cppmodels.qdoc

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// Copyright (C) 2017 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\page qtquick-modelviewsdata-cppmodels.html
\title Using C++ Models with Qt Quick Views
\brief using Qt Quick views with models defined in C++
 
 
\section1 Data Provided In A Custom C++ Model
 
Models can be defined in C++ and then made available to QML. This is useful
for exposing existing C++ data models or otherwise complex datasets to QML.
 
A C++ model class can be defined as a \l QStringList, a \l {QVariant::}{QVariantList}, a
QObjectList or a \l QAbstractItemModel. The first three are useful for exposing
simpler datasets, while QAbstractItemModel provides a more flexible solution for
more complex models.
 
Here is a video tutorial that takes you through the whole process of exposing a C++
model to QML:
 
\youtube 9BcAYDlpuT8
 
\section2 QStringList-based Model
 
A model may be a simple \l QStringList, which provides the contents of the list
via the \e modelData role.
 
Here is a ListView with a delegate that references its model item's
value using the \c modelData role:
 
\snippet models/stringlistmodel/view.qml 0
 
A Qt application can load this QML document and set the value of \c myModel
to a QStringList:
 
\snippet models/stringlistmodel/main.cpp 0
 
The complete source code for this example is available in
\l {models/stringlistmodel}{examples/quick/models/stringlistmodel}
within the Qt install directory.
 
\note There is no way for the view to know that the contents of a QStringList
have changed. If the QStringList changes, it will be necessary to reset
the model by setting the view's \c model property again.
 
\section2 QVariantList-based Model
 
A model may be a single \l {QVariant::}{QVariantList}, which provides the contents
of the list via the \e modelData role.
 
The API works just like with \l QStringList, as shown in the previous section.
 
\note There is no way for the view to know that the contents of a QVariantList
have changed. If the QVariantList changes, it will be necessary to reset
the model.
 
\section2 QObjectList-based Model
 
A list of QObject* values can also be used as a model. A QList<QObject*> provides
the properties of the objects in the list as roles.
 
The following application creates a \c DataObject class with
Q_PROPERTY values that will be accessible as named roles when a
QList<DataObject*> is exposed to QML:
 
\snippet models/objectlistmodel/dataobject.h 0
\dots 4
\snippet models/objectlistmodel/dataobject.h 1
\codeline
\snippet models/objectlistmodel/main.cpp 0
\dots
 
The QObject* is available as the \c modelData property.  As a convenience,
the properties of the object are also made available directly in the
delegate's context. Here, \c view.qml references the \c DataModel properties in
the ListView delegate:
 
\snippet models/objectlistmodel/view.qml 0
 
Note the use of the \c color property. You can require existing properties
by declaring them as \c required in a derived type.
 
The complete source code for this example is available in
\l {models/objectlistmodel}{examples/quick/models/objectlistmodel}
within the Qt install directory.
 
Note: There is no way for the view to know that the contents of a QList
has changed.  If the QList changes, it is necessary to reset
the model by setting the \c model property again.
 
 
\section2 QAbstractItemModel Subclass
 
A model can be defined by subclassing QAbstractItemModel. This is the
best approach if you have a more complex model that cannot be supported
by the other approaches. A QAbstractItemModel can also automatically
notify a QML view when the model data changes.
 
The roles of a QAbstractItemModel subclass can be exposed to QML by
reimplementing QAbstractItemModel::roleNames().
 
Here is an application with a QAbstractListModel subclass named \c AnimalModel,
which exposes the \e type and \e sizes roles. It reimplements
QAbstractItemModel::roleNames() to expose the role names, so that they can be
accessed via QML:
 
\snippet models/abstractitemmodel/model.h 0
\dots
\snippet models/abstractitemmodel/model.h 1
\dots
\snippet models/abstractitemmodel/model.h 2
\codeline
\snippet models/abstractitemmodel/model.cpp 0
\codeline
\snippet models/abstractitemmodel/main.cpp 0
\dots
 
This model is displayed by a ListView delegate that accesses the \e type and \e size
roles:
 
\snippet models/abstractitemmodel/view.qml 0
 
QML views are automatically updated when the model changes. Remember the model
must follow the standard rules for model changes and notify the view when
the model has changed by using QAbstractItemModel::dataChanged(),
QAbstractItemModel::beginInsertRows(), and so on. See the \l {Model subclassing reference} for
more information.
 
The complete source code for this example is available in
\l {models/abstractitemmodel}{examples/quick/models/abstractitemmodel}
within the Qt install directory.
 
QAbstractItemModel presents a hierarchy of tables, but the views currently provided by QML
can only display list data.
In order to display the child lists of a hierarchical model,
use the DelegateModel QML type, which provides the following properties and functions to be used
with list models of QAbstractItemModel type:
 
\list
\li \e hasModelChildren role property to determine whether a node has child nodes.
\li \l DelegateModel::rootIndex allows the root node to be specified
\li \l DelegateModel::modelIndex() returns a QModelIndex which can be assigned to DelegateModel::rootIndex
\li \l DelegateModel::parentModelIndex() returns a QModelIndex which can be assigned to DelegateModel::rootIndex
\endlist
 
\section2 SQL Models
 
Qt provides C++ classes that support SQL data models. These classes work
transparently on the underlying SQL data, reducing the need to run SQL
queries for basic SQL operations such as create, insert, or update.
For more details about these classes, see \l{Using the SQL Model Classes}.
 
Although the C++ classes provide complete feature sets to operate on SQL
data, they do not provide data access to QML. So you must implement a
C++ custom data model as a subclass of one of these classes, and expose it
to QML either as a type or context property.
 
\section3 Read-only Data Model
 
The custom model must reimplement the following methods to enable read-only
access to the data from QML:
 
\list
\li \l{QAbstractItemModel::}{roleNames}() to expose the role names to the
    QML frontend. For example, the following version returns the selected
    table's field names as role names:
    \code
     QHash<int, QByteArray> SqlQueryModel::roleNames() const
     {
        QHash<int, QByteArray> roles;
        // record() returns an empty QSqlRecord
        for (int i = 0; i < this->record().count(); i ++) {
            roles.insert(Qt::UserRole + i + 1, record().fieldName(i).toUtf8());
        }
        return roles;
    }
    \endcode
\li \l{QSqlQueryModel::}{data}() to expose SQL data to the QML frontend.
    For example, the following implementation returns data for the given
    model index:
    \code
    QVariant SqlQueryModel::data(const QModelIndex &index, int role) const
    {
        QVariant value;
 
        if (index.isValid()) {
            if (role < Qt::UserRole) {
                value = QSqlQueryModel::data(index, role);
            } else {
                int columnIdx = role - Qt::UserRole - 1;
                QModelIndex modelIndex = this->index(index.row(), columnIdx);
                value = QSqlQueryModel::data(modelIndex, Qt::DisplayRole);
            }
        }
        return value;
    }
    \endcode
\endlist
 
The QSqlQueryModel class is good enough to implement a custom read-only
model that represents data in an SQL database. The
\l{Qt Quick Controls - Chat Tutorial}{chat tutorial} example
demonstrates this very well by implementing a custom model to fetch the
contact details from an SQLite database.
 
\section3 Editable Data Model
 
QSqlTableModel implements setData() as described \l{#changing-model-data}{below}.
 
Depending on the \l{QSqlTableModel::}{EditStrategy} used by the model, the
changes are either queued for submission later or submitted immediately.
 
You can also insert new data into the model by calling
\l {QSqlTableModel::insertRecord}(). In the following example snippet,
a QSqlRecord is populated with book details and appended to the
model:
 
\code
    ...
    QSqlRecord newRecord = record();
    newRecord.setValue("author", "John Grisham");
    newRecord.setValue("booktitle", "The Litigators");
    insertRecord(rowCount(), newRecord);
    ...
\endcode
 
\section2 Exposing C++ Data Models to QML
 
The above examples use required properties on the view to set
model values directly in QML components. An alternative to this is to
register the C++ model class as a QML type (see
\l{Defining QML Types from C++}). This allows the model classes to be
created directly as types within QML:
 
\table
\row
 
\li C++
\li
\code
class MyModel : public QAbstractItemModel
{
    Q_OBJECT
    QML_ELEMENT
 
    // [...]
}
\endcode
\row
\li QML
\li
\qml
MyModel {
    id: myModel
}
\endqml
 
\qml
ListView {
    width: 200; height: 250
    model: myModel
    delegate: Text {
        required property string someProperty
        text: someProperty
    }
}
\endqml
 
\endtable
 
See \l {Writing QML Extensions with C++} for details on writing QML types
in C++.
 
\section2 Changing Model Data
 
Besides the \c roleNames() and \c data(), editable models must reimplement
the \l{QAbstractItemModel::}{setData} method to save changes to existing model data.
The following version of the method checks if the given model index is valid
and the \c role is equal to \l Qt::EditRole:
 
\code
bool EditableModel::setData(const QModelIndex &index, const QVariant &value, int role)
{
    if (index.isValid() && role == Qt::EditRole) {
        // Set data in model here. It can also be a good idea to check whether
        // the new value actually differs from the current value
        if (m_entries[index.row()] != value.toString()) {
            m_entries[index.row()] = value.toString();
            emit dataChanged(index, index, { Qt::EditRole, Qt::DisplayRole });
            return true;
        }
    }
    return false;
}
\endcode
 
\note It is important to emit the \l{QAbstractItemModel::}{dataChanged}()
signal after saving the changes.
 
Unlike the C++ item views such as QListView or QTableView, the \c setData()
method must be explicitly invoked from QML delegates whenever appropriate. This is done
by simply assigning a new value to the corresponding model property.
 
\qml
ListView {
    anchors.fill: parent
    model: EditableModel {}
    delegate: TextField {
        width: ListView.view.width
        text: model.edit
        onAccepted: model.edit = text
    }
}
\endqml
 
\note The \c edit role is equal to \l Qt::EditRole. See \l{QAbstractItemModel::}{roleNames}()
for the built-in role names. However, real life models would usually register custom roles.
 
*/