扩展 QML（高级）- 默认属性

这是 6 个示例系列的第三个，该系列通过生日派对示例演示了 QML 的一些高级功能。

目前，在 QML 文件中，每个属性都需要显式分配。例如，host 属性被分配给 Boy，而 guests 属性被分配为一个包含 Boy 或 Girl 对象的列表。这样做虽然简单，但针对这种特定场景可以简化处理。我们不需要显式分配 guests 属性，可以直接在派对中添加 Boy 和 Girl 对象，并自动将它们赋给 guests。体面的说法是，我们指定的所有参与者，除主持人以外，都是客人。这种改变纯粹是语法的，但在许多情况下可以使感觉更自然。

可以将 guests 属性指定为 BirthdayParty 的默认属性。这意味着在 BirthdayParty 内创建的每个对象都会隐式地附加到默认属性 guests 上。结果 QML 看起来如下。

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

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

要启用此行为，只需将 DefaultProperty 类信息注释添加到 BirthdayParty 中，将 guests 指定为其默认属性。

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

你可能已经熟悉这种机制。在 QML 中，所有 Item 后代的默认属性是 data 属性。所有未显式添加到 Item 属性中的元素都将添加到 data 中。这使得结构更清晰，并在代码中减少了不必要的噪声。

下载 此 示例

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/tutorials/extending-qml-advanced/default advanced3-Default-properties example
   from Qt v6.x"""

from pathlib import Path
import sys

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

from person import Boy, Girl  # noqa: F401
from birthdayparty import BirthdayParty  # noqa: F401


app = QCoreApplication(sys.argv)
engine = QQmlEngine()
engine.addImportPath(Path(__file__).parent)
component = QQmlComponent(engine)
component.loadFromModule("People", "Main")
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, Signal
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 = "People"
QML_IMPORT_MAJOR_VERSION = 1


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

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

    @Property(Person, notify=host_changed, final=True)
    def host(self):
        return self._host

    @host.setter
    def host(self, h):
        if self._host != h:
            self._host = h
            self.host_changed.emit()

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

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

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

    guests = ListProperty(Person, appendGuest, notify=guests_changed, final=True)

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, Signal
from PySide6.QtQml import QmlAnonymous, QmlElement

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


@QmlAnonymous
class Person(QObject):
    name_changed = Signal()
    shoe_size_changed = Signal()

    def __init__(self, parent=None):
        super().__init__(parent)
        self._name = ''
        self._shoe_size = 0

    @Property(str, notify=name_changed, final=True)
    def name(self):
        return self._name

    @name.setter
    def name(self, n):
        if self._name != n:
            self._name = n
            self.name_changed.emit()

    @Property(int, notify=shoe_size_changed, final=True)
    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)

People/Main.qml

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

import People

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

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

People/qmldir

module People
typeinfo coercion.qmltypes
Main 1.0 Main.qml