Swapping Between multiple QGraphicsViews / QGraphicsScenes

S Tozer

Hello -

In my application, I have a class which contains both a QGraphicsView and an associated QGraphics scene (I would like to have one view per graphics scene in order to retain things like pan/zoom per scene).

I would like to swap between multiple instances of this class in my application's main viewport, but I'm unsure what to actually set as the Main window's central widget here... if I set the first instance of my custom class as the central widget, and then try to to re-set the central widget with another instance of that class, I produce this error:

wrapped C/C++ object of type SceneAndView has been deleted

Obviously this is not the right approach and the error is expected, since Qt will apparently delete the original central widget as it is replaced by another widget, as written about here:

https://stackoverflow.com/questions/5339062/python-pyside-internal-c-object-already-deleted

In the above link, there is a reference to QStackedWidget. I played around with it but was unable to get a workable result. So, I was wondering if anyone could help out with the code below? The result I'm looking for is to be able to toggle between 'self.graphics_scene_1' and 'self.graphics_scene_2' in the main viewport.

Any advice or help would be greatly appreciated, thankyou,
Ekar

import sys

from PyQt5.QtCore import Qt
from PyQt5.QtGui import QBrush, QPen
from PyQt5.QtWidgets import *

class SceneAndView(QWidget): 

    """
    Class containing both a QGraphicsView and an associated QGraphicsScene
    """

    def __init__(self, parent=None):
        super().__init__(parent)

        # setup layout
        self.layout = QVBoxLayout()
        self.setLayout(self.layout)

        # create graphics scene
        self.gfx_scene = QGraphicsScene(self)
        self.gfx_scene.setSceneRect(0, 0, 200, 200)
        
        # create graphics view
        self.view = QGraphicsView(self.gfx_scene)
        self.layout.addWidget(self.view)

        # define and add some graphics items
        rect = QGraphicsRectItem(0, 0, 200, 50)
        rect.setPos(50, 20)
        brush = QBrush(Qt.red)
        rect.setBrush(brush)
        pen = QPen(Qt.cyan)
        pen.setWidth(10)
        rect.setPen(pen)
        ellipse = QGraphicsEllipseItem(0, 0, 100, 100)
        ellipse.setPos(75, 30)
        brush = QBrush(Qt.blue)
        ellipse.setBrush(brush)
        pen = QPen(Qt.green)
        pen.setWidth(5)
        ellipse.setPen(pen)
        self.gfx_scene.addItem(ellipse)
        self.gfx_scene.addItem(rect)

        # Set all items as moveable and selectable
        for item in self.gfx_scene.items():
            item.setFlag(QGraphicsItem.ItemIsMovable)
            item.setFlag(QGraphicsItem.ItemIsSelectable)


class Window(QMainWindow):
    def __init__(self):
        super().__init__()

        self.toggle = 1  # var to toggle between scenes 1 and 2

        self.graphics_scene_1 = SceneAndView()
        self.graphics_scene_2 = SceneAndView()

        # add a button to enable toggling between scenes
        self.test_toolbar = QToolBar(self)
        self.addToolBar(Qt.TopToolBarArea, self.test_toolbar)
        self.toggle_view_and_scene = QAction('Toggle View and GFX Scene', self)
        self.toggle_view_and_scene.triggered.connect(self.toggle_scene)
        self.test_toolbar.addAction(self.toggle_view_and_scene)

        """
        # what should I be setting as the central widget here? Getting error when toggling back and forth between gfx scene 1 and 2:  
        # 'wrapped C/C++ object of type SceneAndView has been deleted'
        """
        
        self.setCentralWidget(self.graphics_scene_1)

    def toggle_scene(self):
        print('toggling scene')

        # I want to be able to swap between gfx scene and view, 1 and 2
        if self.toggle == 1:
            self.setCentralWidget(self.graphics_scene_2)
            self.toggle = 0
        else:
            self.setCentralWidget(self.graphics_scene_1)
            self.toggle = 1


app = QApplication(sys.argv)

w = Window()
w.show()

app.exec()

JonB

@S-Tozer
A QGraphicsView is just QWidget, so use QStackedWidget. You add your two views onto the QSW and use setCurrentIndex/Widget() to swap which one it currently shows. Make your central widget be that QSW.

S Tozer

Thankyou JonB- this is working fine now. Terrific!

Here's how it looks now in the example code:

self.stacked_widget = QStackedWidget()
self.setCentralWidget(self.stacked_widget)
self.stacked_widget.addWidget(self.graphics_scene_1)
self.stacked_widget.addWidget(self.graphics_scene_2)

Now I can swap between scenes without issues with:

self.stacked_widget.setCurrentIndex(1)
self.stacked_widget.setCurrentIndex(0)

etc...

Cheers!