QQuickListView: implement support for reusing items

This patch will implement delegate item recycling in
ListView. The API will be the same as used in TableView, except
that it will be off by default since the behavior of a delegate
that is reused is not compatible with legacy applications
which were not written with this in mind.

Most importantly:

- Component.onCompleted will only be called on a delegate the
first time it's created, and never when it's reused.

- Any user-declared properties in the delegate (that is, not
model roles, index, etc) will not be cleared or updated when an
item is reused. The application must do this manually upon
receiving the pooled or reused signal in the item.

[ChangeLog][ListView] ListView now has support for reusing delegate
items. This can be switched on by setting the reuseItems property of
ListView to true.

Task-number: QTBUG-80507
Change-Id: I68cc8300b050e4a1f89feebb1d31a2fd9189c793
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

1 files changed, 103 insertions, 0 deletions

diff --git a/tests/manual/listview/main.cpp b/tests/manual/listview/main.cpp
new file mode 100644
index 0000000000..8b28b91ee8
--- /dev/null
+++ b/tests/manual/listview/main.cpp
@@ -0,0 +1,103 @@
+/****************************************************************************
+**
+** Copyright (C) 2019 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtQuick module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** 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.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QGuiApplication>
+#include <QQmlApplicationEngine>
+#include <QAbstractTableModel>
+
+typedef QPair<QString, bool> CellData;
+
+class TestModel : public QAbstractListModel
+{
+    Q_OBJECT
+    Q_PROPERTY(int rowCount READ rowCount WRITE setRowCount NOTIFY rowCountChanged)
+
+public:
+    TestModel(QObject *parent = nullptr) : QAbstractListModel(parent) { }
+
+    int rowCount(const QModelIndex & = QModelIndex()) const override { return m_rows; }
+    void setRowCount(int count) {
+        m_rows = count;
+        emit rowCountChanged();
+    }
+
+    QVariant data(const QModelIndex &index, int role) const override
+    {
+        if (!index.isValid())
+            return QVariant();
+
+        switch (role) {
+        case Qt::DisplayRole:
+            return index.row() % 2 ? QStringLiteral("type2") : QStringLiteral("type1");
+        default:
+            break;
+        }
+
+        return QVariant();
+    }
+
+    QHash<int, QByteArray> roleNames() const override
+    {
+        return {
+            {Qt::DisplayRole, "delegateType"},
+        };
+    }
+
+signals:
+    void rowCountChanged();
+
+private:
+    int m_rows = 0;
+};
+
+#include "main.moc"
+
+int main(int argc, char *argv[])
+{
+    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+
+    QGuiApplication app(argc, argv);
+
+    qmlRegisterType<TestModel>("TestModel", 0, 1, "TestModel");
+
+    QQmlApplicationEngine engine;
+    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
+
+    return app.exec();
+}