Can't manually update QGraphicsScene

[aladagemre]

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.

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)

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)

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

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()

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()

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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?

[wysota]

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

[aladagemre]

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

[wysota]

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

[aladagemre]

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:

self.setCacheMode(QGraphicsView.CacheBackground)

self.setCacheMode(QGraphicsView.CacheBackground)

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

Thank you for your help.