QQuickItemView: Skip instantiating delegates if size is 0

If the area of a (List|Grid)View is 0, then instantiating delegates is
pointless, as they couldn't be shown anyway. However, our current logic
could not handle this case well, and would end up instantiating a
delegate for every delegate entry if their size also ended up being 0 -
you can after all fit infinitely many 0 sized items into a zero sized
container.
Detect this situation in QQuickItemViewPrivate::refill and the
applyInsertionChange implementations. Note that we only exit early if
there are no visible items and the view is zero-sized; if there are
visible items, we still want to ensure that they are removed after all.

We also need to adjust a few tests which had zero sized views to no
longer be zero sized; otherwise they wouldn't have created their
delegates in time.

[ChangeLog][QtQuick][ListView][Important Behavior Change] If a ListView
has size zero, it won't instantiate any delegates until its size becomes
non-zero.

Fixes: QTBUG-110625
Fixes: QTBUG-89568
Fixes: QTBUG-51773
Change-Id: Ibe0e6fa5f01784016882522c120d2fee38df285b
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
(cherry picked from commit 37c25c6e74f4f74d7cca8f5b0f12a40ec0354f7e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

9 files changed, 64 insertions, 3 deletions

diff --git a/tests/auto/qml/qqmllistmodel/tst_qqmllistmodel.cpp b/tests/auto/qml/qqmllistmodel/tst_qqmllistmodel.cpp
index e522f280a3..6fd173fb0c 100644
--- a/tests/auto/qml/qqmllistmodel/tst_qqmllistmodel.cpp
+++ b/tests/auto/qml/qqmllistmodel/tst_qqmllistmodel.cpp
@@ -1549,6 +1549,8 @@ void tst_qqmllistmodel::modify_through_delegate()
         "       ListElement { name: \"Doe\"; age: 33 }\n"
         "   }\n"
         "   ListView {\n"
+        "       height: 100\n" \
+        "       width: 100\n" \
         "       model: testModel\n"
         "       delegate: Item {\n"
         "           Component.onCompleted: model.age = 18;\n"
diff --git a/tests/auto/quick/qquicklistview/data/objectModelCulling.qml b/tests/auto/quick/qquicklistview/data/objectModelCulling.qml
index c0a70ec485..9ca79ec8e4 100644
--- a/tests/auto/quick/qquicklistview/data/objectModelCulling.qml
+++ b/tests/auto/quick/qquicklistview/data/objectModelCulling.qml
@@ -3,6 +3,7 @@ import QtQuick 2.12
 import QtQml.Models 2.12
 
 Item {
+    anchors.fill: parent
 
     ObjectModel {
         id: model1
diff --git a/tests/auto/quick/qquicklistview/data/snapOneItemResize.qml b/tests/auto/quick/qquicklistview/data/snapOneItemResize.qml
index 7ecc833a64..4603cd9f34 100644
--- a/tests/auto/quick/qquicklistview/data/snapOneItemResize.qml
+++ b/tests/auto/quick/qquicklistview/data/snapOneItemResize.qml
@@ -2,6 +2,7 @@ import QtQuick 2.0
 
 ListView {
     id: list
+    anchors.fill: parent
     currentIndex: 5
     snapMode: ListView.SnapOneItem
     orientation: ListView.Horizontal
diff --git a/tests/auto/quick/qquicklistview2/data/areaZeroView.qml b/tests/auto/quick/qquicklistview2/data/areaZeroView.qml
new file mode 100644
index 0000000000..e0329f4e83
--- /dev/null
+++ b/tests/auto/quick/qquicklistview2/data/areaZeroView.qml
@@ -0,0 +1,22 @@
+import QtQuick
+
+Window {
+    width: 600
+    height: 600
+    visible: true
+    property int delegateCreationCounter: 0
+
+    ListView {
+        id: lv
+        anchors.fill: parent
+        model: 6000
+
+        delegate: Rectangle {
+            width: ListView.view.width
+            height: ListView.view.width / 6
+            color: "white"
+            border.width: 1
+            Component.onCompleted: ++delegateCreationCounter
+        }
+    }
+}
diff --git a/tests/auto/quick/qquicklistview2/data/boundDelegateComponent.qml b/tests/auto/quick/qquicklistview2/data/boundDelegateComponent.qml
index 6fb84279b0..5e4de1a08d 100644
--- a/tests/auto/quick/qquicklistview2/data/boundDelegateComponent.qml
+++ b/tests/auto/quick/qquicklistview2/data/boundDelegateComponent.qml
@@ -6,6 +6,8 @@ Item {
     objectName: "outer"
     ListView {
         id: listView
+        width: 100
+        height: 100
         model: 1
         property string foo: "foo"
         delegate: Text {
@@ -16,6 +18,8 @@ Item {
 
     ListView {
         id: listView2
+        width: 100
+        height: 100
         model: 1
         delegate: Text {
             required property int index
@@ -49,6 +53,8 @@ Item {
             }
 
             ListView {
+                width: 100
+                height: 100
                 id: innerListView
                 model: listModel
                 delegate: innerComponent
diff --git a/tests/auto/quick/qquicklistview2/data/innerRequired.qml b/tests/auto/quick/qquicklistview2/data/innerRequired.qml
index b45181c0d2..c0862cec0d 100644
--- a/tests/auto/quick/qquicklistview2/data/innerRequired.qml
+++ b/tests/auto/quick/qquicklistview2/data/innerRequired.qml
@@ -25,6 +25,8 @@ Item {
     ListView {
         id: listView
         model: myModel
+        width: 100
+        height: 100
         delegate: AnotherDelegate {
             age: model.age
             text: model.noise
diff --git a/tests/auto/quick/qquicklistview2/data/metaSequenceAsModel.qml b/tests/auto/quick/qquicklistview2/data/metaSequenceAsModel.qml
index 2b4651feaa..461450239f 100644
--- a/tests/auto/quick/qquicklistview2/data/metaSequenceAsModel.qml
+++ b/tests/auto/quick/qquicklistview2/data/metaSequenceAsModel.qml
@@ -2,6 +2,8 @@ import QtQuick
 
 ListView {
     id: view
+    width: 100
+    height: 100
     property list<rect> rects: [ Qt.rect(1, 2, 3, 4), Qt.rect(5, 6, 7, 8) ]
     property list<string> texts
 
diff --git a/tests/auto/quick/qquicklistview2/data/sectionBoundComponent.qml b/tests/auto/quick/qquicklistview2/data/sectionBoundComponent.qml
index dbb966ae28..74ab6b59fa 100644
--- a/tests/auto/quick/qquicklistview2/data/sectionBoundComponent.qml
+++ b/tests/auto/quick/qquicklistview2/data/sectionBoundComponent.qml
@@ -2,7 +2,9 @@ pragma ComponentBehavior: Bound
 import QtQuick
 ListView {
    id: view
-    model: ListModel {
+   width: 100
+   height: 100
+   model: ListModel {
         ListElement { name: "foo"; age: 42  }
         ListElement { name: "bar"; age: 13  }
     }
diff --git a/tests/auto/quick/qquicklistview2/tst_qquicklistview2.cpp b/tests/auto/quick/qquicklistview2/tst_qquicklistview2.cpp
index 684a12160b..c83040fb11 100644
--- a/tests/auto/quick/qquicklistview2/tst_qquicklistview2.cpp
+++ b/tests/auto/quick/qquicklistview2/tst_qquicklistview2.cpp
@@ -54,6 +54,7 @@ private slots:
     void highlightWithBound();
     void sectionIsCompatibleWithBoundComponents();
     void sectionGeometryChange();
+    void areaZeroviewDoesNotNeedlesslyPopulateWholeModel();
 
 private:
     void flickWithTouch(QQuickWindow *window, const QPoint &from, const QPoint &to);
@@ -334,7 +335,7 @@ void tst_QQuickListView2::boundDelegateComponent()
     QVERIFY2(c.isReady(), qPrintable(c.errorString()));
 
     QTest::ignoreMessage(
-            QtWarningMsg, qPrintable(QLatin1String("%1:12: ReferenceError: index is not defined")
+            QtWarningMsg, qPrintable(QLatin1String("%1:14: ReferenceError: index is not defined")
                                              .arg(url.toString())));
 
     QScopedPointer<QObject> o(c.create());
@@ -365,7 +366,7 @@ void tst_QQuickListView2::boundDelegateComponent()
     for (int i = 0; i < 3; ++i) {
         QTest::ignoreMessage(
                 QtWarningMsg,
-                qPrintable(QLatin1String("%1:47:21: ReferenceError: model is not defined")
+                qPrintable(QLatin1String("%1:51:21: ReferenceError: model is not defined")
                                    .arg(url.toString())));
     }
 
@@ -988,6 +989,28 @@ void tst_QQuickListView2::sectionGeometryChange()
     QTRY_COMPARE(element1->y(), section1->y() + section1->height());
 }
 
+void tst_QQuickListView2::areaZeroviewDoesNotNeedlesslyPopulateWholeModel()
+{
+    QTest::failOnWarning(QRegularExpression(".*"));
+    QQmlEngine engine;
+    QQmlComponent c(&engine, testFileUrl("areaZeroView.qml"));
+    QVERIFY2(c.isReady(), qPrintable(c.errorString()));
+    std::unique_ptr<QObject> root(c.create());
+    QVERIFY(root);
+    auto delegateCreationCounter = [&]() {
+        return root->property("delegateCreationCounter").toInt();
+    };
+    // wait for onComplete to be settled
+    QTRY_VERIFY(delegateCreationCounter() != 0);
+    auto view = qobject_cast<QQuickListView *>(qmlContext(root.get())->objectForName("lv"));
+    QVERIFY(view);
+    QCOMPARE(view->count(), 6'000);
+    // we use 100, which is < 6000, but larger than the actual expected value
+    // that's to give the test some leniency in case the ListView implementation
+    // changes in the future to instantiate a few more items outside of the viewport
+    QVERIFY(delegateCreationCounter() < 100);
+}
+
 QTEST_MAIN(tst_QQuickListView2)
 
 #include "tst_qquicklistview2.moc"