python - several questions about QML and PySide2

Question

I have the following cases, I want to use several Qml: "welcome.qml", "create.qml", "dashboard.qml"

in which cases to use QQuickview or QqmlApplicationEngine.?

I am using "QQmlAplicatiobEngine" and search in the object with findChild to get the signal, and handle the logic, If the signal completes a condition, I use the engine.load to load another QML.

python:

class guiBackend(QObject):

    def __init__(self):
        self.engine = QQmlApplicationEngine()
        self.context = self.engine.rootContext()
        self.context.setContextProperty("main", self.engine)
        self.welcome()

    def welcome(self):
        self.engine.load("welcome.qml")
        self.engine.rootObjects()[0].show()
        ob = self.engine.rootObjects()[0]
        next = ob.findChild(QObject, "timer")
        print(dir(next))
        if  path.exists('data.some'):
            next.change.connect(self.open_account)
        else:
            next.change.connect(self.create_account)
    def create(self):
        self.engine.rootObjects()[0].close()
        self.engine.load("create.qml")
        self.engine.rootObjects()[1].show()
        add = ob.findChild(QObject, "addWallet")
        recovery = ob.findChild(QObject, "recovery")
        add.change.connect(self.add_account)
        recovery.change.connect(self.recovery)
        #self.open_account load dashboard.qml and self.account load form.qml

if __name__ == "__main__":
    app = QGuiApplication(sys.argv)
    gui = guiBackend()
    sys.exit(app.exec_())

Qml:

ApplicationWindow {
    id: welcome
    width: 650; height: 390
    opacity: 1
    visible: true
    minimumHeight: 232
    minimumWidth:226
    title: "open account"
    flags: Qt.SplashScreen
    Item {

        id: element
        x: 9
        y: 88
        width: 560
        height: 300
        anchors.horizontalCenter: parent.horizontalCenter
        anchors.verticalCenter: parent.verticalCenter
        Timer {
        signal change
        objectName: "timer"
        interval: 5000; running: true; repeat: false
        onTriggered: change()
        }
        Image {
            id: image
            x: 130
            y: 60
            width: 342
            height: 188
            anchors.verticalCenterOffset: -56
            anchors.horizontalCenterOffset: 0
            anchors.verticalCenter: parent.verticalCenter
            anchors.horizontalCenter: parent.horizontalCenter
            source: "neirons_logo.png"
            fillMode: Image.PreserveAspectFit
        }

        AnimatedImage {
            id: animatedImage
            x: 236
            y: 200
            width: 100
            height: 100
            source: "loading.gif"
        }
    }
}

create.qml:

ApplicationWindow {
    id: create
    width: 210; height: 210

    Rectangle {
        id: rectangle
        x: 70
        y: 132
        width: 200
        height: 200
        color: "#72ded8"
        anchors.verticalCenterOffset: 0
        anchors.horizontalCenterOffset: 0
        anchors.horizontalCenter: parent.horizontalCenter
        anchors.verticalCenter: parent.verticalCenter

        ColumnLayout {
            x: 60
            y: 73
            width: 109
            height: 128
            anchors.verticalCenter: parent.verticalCenter
            anchors.horizontalCenter: parent.horizontalCenter

            TextInput {
                id: nameAccount
                objectName: textAccount
                text: qsTr("nameAccount")
                Layout.fillWidth: true
                Layout.alignment: Qt.AlignHCenter | Qt.AlignTop
                Layout.preferredHeight: 20
                Layout.preferredWidth: 80
                verticalAlignment: Text.AlignVCenter
                font.pixelSize: 12
            }

            TextInput {
                id: nameAccount
                objectName: textAccount
                text: qsTr("Name Account")
                Layout.fillWidth: true
                Layout.alignment: Qt.AlignHCenter | Qt.AlignTop
                Layout.preferredHeight: 20
                Layout.preferredWidth: 80
                font.pixelSize: 12
            }
        }

        Button {
            signal addinfo
            id: submitNameAccount
            objectName: submit
            x: 50
            y: 134
            width: 81
            height: 23
            text: qsTr("Add")
            font.bold: true
            font.pointSize: 12
            anchors.horizontalCenter: parent.horizontalCenter
            onClicked: addinfo()
    }


}

It's better with QQuickview or QQmlAplicationEngine.

Answer 1

From what the OP provides, I will point out the following aspects:

• When should QQmlAplicationEngine or QQuickview be used? is better QQmlAplicationEngine or QQuickview ?

  The use of one or the other depends on the root element of the QML, if the root is Window or ApplicationWindow then you must use QQmlAplicationEngine, if instead it is an Item or its derivatives you can use QQuickView. So for the above one is not better than another. What happens if I load a QML with root Window or ApplicationWindow with QQuickView? Then it will show 2 windows: One will be from the QQuickView and the other from the Window or ApplicationWindow. What if I load a QML with Item with QQmlApplicationEngine? Well you will need to place it inside a Window as indicated by the docs:

  Unlike QQuickView, QQmlApplicationEngine does not automatically create a root window. If you are using visual items from Qt Quick, you will need to place them inside of a Window.

• Do not access QML elements from python/C++

  QML has its own handling of the variables so you could delete it or create it at any time, there is nothing determined, so accessing these objects can be dangerous since they may not have allocated memory. The right thing is to do the opposite, export a QObject to QML and make connections in QML.

---

We will apply the above concepts to the code provided by the OP (there are types that I have corrected).

• First of all, since the roots are ApplicationWindow, then QQmlApplicationEngine should be used.

• Instead of creating the "change" signal in each element, you can export the backend to QML using setContextProperty and then call the slots directly. To obtain the object using rootObjects() is dangerous since there are qmls that are loaded asynchronously, instead use the objectCreated signal.

main.py

import os

from PyQt5 import QtCore, QtGui, QtQml

CURRENT_DIR = os.path.dirname(os.path.realpath(__file__))


class Backend(QtCore.QObject):
    def __init__(self, parent=None):
        super().__init__(parent)

        self._engine = QtQml.QQmlApplicationEngine()
        self._welcome = None
        self._wallet = None
        self.engine.objectCreated.connect(self.on_object_created)
        self.engine.rootContext().setContextProperty("backend", self)

        self.load_welcome()

    @property
    def engine(self):
        return self._engine

    @property
    def welcome(self):
        return self._welcome

    @property
    def wallet(self):
        return self._wallet

    @staticmethod
    def create_url(qml):
        return QtCore.QUrl.fromLocalFile(os.path.join(CURRENT_DIR, qml))

    def load_welcome(self):
        self.engine.load(self.create_url("welcome.qml"))

    @QtCore.pyqtSlot(QtCore.QObject, QtCore.QUrl)
    def on_object_created(self, obj, url):
        if url == self.create_url("welcome.qml"):
            self._welcome = obj
        elif url == self.create_url("wallet.qml"):
            self._wallet = obj

    @QtCore.pyqtSlot()
    def create_wallet(self):
        self.welcome.close()
        self.engine.load(self.create_url("wallet.qml"))

    @QtCore.pyqtSlot()
    def add_info(self):
        print("add_info")


if __name__ == "__main__":
    import sys

    app = QtGui.QGuiApplication(sys.argv)
    backend = Backend()

    sys.exit(app.exec_())

welcome.qml

import QtQuick 2.14
import QtQuick.Controls 2.14

ApplicationWindow {
    id: root
    width: 650; height: 390
    opacity: 1
    visible: true
    minimumHeight: 232
    minimumWidth:226
    title: "open account"
    flags: Qt.SplashScreen
    Item {
        id: element
        x: 9
        y: 88
        width: 560
        height: 300
        anchors.horizontalCenter: parent.horizontalCenter
        anchors.verticalCenter: parent.verticalCenter
        Timer {
            interval: 5000; running: true; repeat: false
            onTriggered: backend.create_wallet()
        }
        Image {
            id: image
            x: 130
            y: 60
            width: 342
            height: 188
            anchors.verticalCenterOffset: -56
            anchors.horizontalCenterOffset: 0
            anchors.verticalCenter: parent.verticalCenter
            anchors.horizontalCenter: parent.horizontalCenter
            source: "neirons_logo.png"
            fillMode: Image.PreserveAspectFit
        }

        AnimatedImage {
            id: animatedImage
            x: 236
            y: 200
            width: 100
            height: 100
            source: "loading.gif"
        }
    }
}

wallet.qml

import QtQuick 2.14
import QtQuick.Controls 2.14
import QtQuick.Layouts 1.14


ApplicationWindow {
    id: root
    visible: true
    width: 210; height: 210

    Rectangle {
        id: rectangle
        x: 70
        y: 132
        width: 200
        height: 200
        color: "#72ded8"
        anchors.verticalCenterOffset: 0
        anchors.horizontalCenterOffset: 0
        anchors.horizontalCenter: parent.horizontalCenter
        anchors.verticalCenter: parent.verticalCenter

        ColumnLayout {
            x: 60
            y: 73
            width: 109
            height: 128
            anchors.verticalCenter: parent.verticalCenter
            anchors.horizontalCenter: parent.horizontalCenter

            TextInput {
                id: nameAccount1
                text: qsTr("nameAccount")
                Layout.fillWidth: true
                Layout.alignment: Qt.AlignHCenter | Qt.AlignTop
                Layout.preferredHeight: 20
                Layout.preferredWidth: 80
                verticalAlignment: Text.AlignVCenter
                font.pixelSize: 12
            }

            TextInput {
                id: nameAccount2
                text: qsTr("Name Account")
                Layout.fillWidth: true
                Layout.alignment: Qt.AlignHCenter | Qt.AlignTop
                Layout.preferredHeight: 20
                Layout.preferredWidth: 80
                font.pixelSize: 12
            }
        }

        Button {
            id: submitNameAccount
            x: 50
            y: 134
            width: 81
            height: 23
            text: qsTr("Add")
            font.bold: true
            font.pointSize: 12
            anchors.horizontalCenter: parent.horizontalCenter
            onClicked: backend.add_info()
        }
    }
}