Extending QML - Default Property Example

This example builds on the Extending QML - Adding Types Example, the Extending QML - Object and List Property Types Example and the Extending QML - Inheritance and Coercion Example.

The Default Property Example is a minor modification of the Extending QML - Inheritance and Coercion Example that simplifies the specification of a BirthdayParty through the use of a default property.

Declaring the BirthdayParty Class

The only difference between this example and the last, is the addition of a DefaultProperty class info annotation.

The default property specifies the property to assign to whenever an explicit property is not specified, in the case of the BirthdayParty type the guest property. It is purely a syntactic simplification, the behavior is identical to specifying the property by name, but it can add a more natural feel in many situations. The default property must be either an object or list property.

Running the Example

The main.py file in the example includes a simple shell application that loads and runs the QML snippet shown below.

main.py

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

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

from pathlib import Path
import sys

from PySide6.QtCore import QCoreApplication, QUrl
from PySide6.QtQml import QQmlComponent, QQmlEngine

from person import Boy, Girl
from birthdayparty import BirthdayParty


app = QCoreApplication(sys.argv)
qml_file = Path(__file__).parent / "example.qml"
url = QUrl.fromLocalFile(qml_file)
engine = QQmlEngine()
component = QQmlComponent(engine, url)
party = component.create()
if not party:
    print(component.errors())
    del engine
    sys.exit(-1)
host = party.host
print(f"{host.name} is having a birthday!")
if isinstance(host, Boy):
    print("He is inviting:")
else:
    print("She is inviting:")
for g in range(party.guestCount()):
    name = party.guest(g).name
    print(f"    {name}")
del engine
sys.exit(0)

birthdayparty.py

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

from PySide6.QtCore import QObject, ClassInfo, Property
from PySide6.QtQml import QmlElement, ListProperty

from person import Person


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


@QmlElement
@ClassInfo(DefaultProperty="guests")
class BirthdayParty(QObject):

    def __init__(self, parent=None):
        super().__init__(parent)
        self._host = None
        self._guests = []

    @Property(Person)
    def host(self):
        return self._host

    @host.setter
    def host(self, h):
        self._host = h

    def guest(self, n):
        return self._guests[n]

    def guestCount(self):
        return len(self._guests)

    def appendGuest(self, guest):
        self._guests.append(guest)

    guests = ListProperty(Person, appendGuest)

person.py

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

from PySide6.QtCore import QObject, Property
from PySide6.QtQml import QmlAnonymous, QmlElement

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


@QmlAnonymous
class Person(QObject):
    def __init__(self, parent=None):
        super().__init__(parent)
        self._name = ''
        self._shoe_size = 0

    @Property(str)
    def name(self):
        return self._name

    @name.setter
    def name(self, n):
        self._name = n

    @Property(int)
    def shoe_size(self):
        return self._shoe_size

    @shoe_size.setter
    def shoe_size(self, s):
        self._shoe_size = s


@QmlElement
class Boy(Person):
    def __init__(self, parent=None):
        super().__init__(parent)


@QmlElement
class Girl(Person):
    def __init__(self, parent=None):
        super().__init__(parent)

example.qml

// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

import examples.default.people

BirthdayParty {
    host: Boy {
        name: "Bob Jones"
        shoe_size: 12
    }

    Boy { name: "Leo Hodges" }
    Boy { name: "Jack Smith" }
    Girl { name: "Anne Brown" }
}