Can't manually update QGraphicsScene

I'm working with PyQt4.5.4.

Summary: QGraphicsScene.update() method doesn't update the scene immediately.

Detailed Description:
I have a QGraphicsScene, with drawBackground method to draw background grids.

Code:

---

class Scene(QGraphicsScene):
    ...............
 
    def drawBackground(self, painter, rect):
        if self.gridActive:
            gridSize = 50
            left = int(rect.left()) - (int(rect.left()) % gridSize)
            top = int(rect.top()) - (int(rect.top()) % gridSize)
            lines = []
            right = int(rect.right())
            bottom = int(rect.bottom())
            for x in range(left, right, gridSize):
                lines.append(QLineF(x, rect.top(), x, rect.bottom()))
            for y in range(top, bottom, gridSize):
                lines.append(QLineF(rect.left(), y, rect.right(),y))
 
            painter.setPen(QPen(Qt.lightGray))
            painter.drawLines(lines)

---

I want to be able to toggle the grid by using the menu:

Code:

---

class MainWindow(QMainWindow):
     .......
 
    def createActions(self):
        displayGrid = QAction('Display Grid', self)
        displayGrid.setCheckable(True)
        displayGrid.setChecked(self.scene.gridActive)
        displayGrid.setShortcut('Ctrl+G')
        displayGrid.setStatusTip('Display or Hide the grid')
 
        self.connect(displayGrid, SIGNAL('toggled(bool)'), self.setDisplayGrid)
        viewMenu = self.menubar().addMenu('&View')
        viewMenu.addAction(displayGrid)
 
    def setDisplayGrid(self, value):
        self.scene.gridActive = value
        self.scene.update()

---

(scene.gridActive is initially True)

But the self.scene.update() method doesn't update it immediately. But rather I have to zoom in/out or resize the window to make the toggle effect get work.

I used to be able to do it but somehow corrupted it and can't find the source of the problem. Any ideas?

We'd have to see the bigger picture. It is likely you are drawing outside boundingRects of your items or something like that.

Hmm,

I tried self.view.fitInView(self.scene.itemsBoundingRect() ,Qt.KeepAspectRatio) but it also didn't work.

I don't know if you'd mind looking at the big picture but here is the complete code:
(unfortunately I'm using some other lib for data import. The Scene/View and MainWindow implementations are at the bottom. Top classes are not important)

http://pastebin.com/m264f6728

It would be best to isolate the problem using a minimal compilable example.

Oopps, I'm a bit dizzy late at night. I've produced a minimal example and figured out that the problem is with the line:

Code:

---

self.setCacheMode(QGraphicsView.CacheBackground)

---

Removing it solved the problem. I just tought it could speed up the performance of drawing grid.

Thank you for your help.