Cannot copy the text of TextField by the copy method

[tham]

Try to implement a right click menu like QLineEdit, but I can't make the copy method work, I guess this is because the selectedText of the TextField always empty.

import QtQuick 2.9

import QtQuick.Controls 2.2
import QtQuick.Layouts 1.3

TextField{
    id: textField

    height: 20
    width: 100
    selectByMouse: true

    MouseArea{
        anchors.fill: parent
        propagateComposedEvents: true
        acceptedButtons: Qt.RightButton
        onClicked: {
            menuDialog.open()
        }
    }

    Dialog {
        id: menuDialog

        width: 300

        x: (parent.width - width) / 2
        y: (parent.height - height) / 2

        title: qsTr("Menu")

        ColumnLayout{
            anchors.fill: parent
            spacing: 10

            Button{
                id: copyBtn

                text: qsTr("Copy")

                Layout.fillWidth: true

                onClicked: {
                    textField.copy() //copy do not work
                    console.log("copy text:" + textField.selectedText) //selectedText always empty
                    menuDialog.close()
                }
            }

            Button{
                id: pasteBtn

                text: qsTr("Paste")

                Layout.fillWidth: true

                onClicked: {
                    console.log("paste text")
                    menuDialog.close()
                    textField.paste() //paste function work, copy by keyboard
                }
            }
        }
    }
}

How could I select the text after I press on the "copy" button(opened by the right button of the mouse)?

[tham]

Just find out why, problem is, every time I opened the Dialog, the selectionText will become empty, one of the solution is set persistentSelection to true.