QQmlApplicationEngine 列表索引超出范围问题

Posted 2023-02-15

【中文标题】QQmlApplicationEngine 列表索引超出范围问题

【英文标题】：QQmlApplicationEngine list index out of range issue

【发布时间】：2021-06-29 13:57:21

【问题描述】：

我正在尝试运行此 website 中的最后两个示例并最终得到 root = engine.rootObjects()[0] # type: QObject IndexError: list index out of range 两个示例的错误（根布局的信号连接）（其他 qml 文件的信号连接）

如果有帮助，我将所有示例文件放在同一目录级别，并将导入语句更改为 Pyside6（来自 PySide6.QtCore...）而不是 PySide2。如果代码更易于查看，我还添加了我的代码版本：

main.py:

import sys

from PySide6.QtCore import *
from PySide6.QtQml import QQmlApplicationEngine
from PySide6.QtWidgets import QApplication


def say(s):
    print(s)


if __name__ == '__main__':
    app = QApplication()
    engine = QQmlApplicationEngine()
    engine.load(QUrl.fromLocalFile('main.qml'))
    # Get the root object.
    root = engine.rootObjects()[0]  # type: QObject
    # Find the target object. Since our target object is Window, which is the root object. So use it directly.
    target_view = root
    # Bind signal.
    target_view.say.connect(say)  # The former one is the signal of qml, and the latter one is from Python
    # say() method.
    
    sys.exit(app.exec())

main.qml:

import QtQuick 2.14
import QtQuick.Controls 2.14
import QtQuick.Window 2.12

Window 
    visible: true
    width: 600; height: 400

    signal say(string s)

    Button 
        text: "hello"
        onClicked: say(text)
    

【问题讨论】：

通常这意味着您的 QML 代码存在问题，并且通常会打印出来。您在输出中看到任何错误吗？在任何情况下，您都应该在访问它之前检查 rootObjects() 是否返回空列表。在 C++ 中，处理错误的常用方法是连接到引擎的 objectCreated 信号并在对象为空时退出应用程序。不确定 Python 中的等价物是什么。

【参考方案1】：

错误的可能原因有：

.qml 的路径不正确，因此 .qml 未加载。 .qml 的加载并不像您假设的那样同步。 .qml 有一些错误（例如语法）

综合以上情况，解决办法是：

import os
import sys
from pathlib import Path

from PySide6.QtCore import QCoreApplication, Qt, QUrl
from PySide6.QtQml import QQmlApplicationEngine
from PySide6.QtWidgets import QApplication

CURRENT_DIRECTORY = Path(__file__).resolve().parent


def say(s):
    print(s)


if __name__ == "__main__":
    app = QApplication()
    engine = QQmlApplicationEngine()
    filename = os.fspath(CURRENT_DIRECTORY / "main.qml")
    url = QUrl.fromLocalFile(filename)

    def handle_object_created(obj, obj_url):
        if obj is None and url == obj_url:
            QCoreApplication.exit(-1)
        else:
            root = engine.rootObjects()[0]
            target_view = root
            target_view.say.connect(say)

    engine.objectCreated.connect(handle_object_created, Qt.QueuedConnection)
    engine.load(url)

    sys.exit(app.exec())

但无论如何我不喜欢使用 rootObjects 而是导出一个 QObject：

import os
import sys
from pathlib import Path

from PySide6.QtCore import QCoreApplication, QObject, Qt, QUrl, Signal
from PySide6.QtQml import QQmlApplicationEngine
from PySide6.QtWidgets import QApplication

CURRENT_DIRECTORY = Path(__file__).resolve().parent


class Helper(QObject):
    say = Signal(str)


def say(s):
    print(s)


if __name__ == "__main__":
    app = QApplication()
    engine = QQmlApplicationEngine()

    helper = Helper()
    helper.say.connect(say)

    engine.rootContext().setContextProperty("helper", helper)

    filename = os.fspath(CURRENT_DIRECTORY / "main.qml")
    url = QUrl.fromLocalFile(filename)

    def handle_object_created(obj, obj_url):
        if obj is None and url == obj_url:
            QCoreApplication.exit(-1)

    engine.objectCreated.connect(handle_object_created, Qt.QueuedConnection)
    engine.load(url)

    sys.exit(app.exec())

import QtQuick 2.14
import QtQuick.Controls 2.14
import QtQuick.Window 2.12

Window 
    visible: true
    width: 600
    height: 400

    Button 
        text: "hello"
        onClicked: helper.say(text)