Default Prototypes Example | Qt Script

- Qt 5.5
- Qt Script
- Default Prototypes Example
- Qt 5.5.1 Reference Documentation

# Default Prototypes Example

This Qt Script example shows how to use default prototypes to make a non-QObject-based type scriptable.

With QScriptEngine::setDefaultPrototype() you can specify a Qt Script object that defines a scripting interface for a C++ type; Qt Script operations on values of such types will then be delegated to your prototype object. In this example, a simple scripting interface for QListWidgetItem is defined, so that the text of items can easily be accessed from script code.

To define a scripting API for QListWidgetItem in terms of Qt properties and slots, we subclass QObject and QScriptable.

    class ListWidgetItemPrototype : public QObject, public QScriptable
    {
        Q_OBJECT
        Q_PROPERTY(QString text READ text WRITE setText)
    public:
        ListWidgetItemPrototype(QObject *parent = 0);

        QString text() const;
        void setText(const QString &text);

    public slots:
        QString toString() const;
    };

A single property, `text`, is defined, along with a slot, `toString`.

    ListWidgetItemPrototype::ListWidgetItemPrototype(QObject *parent)
        : QObject(parent)
    {
    }

    QString ListWidgetItemPrototype::text() const
    {
        QListWidgetItem *item = qscriptvalue_cast<QListWidgetItem*>(thisObject());
        if (item)
            return item->text();
        return QString();
    }

    void ListWidgetItemPrototype::setText(const QString &text)
    {
        QListWidgetItem *item = qscriptvalue_cast<QListWidgetItem*>(thisObject());
        if (item)
            item->setText(text);
    }

    QString ListWidgetItemPrototype::toString() const
    {
        return QString("ListWidgetItem(text = %0)").arg(text());
    }

The implementation of the property accessors use the qscriptvalue_cast() function to cast the script object to a QListWidgetItem pointer. The normal C++ QListWidgetItem API is then used to implement the desired functionality.

Although not shown here, it is possible to throw a script exception from a prototype function; for example, you could throw a TypeError exception if the qscriptvalue_cast() fails.

QListWidgetItems are usually added to a QListWidget. While QListWidget is a QObject-based class, not all the functionality needed for this example are present. We can solve this by creating a default prototype for the QListWidget class as well. The prototype will augment the functionality already provided by the Qt Script QObject integration; i.e. if a property or slot is not found in the QListWidget object itself, the prototype will be used as a fallback.

    class ListWidgetPrototype : public QObject, public QScriptable
    {
        Q_OBJECT
    public:
        ListWidgetPrototype(QObject *parent = 0);

    public slots:
        void addItem(const QString &text);
        void addItems(const QStringList &texts);
        void setBackgroundColor(const QString &colorName);
    };

The additional slots will make it possible to add items to a QListWidget from script code, and to set the background color of the widget from a string.

    ListWidgetPrototype::ListWidgetPrototype(QObject *parent)
        : QObject(parent)
    {
    }

    void ListWidgetPrototype::addItem(const QString &text)
    {
        QListWidget *widget = qscriptvalue_cast<QListWidget*>(thisObject());
        if (widget)
            widget->addItem(text);
    }

    void ListWidgetPrototype::addItems(const QStringList &texts)
    {
        QListWidget *widget = qscriptvalue_cast<QListWidget*>(thisObject());
        if (widget)
            widget->addItems(texts);
    }

    void ListWidgetPrototype::setBackgroundColor(const QString &colorName)
    {
        QListWidget *widget = qscriptvalue_cast<QListWidget*>(thisObject());
        if (widget) {
    #ifdef Q_WS_MAEMO_5
            QString style = QString("QListWidget::item {background-color: %1;}").arg(colorName);
            style += "QListWidget::item {selection-color: black;}";
            widget->setStyleSheet(style);
    #else
            QPalette palette = widget->palette();
            QColor color(colorName);
            palette.setBrush(QPalette::Base, color);
            widget->setPalette(palette);
    #endif
        }
    }

Again, we use qscriptvalue_cast() to cast the script object to the relevant C++ type, in this case a QListWidget pointer. The addItem() and addItems() functions simply forward their arguments to the corresponding functions in the QListWidget class. setBackgroundColor() gets the widget's palette, creates a QColor from the given string argument and changes the palette accordingly.

    Q_DECLARE_METATYPE(QListWidgetItem*)
    Q_DECLARE_METATYPE(QListWidget*)

The relevant C++ types must be made known to Qt's meta type system.

        QScriptEngine engine;

        ListWidgetItemPrototype lwiProto;
        engine.setDefaultPrototype(qMetaTypeId<QListWidgetItem*>(),
                                   engine.newQObject(&lwiProto));

        ListWidgetPrototype lwProto;
        engine.setDefaultPrototype(qMetaTypeId<QListWidget*>(),
                                   engine.newQObject(&lwProto));

For each type that we want to associate a prototype object with, we create an instance of the prototype class, pass it to QScriptEngine::newQObject(), and then create the link between the C++ type and the resulting script object by calling QScriptEngine::setDefaultPrototype().

        QListWidget listWidget;
        engine.globalObject().setProperty("listWidget",
                                          engine.newQObject(&listWidget));

In this example, a single QListWidget object is added as a global script variable, called `listWidget`. Script code can add items to this widget by calling addItem() or addItems().

    listWidget.addItem("Red");
    listWidget.addItem("Blue");
    listWidget.addItem("Green");
    listWidget.addItem("Cyan");
    listWidget.addItem("Yellow");
    listWidget.addItem("Purple");
    listWidget.addItems(["Orange", "Gray"]);

Script code can connect to signals of the QListWidget object; signal handlers can use the interface defined in the QListWidgetItem prototype to manipulate item arguments.

    listWidget.currentItemChanged.connect(
        function(item)
        {
            listWidget.setBackgroundColor(item.text);
        }
    );

Not shown in this example is how to make QListWidgetItem constructible from Qt Script code, i.e. to be able to write "new QListWidgetItem()" in a script. In order to do this, you have to define your own script constructor for the type. The constructor would just be a factory function that constructs a new C++ QListWidgetItem and returns it back to the script. See QScriptEngine::newFunction() for more information.

Files:

- script/defaultprototypes/code.js
- script/defaultprototypes/prototypes.cpp
- script/defaultprototypes/prototypes.h
- script/defaultprototypes/main.cpp
- script/defaultprototypes/defaultprototypes.pro
- script/defaultprototypes/defaultprototypes.qrc