List the labels of all lodgings in QML

To query the database, i have to run the query inside the same thread
where the database was created, which means that Database should be a
singleton not only within QML, but also in C++, and has to be the _same_
singleton in both worlds.

Although i expose an object that i have created, i followed the same
section titled “Exposing an existing object as a singleton” from Qt’s
documentation[0].  The only difference is that i do not have to declare
the element as a foreign type, because it is a bona fide QObject.

[0]: https://doc.qt.io/qt-6/qml-singleton.html#exposing-an-existing-
object-as-a-singleton

src/CMakeLists.txt

@@ -9,6 +9,7 @@ qt_add_qml_module(${PROJECT_NAME}
         QtCore
         QtQuick
     SOURCES
+        calendarlistmodel.cpp calendarlistmodel.h
         database.cpp database.h
         mnemonicattached.cpp mnemonicattached.h
     QML_FILES

src/ReservationsPage.qml

@@ -1,4 +1,5 @@
 pragma ComponentBehavior: Bound
+import QtQuick
 import QtQuick.Controls
 import QtQuick.Layouts
 
@@ -15,6 +16,18 @@ Page {
         }
     }
 
+    ListView {
+        anchors.fill: parent
+
+        delegate: Text {
+            required property string name
+
+            text: name
+        }
+        model: CalendarListModel {
+        }
+    }
+
     MnemonicAction {
         id: logoutAction
 

src/calendarlistmodel.cpp

@@ -0,0 +1,86 @@
+#include "calendarlistmodel.h"
+#include <QSqlQuery>
+#include <QVariant>
+#include "database.h"
+
+struct CalendarListModel::Calendar
+{
+    QString name;
+};
+
+CalendarListModel::CalendarListModel(QObject *parent)
+    : QAbstractListModel{parent}
+    , m_calendars{}
+{
+    fetch();
+}
+
+CalendarListModel::~CalendarListModel()
+{
+    qDeleteAll(m_calendars);
+}
+
+QVariant CalendarListModel::headerData(int section, Qt::Orientation orientation, int role) const
+{
+    if (role != Qt::DisplayRole) {
+        return {};
+    }
+    return section;
+}
+
+int CalendarListModel::rowCount(const QModelIndex &parent) const
+{
+    if (parent.isValid()) {
+        return 0;
+    }
+
+    return m_calendars.count();
+}
+
+QHash<int, QByteArray> CalendarListModel::roleNames() const
+{
+    QHash<int, QByteArray> roles = QAbstractItemModel::roleNames();
+    roles[Name] = "name";
+    return roles;
+}
+
+void CalendarListModel::fetch()
+{
+    Database::query([]() {
+        QSqlQuery query("select label from campsite order by label");
+        QVector<Calendar> calendars;
+        while (query.next()) {
+            Calendar calendar{query.value(0).toString()};
+            calendars.append(calendar);
+        }
+        return calendars;
+    }).then([this](const QVector<Calendar> &calendars) {
+        if (calendars.empty()) {
+            return;
+        }
+        beginInsertRows(QModelIndex(), 0, calendars.count() - 1);
+        for (const Calendar &calendar : calendars) {
+            m_calendars.append(new Calendar(calendar));
+        }
+        endInsertRows();
+    });
+}
+
+QVariant CalendarListModel::data(const QModelIndex &index, int role) const
+{
+    if (!checkIndex(index,
+                    QAbstractItemModel::CheckIndexOption::IndexIsValid
+                        | QAbstractItemModel::CheckIndexOption::ParentIsInvalid)) {
+        return {};
+    }
+
+    Calendar *calendar = m_calendars.at(index.row());
+    switch (role) {
+    case Name:
+        return calendar->name;
+    }
+
+    return {};
+}
+
+#include "moc_calendarlistmodel.cpp"

src/calendarlistmodel.h

@@ -0,0 +1,40 @@
+#ifndef CALENDARLISTMODEL_H
+#define CALENDARLISTMODEL_H
+
+#include <QAbstractListModel>
+#include <QtQmlIntegration>
+
+class CalendarListModel : public QAbstractListModel
+{
+    Q_OBJECT
+    QML_ELEMENT
+
+public:
+    enum Roles {
+        Name = Qt::UserRole,
+    };
+
+    explicit CalendarListModel(QObject *parent = nullptr);
+    ~CalendarListModel() override;
+
+    QVariant headerData(int section,
+                        Qt::Orientation orientation,
+                        int role = Qt::DisplayRole) const override;
+
+    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;
+
+    int rowCount(const QModelIndex &parent = QModelIndex()) const override;
+
+    QHash<int, QByteArray> roleNames() const override;
+
+private:
+    struct Calendar;
+
+    Q_DISABLE_COPY_MOVE(CalendarListModel)
+
+    void fetch();
+
+    QVector<Calendar *> m_calendars;
+};
+
+#endif // CALENDARLISTMODEL_H

src/database.cpp

@@ -2,7 +2,6 @@
 #include <QSqlDatabase>
 #include <QSqlError>
 #include <QSqlQuery>
-#include <QtConcurrent>
 
 Database::Database(QObject *parent)
     : QObject{parent}
@@ -12,6 +11,29 @@ Database::Database(QObject *parent)
     m_pool.setExpiryTimeout(-1);
 }
 
+Database &Database::getInstance()
+{
+    static Database instance;
+    return instance;
+}
+
+Database *Database::create(QQmlEngine *qmlEngine, QJSEngine *)
+{
+    Database &instance = getInstance();
+    Q_ASSERT(qmlEngine->thread() == instance.thread());
+
+    static QQmlEngine *engine = nullptr;
+    if (engine) {
+        Q_ASSERT(qmlEngine == engine);
+    } else {
+        engine = qmlEngine;
+    }
+
+    QJSEngine::setObjectOwnership(&instance, QJSEngine::CppOwnership);
+
+    return &instance;
+}
+
 QFuture<void> Database::open(const QString &user,
                              const QString &password,
                              const QString &hostName,

src/database.h

@@ -3,8 +3,9 @@
 
 #include <QFuture>
 #include <QObject>
+#include <QQmlEngine>
 #include <QThreadPool>
-#include <QtQmlIntegration>
+#include <QtConcurrent>
 
 class Database : public QObject
 {
@@ -13,7 +14,9 @@ class Database : public QObject
     QML_ELEMENT
 
 public:
-    explicit Database(QObject *parent = nullptr);
+    static Database &getInstance();
+
+    static Database *create(QQmlEngine *qmlEngine, QJSEngine *);
 
     Q_INVOKABLE QFuture<void> open(const QString &user,
                                    const QString &password,
@@ -24,12 +27,21 @@ public:
                                    const QString &connectOptions);
     Q_INVOKABLE QFuture<void> close();
 
+    template<class Function>
+    static auto query(Function &&f)
+    {
+        return QtConcurrent::run(&getInstance().m_pool, f);
+    }
+
 signals:
     void closed();
     void errorOcurred(const QString &errorMessage);
     void opened();
 
 private:
+    explicit Database(QObject *parent = nullptr);
+    Q_DISABLE_COPY_MOVE(Database)
+
     QThreadPool m_pool;
 };
 

src/main.cpp

@@ -1,5 +1,6 @@
 #include <QGuiApplication>
 #include <QQmlApplicationEngine>
+#include "database.h"
 
 int main(int argc, char *argv[])
 {
@@ -8,6 +9,7 @@ int main(int argc, char *argv[])
 
     QGuiApplication app(argc, argv);
 
+    Database::getInstance(); // return ignored; create instance before QML engine
     QQmlApplicationEngine engine;
     QObject::connect(
         &engine,