QQuickListView/GridView: load items that results from model changes synchronously

The current implementation created new items with default incubation mode, which
is AsynchronousIfNested.  But from reading the code, it seems like changes to the
model were really expected to be handled synchronously, since there aren't any
sections in the code that will recover from situations were requested items ends
up incubating async. This is also backed by the fact that the second argument
used to be a bool set to 'synchronous'. The fact that this was translated to
AsynchronousIfNested later down the chain didn't seems to be taken into account.

A bug with this is found in ListView when it's embedded inside an async Loader.
In that case, all list items will be incubating async at startup, which is normally
expected and fine. But if the model assigned to the ListView is modified before the
loader has finished, async loading will also happen to the items created because of
the change. And then the situation described above will occur.

This patch will force any items loaded because of a model change to be synchronous.
This seems to be in line with the synchronous logic that already exists. And its
also quite acceptable, since changing the model before everything is completed is
a corner case, and can naturally lead to some stuttering in the list view anyway.

A potential for improvement on this logic is to try to recover whenever creating an
item fails. But this takes a lot of work because of the way model changes are
structured and stored, and can easily cause regressions. Since this is a corner
case, it is probably not worth it.

[ChangeLog][QtQml][ListView] Fixed a bug in ListView that sometimes make items
end up with wrong geometry when changes to its model happens while the ListView
is being loaded async (e.g if wrapped inside an async Loader).

Task-number: QTBUG-61537
Change-Id: I8d6857beaf8ef98b02bb46282a1312749b0fb801
Reviewed-by: J-P Nurmi <jpnurmi@qt.io>

2 files changed, 153 insertions, 0 deletions

diff --git a/tests/auto/quick/qquicklistview/data/qtbug61537_modelChangesAsync.qml b/tests/auto/quick/qquicklistview/data/qtbug61537_modelChangesAsync.qml
new file mode 100644
index 0000000000..0dc9e6fdb5
--- /dev/null
+++ b/tests/auto/quick/qquicklistview/data/qtbug61537_modelChangesAsync.qml
@@ -0,0 +1,125 @@
+/****************************************************************************
+**
+** Copyright (C) 2017 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** BSD License Usage
+** Alternatively, you may use this file under the terms of the BSD license
+** as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
+**   * Redistributions in binary form must reproduce the above copyright
+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
+**   * Neither the name of The Qt Company Ltd nor the names of its
+**     contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+import QtQuick 2.0
+
+Item {
+    visible: true
+    width: 640
+    height: 480
+
+    property ListView listView
+
+    Loader {
+        id: loader
+        anchors.fill: parent
+        asynchronous: true
+        sourceComponent: comp
+
+        onStatusChanged: {
+            if (status == Loader.Ready) {
+                // Assign the listview to the root prop late, so
+                // that the c++ part doesn't start before everything is ready.
+                listView = item.listView
+            }
+        }
+    }
+
+    Component {
+        id: comp
+        Item {
+            property alias listView: listView
+
+            ListView {
+                id: listView
+
+                model: ListModel {
+                    id: listModel
+                    ListElement { title: "one" }
+                    ListElement { title: "two" }
+                }
+
+                anchors.fill: parent
+                orientation: ListView.Horizontal
+
+                delegate: Item {
+                    id: delegateRoot
+                    objectName: "delegate"
+
+                    width: 200
+                    height: 200
+
+                    Component.onCompleted: {
+                        if (index === listModel.count - 1) {
+                            // Add a new item while the outer Loader is still incubating async. If the new model item
+                            // incubates using e.g QQmlIncubator::AsynchronousIfNested, it will also be loaded async, which
+                            // is not currently supported (the item will not be added to the listview, or end up the wrong
+                            // position, depending on its index and the current state of the refill/layout logic in
+                            // QQuickListView).
+                            // We add the new item here at the last delegates Component.onCompleted to hit the point in time
+                            // when the listview is not expecting any more async items. In that case, the item will only be
+                            // added to the list of visible items if incubated synchronously, which gives us something we
+                            // can test for in the auto-test.
+                            listModel.insert(0, {title: "zero"});
+                        }
+                    }
+
+                    Rectangle {
+                        anchors.fill: parent
+                        border.width: 1
+                        Text {
+                            anchors.centerIn: parent
+                            text: index
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
diff --git a/tests/auto/quick/qquicklistview/tst_qquicklistview.cpp b/tests/auto/quick/qquicklistview/tst_qquicklistview.cpp
index f06a118976..223f6004ff 100644
--- a/tests/auto/quick/qquicklistview/tst_qquicklistview.cpp
+++ b/tests/auto/quick/qquicklistview/tst_qquicklistview.cpp
@@ -261,6 +261,7 @@ private slots:
     void releaseItems();
 
     void QTBUG_34576_velocityZero();
+    void QTBUG_61537_modelChangesAsync();
 
 private:
     template <class T> void items(const QUrl &source);
@@ -8694,6 +8695,33 @@ void tst_QQuickListView::QTBUG_34576_velocityZero()
     delete window;
 }
 
+void tst_QQuickListView::QTBUG_61537_modelChangesAsync()
+{
+    // The purpose of this test if to check that any model changes that happens
+    // during start-up, while a loader higher up in the chain is still incubating
+    // async, will not fail.
+    QQuickView window;
+    window.setGeometry(0,0,640,480);
+
+    QString filename(testFile("qtbug61537_modelChangesAsync.qml"));
+    window.setSource(QUrl::fromLocalFile(filename));
+    window.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&window));
+
+    // The qml file will assign the listview to the 'listView' property once the
+    // loader is ready with async incubation. So we need to wait for it.
+    QObject *root = window.rootObject();
+    QTRY_VERIFY(root->property("listView").value<QQuickListView *>());
+    QQuickListView *listView = root->property("listView").value<QQuickListView *>();
+    QVERIFY(listView);
+
+    // Check that the number of delegates we expect to be visible in
+    // the listview matches the number of items we find if we count.
+    int reportedCount = listView->count();
+    int actualCount = findItems<QQuickItem>(listView, "delegate").count();
+    QCOMPARE(reportedCount, actualCount);
+}
+
 QTEST_MAIN(tst_QQuickListView)
 
 #include "tst_qquicklistview.moc"