# ref: https://blenderartists.org/t/parent-pyqt-window-widget-to-blenders-window/700722/2

# install info
# note: here we assume you already pip install PySide2 already, no check pyside2 code here
bl_info={
    "name":"MyToolUI Launcher Addon",
    "description": "MyToolUI Launcher",
    "author": "YourName",
    "version": (1, 0, 0),
    "blender": (2, 80, 0),
    "location": "3D View > Tools",
    "category":"Development"
}

import bpy
from PySide2 import QtWidgets, QtCore

# optional single instant of MyToolUI
single_MyToolUI = None

# QT UI PART
class MyToolUI(QtWidgets.QWidget):
    
    def __init__(self, parent=None):
        super(MyToolUI, self).__init__(parent)
        
        main_layout = QtWidgets.QVBoxLayout()
        self.setLayout(main_layout)
        
        main_label = QtWidgets.QLabel("Example")
        main_input = QtWidgets.QLineEdit()
        main_button = QtWidgets.QPushButton("QButton to Toggle Always Top")
        main_layout.addWidget(main_label)
        main_layout.addWidget(main_input)
        main_layout.addWidget(main_button)
        
        main_button.clicked.connect(self.toggleAlwaysTopUI)
        
    def toggleAlwaysTopUI(self):
        self.setWindowFlags(self.windowFlags() ^ QtCore.Qt.WindowStaysOnTopHint)
        self.show()

# Blender UI PART
class MYTOOL_PT_MyTool_launcher_panel(bpy.types.Panel):
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = 'CoolTools' # the tab name
    bl_label = "MyTool Launcher" # tab tab > panel name
    
    def draw(self, context):
        layout = self.layout
        layout.operator('custom.mytool_show_op')
        
# Blender Operator Button to launch QT UI
class MYTOOL_OT_show_op(bpy.types.Operator):
    bl_idname = 'custom.mytool_show_op'
    bl_label = "Launch MyTool"
    bl_options = {'REGISTER'}
    
    def execute(self, context):
        self.app = QtWidgets.QApplication.instance()
        if not self.app:
            self.app = QtWidgets.QApplication(['blender'])
        self.event_loop = QtCore.QEventLoop()
        
        # single UI
        global single_MyToolUI
        if single_MyToolUI is None:
            single_MyToolUI = MyToolUI()
            
        self.widget = single_MyToolUI
        self.widget.show()
        return {'FINISHED'}

CLASSES = [
    MYTOOL_PT_MyTool_launcher_panel, 
    MYTOOL_OT_show_op
]

def register():
    for cls in CLASSES:
        bpy.utils.register_class(cls)
        
def unregister():
    for cls in CLASSES:
        bpy.utils.unregister_class(cls)
        
if __name__ == "__main__":
    register()