PyQt5 中的前后端分离与 QML

Question

我想将 QML UI 添加到现有的 Python 线程任务中，但我不知道该怎么做才能不过多更改任务以保持前端和后端分离。

让我用一个最小化的例子来解释这个问题。我想用 QML 添加 GUI 来控制进程（或线程）的启动和停止，以及从进程中获取和显示信息。例如，以下是一些繁重的工作流程。

class Task(threading.Thread):
    def __init__(self):
        super().__init__()
        self.num = 0

    def run(self):
        for i in range(35):
            self.num = fib(i)
def fib(N):
    if N <= 1:
        return 1
    return fib(N - 1) + fib(N - 2)

现在，我尝试创建一个 QML 文件，以在 Task 之后的 Task().start() 中获取 self.num 而不会阻塞。

import QtQuick 2.12
import QtQuick.Controls 2.12
ApplicationWindow {
    visible: true
    Column {
        Button {
            text: 'hello'
            onClicked: {
                backend.start_process()
            }
        }
        Text {
            text: backend.num
        }
    }
}

为了保持前后端分离，有什么方法可以在不更改Task 中的任何脚本内容的情况下将Task 连接到QML UI？根据this question，我尝试创建一个Backend 类来处理前端和后端之间的任何通信，并将上下文属性设置为QML (engine.rootContext().setContextProperty("backend", Backend()))。例如，

class Backend(PyQt5.QtCore.QObject):
    changed = PyQt5.QtCore.pyqtSignal(int)

    def __init__(self):
        super().__init__()
        self._num = 0

    @PyQt5.QtCore.pyqtProperty(int, notify=changed)
    def num(self):
        return self._num

    @num.setter
    def num(self, val):
        if self._num == val:
            return
        self._num = val
        self.changed.emit(self._num)

    @PyQt5.QtCore.pyqtSlot()
    def start_process(self):
        t = Task()
        t.start()

不过，我不知道如何双向绑定backend 中的self.num 和t.num。因此，在 QML UI 中的按钮调用 start_process() 后，我无法实时更新 t.num。

我想尽量减少对Task 的修改，因为它是旧程序的核心功能。因此，Simple Example in PyQt5 website 无法满足我的要求。

此外，如果我想将 Python threading.Thread 更改为 multiprocessing.Process，我应该怎么做才能在每次更新过程中获取变量以便在 QML UI 上显示？

Answer 1

首先，您不需要像 qproperty 这样的设置器，因为根据您的逻辑，您不应该从 QML 修改 qproperty。另一方面，您必须通过 args 或 kwargs 将后端传递给您的线程，并发送调用插槽的信息：

import threading
import time
from PyQt5 import QtCore, QtGui, QtQml

class Task(threading.Thread):
    def run(self):
        self.backend = self._kwargs.get('backend')
        for i in range(35):
            num = fib(i)
            if self.backend is not None:
                QtCore.QMetaObject.invokeMethod(
                    self.backend, 
                    "set_num",
                    QtCore.Qt.QueuedConnection, 
                    QtCore.Q_ARG(int, num)
                )
                time.sleep(0.01)
def fib(N):
    if N <= 1:
        return 1
    return fib(N - 1) + fib(N - 2)

class Backend(QtCore.QObject):
    changed = QtCore.pyqtSignal(int)

    def __init__(self):
        super().__init__()
        self._num = 0

    @QtCore.pyqtProperty(int, notify=changed)
    def num(self):
        return self._num

    @QtCore.pyqtSlot(int)
    def set_num(self, val):
        if self._num == val:
            return
        self._num = val
        self.changed.emit(self._num)

    @QtCore.pyqtSlot()
    def start_process(self):
        Task(kwargs=dict(backend=self), daemon=True).start()

if __name__ == "__main__":
    import os
    import sys

    app = QtGui.QGuiApplication(sys.argv)
    backend = Backend()
    engine = QtQml.QQmlApplicationEngine()
    engine.rootContext().setContextProperty("backend", backend)
    qml_path = os.path.join(os.path.dirname(__file__), "main.qml")
    engine.load(QtCore.QUrl.fromLocalFile(qml_path))
    if not engine.rootObjects():
        sys.exit(-1)
    engine.quit.connect(app.quit)
    sys.exit(app.exec_())

---

另一种方法是创建一个包含信息的 QEvent：

import threading
import time
from PyQt5 import QtCore, QtGui, QtQml

class InfoEvent(QtCore.QEvent):
    EventType = QtCore.QEvent.User

    def __init__(self, info):
        super(InfoEvent, self).__init__(InfoEvent.EventType)
        self._info = info

    @property
    def info(self):
        return self._info

class Task(threading.Thread):
    def run(self):
        self.backend = self._kwargs.get('backend')
        for i in range(35):
            num = fib(i)
            if self.backend is not None:
                event = InfoEvent(num)
                QtCore.QCoreApplication.sendEvent(self.backend, event)
                time.sleep(0.01)
def fib(N):
    if N <= 1:
        return 1
    return fib(N - 1) + fib(N - 2)

class Backend(QtCore.QObject):
    changed = QtCore.pyqtSignal(int)

    def __init__(self):
        super().__init__()
        self._num = 0

    def event(self, e):
        if e.type() == InfoEvent.EventType:
            self.set_num(e.info)
            return True
        return super(Backend, self).event(e)

    @QtCore.pyqtProperty(int, notify=changed)
    def num(self):
        return self._num

    @QtCore.pyqtSlot(int)
    def set_num(self, val):
        if self._num == val:
            return
        self._num = val
        self.changed.emit(self._num)

    @QtCore.pyqtSlot()
    def start_process(self):
        Task(kwargs=dict(backend=self), daemon=True).start()

if __name__ == "__main__":
    import os
    import sys

    app = QtGui.QGuiApplication(sys.argv)
    backend = Backend()
    engine = QtQml.QQmlApplicationEngine()
    engine.rootContext().setContextProperty("backend", backend)
    qml_path = os.path.join(os.path.dirname(__file__), "main.qml")
    engine.load(QtCore.QUrl.fromLocalFile(qml_path))
    if not engine.rootObjects():
        sys.exit(-1)
    engine.quit.connect(app.quit)
    sys.exit(app.exec_())

---

另一种解决方案是它不再继承自threading.Thread，而是继承自QThread，它实现了一些向主线程发送信息的信号。

from PyQt5 import QtCore, QtGui, QtQml

class Task(QtCore.QThread):
    valueChanged = QtCore.pyqtSignal(int)

    def run(self):
        for i in range(35):
            num = fib(i)
            self.valueChanged.emit(num)
            QtCore.QThread.msleep(1)

def fib(N):
    if N <= 1:
        return 1
    return fib(N - 1) + fib(N - 2)

class Backend(QtCore.QObject):
    changed = QtCore.pyqtSignal(int)

    def __init__(self):
        super().__init__()
        self._num = 0

    @QtCore.pyqtProperty(int, notify=changed)
    def num(self):
        return self._num

    @QtCore.pyqtSlot(int)
    def set_num(self, val):
        if self._num == val:
            return
        self._num = val
        self.changed.emit(self._num)

    @QtCore.pyqtSlot()
    def start_process(self):
        thread = Task(self)
        thread.valueChanged.connect(self.set_num)
        thread.start()

if __name__ == "__main__":
    import os
    import sys

    app = QtGui.QGuiApplication(sys.argv)
    backend = Backend()
    engine = QtQml.QQmlApplicationEngine()
    engine.rootContext().setContextProperty("backend", backend)
    qml_path = os.path.join(os.path.dirname(__file__), "main.qml")
    engine.load(QtCore.QUrl.fromLocalFile(qml_path))
    if not engine.rootObjects():
        sys.exit(-1)
    engine.quit.connect(app.quit)
    sys.exit(app.exec_())