+        d->emitCountChanged();
 }
 
 
@@ -1813,7 +1822,7 @@ void QQuickItemViewPrivate::refill(qreal from, qreal to)
         }
 
         if (prevCount != itemCount)
-            emit q->countChanged();
+            emitCountChanged();
     } while (currentChanges.hasPendingChanges() || bufferedChanges.hasPendingChanges());
     storeFirstVisibleItemPosition();
 }
@@ -1864,6 +1873,16 @@ void QQuickItemViewPrivate::layout()
     // if either dimension is negative, but apparently we support negative-sized
     // views (see tst_QQuickListView::resizeView).
     if ((!isValid() && !visibleItems.size()) || q->size().isNull()) {
+        if (q->size().isNull() && hasPendingChanges()) {
+            // count() refers to the number of items in the model, not in the view
+            // (which is why we don't emit for the !visibleItems.size() case).
+            // If there are pending model changes, emit countChanged in order to
+            // support the use case of QTBUG-129165, where visible is bound to count > 0
+            // and the ListView is in a layout with Layout.preferredHeight bound to
+            // contentHeight. This ensures that a hidden ListView will become visible.
+            emitCountChanged();
+        }
+
         clear();
         setPosition(contentStartOffset());
         updateViewport();
@@ -2138,7 +2157,7 @@ bool QQuickItemViewPrivate::applyModelChanges(ChangeResult *totalInsertionResult
 
     updateSections();
     if (prevItemCount != itemCount)
-        emit q->countChanged();
+        emitCountChanged();
     if (!visibleAffected && viewportChanged)
         updateViewport();
 
diff --git a/src/quick/items/qquickitemview_p_p.h b/src/quick/items/qquickitemview_p_p.h
index 9cb6ea8fd5..b5d70c6d6f 100644
--- a/src/quick/items/qquickitemview_p_p.h
+++ b/src/quick/items/qquickitemview_p_p.h
@@ -229,6 +229,8 @@ public:
             releaseItem(item, reusableFlag);
     }
 
+    void emitCountChanged();
+
     virtual QQuickItemViewAttached *getAttachedObject(const QObject *) const { return nullptr; }
 
     QPointer<QQmlInstanceModel> model;
diff --git a/tests/auto/quick/qquicklistview2/data/visibleBoundToCountGreaterThanZero.qml b/tests/auto/quick/qquicklistview2/data/visibleBoundToCountGreaterThanZero.qml
new file mode 100644
index 0000000000..e75c779584
--- /dev/null
+++ b/tests/auto/quick/qquicklistview2/data/visibleBoundToCountGreaterThanZero.qml
@@ -0,0 +1,31 @@
+import QtQuick
+import QtQuick.Layouts
+
+ColumnLayout {
+    property alias listView: listView
+
+    ListView {
+        id: listView
+
+        visible: count > 0 // actual defect. countChanged never fires so this never turns true
+
+        Layout.fillWidth: true
+        Layout.preferredHeight: contentHeight // grow with content, initially 0
+
+        model: ListModel {
+            id: idModel
+        }
+
+        delegate: Text {
+            required property string name
+            text: name
+        }
+
+        Timer {
+            running: true
+            interval: 10
+            repeat: true
+            onTriggered: idModel.append({name:"Hello"})
+        }
+    }
+}
diff --git a/tests/auto/quick/qquicklistview2/tst_qquicklistview2.cpp b/tests/auto/quick/qquicklistview2/tst_qquicklistview2.cpp
index 96dd27e7d2..c07a30d401 100644
--- a/tests/auto/quick/qquicklistview2/tst_qquicklistview2.cpp
+++ b/tests/auto/quick/qquicklistview2/tst_qquicklistview2.cpp
@@ -18,6 +18,8 @@ Q_LOGGING_CATEGORY(lcTests, "qt.quick.tests")
 using namespace QQuickViewTestUtils;
 using namespace QQuickVisualTestUtils;
 
+static const int oneSecondInMs = 1000;
+
 class tst_QQuickListView2 : public QQmlDataTest
 {
     Q_OBJECT
@@ -68,6 +70,7 @@ private slots:
     void bindingDirectlyOnPositionInHeaderAndFooterDelegates();
 
     void clearObjectListModel();
+    void visibleBoundToCountGreaterThanZero();
 
 private:
     void flickWithTouch(QQuickWindow *window, const QPoint &from, const QPoint &to);
@@ -1312,6 +1315,23 @@ void tst_QQuickListView2::clearObjectListModel()
     QVERIFY(!list.itemAtIndex(0));
 }
 
+void tst_QQuickListView2::visibleBoundToCountGreaterThanZero()
+{
+    QQuickView window;
+    QVERIFY(QQuickTest::showView(window, testFileUrl("visibleBoundToCountGreaterThanZero.qml")));
+
+    auto *listView = window.rootObject()->property("listView").value<QQuickListView *>();
+    QVERIFY(listView);
+
+    QSignalSpy countChangedSpy(listView, SIGNAL(countChanged()));
+    QVERIFY(countChangedSpy.isValid());
+
+    QTRY_COMPARE_GT_WITH_TIMEOUT(listView->count(), 1, oneSecondInMs);
+    // Using the TRY variant here as well is necessary.
+    QTRY_COMPARE_GT_WITH_TIMEOUT(countChangedSpy.count(), 1, oneSecondInMs);
+    QVERIFY(listView->isVisible());
+}
+
 QTEST_MAIN(tst_QQuickListView2)
 
 #include "tst_qquicklistview2.moc"