Default Prototypes Example | Qt Script

Qt 5.14 | Qt Script | Default Prototypes Example

Qt 5.14.1 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.

Example project @ code.qt.io