Why QgsMapCanvas only appear after lost focus?

Question

I created the simple frame .UI file, compiled with pyuic4, and tried the ShapeViewer.py to load the shapefile "world_borders.shp". However, the map doesn't display at the first time. I have to click out the window to make it appear. After everything works! I am working in Windows 7. Instalation "osgeo4w-setup-x86". Qgis 2.2.0-Valmiera

Why does QgsMapCanvas in Qgis 2.2.0-Valmiera only appear after losing focus?

Here is my code:

class ShapeViewer(QMainWindow, Ui_MainWindow):
    def __init__(self):
        QMainWindow.__init__(self)
        self.setupUi(self)
        self.canvas = QgsMapCanvas()
        self.canvas.useImageToRender(False)
        self.layout = QVBoxLayout(self.frame)
        self.layout.addWidget(self.canvas)
        file = QFileDialog.getOpenFileName(self,"Open", ".", "Shp(*.shp)")
        layer = QgsVectorLayer(file, "My Shape", "ogr")
        QgsMapLayerRegistry.instance().addMapLayer(layer);
        self.canvas.setExtent(layer.extent())
        cl = QgsMapCanvasLayer(layer)
        self.canvas.setLayerSet([cl])
        self.show()

def main(argv):
    # create Qt application
    app = QApplication(argv)

    # Initialize qgis libraries
    QgsApplication.setPrefixPath(qgis_prefix, True)
    QgsApplication.initQgis()
    # create main window
    wnd = ShapeViewer()
    wnd.show()
    # run!
    retval = app.exec_()

    # exit
    QgsApplication.exitQgis()
    sys.exit(retval)
if __name__ == "__main__":
    main(sys.argv)

Answer 1

This is a bug in 2.2 with the canvas and it's redraw event. The redraw event is called when the widget is first shown and it kicks off a timer which stops the canvas from painting.

The workaround is as simple as calling:

canvas.refresh()
canvas.repaint()

The first time the widget is shown.

Answer 2

I had the same issue. I solve it by calling to mainWindow.show after configuring the qgsmapcanvas. Hope it helps.

Answer 3

I fixed this error by switching to version 2.0.1 I think it's a bug from 2.2. I'm not sure, because I have no experience in qgis.

Answer 4

This bug cost me a day. The workaround is very specific. You must freeze and unfreeze the canvas at just the right time while also refreshing and repainting the canvas:

# Freeze the Canvas
# to make it immune to
# buggy events
canvas.freeze(True)
# Show the frozen canvas
canvas.show()
# Refresh the canvas content
canvas.refresh()
# Now, and only now, we can unfreeze
# the canvas
canvas.freeze(False)
# And finally, we can safely repaint it.
canvas.repaint()