Extending QML - Extension Objects Example

This example builds on the the Extending QML - Adding Types Example.

Shows how to use @QmlExtended to provide an extension object to a QLineEdit without modifying or subclassing it.

Firstly, the LineEditExtension class is registered with the QML system as an extension of QLineEdit. We declare a foreign type to do this as we cannot modify Qt’s internal QLineEdit class.

@QmlNamedElement("QLineEdit")
@QmlExtended(LineEditExtension)
@QmlForeign(QLineEdit)
class LineEditForeign(QObject):

Note the usage of @QmlNamedElement instead of @QmlElement. QmlElement() uses the name of the containing type by default, LineEditExtension in this case. As the class being an extension class is an implementation detail, we choose the more natural name QLineEdit instead.

The QML engine then instantiates a QLineEdit.

In QML, a property is set on the line edit that only exists in the LineEditExtension class:

QLineEdit {
    left_margin: 20
}

The extension type performs calls on the QLineEdit that otherwise will not be accessible to the QML engine.

Download this example

main.py

# Copyright (C) 2022 The Qt Company Ltd.
# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
from __future__ import annotations

"""PySide6 port of the qml/examples/qml/referenceexamples/extended example from Qt v6.x"""

from pathlib import Path
import sys

from PySide6.QtCore import QObject, QUrl, Property
from PySide6.QtWidgets import QApplication, QLineEdit
from PySide6.QtQml import (QQmlComponent, QQmlEngine, QmlForeign, QmlExtended,
                           QmlNamedElement)


# To be used on the @QmlElement decorator
# (QML_IMPORT_MINOR_VERSION is optional)
QML_IMPORT_NAME = "examples.extend"
QML_IMPORT_MAJOR_VERSION = 1


class LineEditExtension(QObject):

    def __init__(self, parent=None):
        super().__init__(parent)
        self._line_edit = parent

    @Property(int)
    def left_margin(self):
        return self._line_edit.textMargins().left()

    @left_margin.setter
    def left_margin(self, m):
        margins = self._line_edit.textMargins()
        margins.setLeft(m)
        self._line_edit.setTextMargins(margins)

    @Property(int)
    def right_margin(self):
        return self._line_edit.textMargins().right()

    @right_margin.setter
    def right_margin(self, m):
        margins = self._line_edit.textMargins()
        margins.setRight(m)
        self._line_edit.setTextMargins(margins)

    @Property(int)
    def top_margin(self):
        return self._line_edit.textMargins().top()

    @top_margin.setter
    def top_margin(self, m):
        margins = self._line_edit.textMargins()
        margins.setTop(m)
        self._line_edit.setTextMargins(margins)

    @Property(int)
    def bottom_margin(self):
        return self._line_edit.textMargins().bottom()

    @bottom_margin.setter
    def bottom_margin(self, m):
        margins = self._line_edit.textMargins()
        margins.setBottom(m)
        self._line_edit.setTextMargins(margins)


@QmlNamedElement("QLineEdit")
@QmlExtended(LineEditExtension)
@QmlForeign(QLineEdit)
class LineEditForeign(QObject):

    def __init__(self, parent=None):
        super().__init__(parent)


if __name__ == '__main__':
    app = QApplication(sys.argv)

    qml_file = Path(__file__).parent / "example.qml"
    url = QUrl.fromLocalFile(qml_file)
    engine = QQmlEngine()
    component = QQmlComponent(engine, url)
    widget = component.create()
    if not widget:
        print(component.errors())
        del engine
        sys.exit(-1)

    widget.show()
    r = app.exec()
    # Deleting the engine before it goes out of scope is required to make sure
    # all child QML instances are destroyed in the correct order.
    del engine
    sys.exit(r)

example.qml

// Copyright (C) 2022 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

import examples.extend 1.0

QLineEdit {
    left_margin: 20
}