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.

Qt Code: 
  1. class Scene(QGraphicsScene):
  2.     ...............
  3.  
  4.     def drawBackground(self, painter, rect):
  5.         if self.gridActive:
  6.             gridSize = 50
  7.             left = int(rect.left()) - (int(rect.left()) % gridSize)
  8.             top = int(rect.top()) - (int(rect.top()) % gridSize)
  9.             lines = []
  10.             right = int(rect.right())
  11.             bottom = int(rect.bottom())
  12.             for x in range(left, right, gridSize):
  13.                 lines.append(QLineF(x, rect.top(), x, rect.bottom()))
  14.             for y in range(top, bottom, gridSize):
  15.                 lines.append(QLineF(rect.left(), y, rect.right(),y))
  16.  
  17.             painter.setPen(QPen(Qt.lightGray))
  18.             painter.drawLines(lines)
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I want to be able to toggle the grid by using the menu:

Qt Code: 
  1. class MainWindow(QMainWindow):
  2.      .......
  3.  
  4.     def createActions(self):
  5.         displayGrid = QAction('Display Grid', self)
  6.         displayGrid.setCheckable(True)
  7.         displayGrid.setChecked(self.scene.gridActive)
  8.         displayGrid.setShortcut('Ctrl+G')
  9.         displayGrid.setStatusTip('Display or Hide the grid')
  10.  
  11.         self.connect(displayGrid, SIGNAL('toggled(bool)'), self.setDisplayGrid)
  12.         viewMenu = self.menubar().addMenu('&View')
  13.         viewMenu.addAction(displayGrid)
  14.  
  15.     def setDisplayGrid(self, value):
  16.         self.scene.gridActive = value
  17.         self.scene.update()
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(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?