mapcanvas.py

        """
        Returns a hint for a SpatialExtent, derived from the first raster layer
        :return: SpatialExtent
        """
        crs = self.crs()

        layers = self.layers()
        if len(layers) > 0:
            e = self.fullExtent()
            ext = SpatialExtent(crs, e)
        else:
            ext = SpatialExtent.world()
        return ext


class CanvasBoundingBoxItem(QgsGeometryRubberBand):

    def __init__(self, mapCanvas):
        assert isinstance(mapCanvas, QgsMapCanvas)
        super(CanvasBoundingBoxItem, self).__init__(mapCanvas)

        self.canvas = mapCanvas
        self.mCanvasExtents = dict()
        self.mShow = True
        self.mShowTitles = True
        self.setIconType(QgsGeometryRubberBand.ICON_NONE)

    def connectCanvas(self, canvas):
        assert isinstance(canvas, QgsMapCanvas)
        assert canvas != self.canvas
        if canvas not in self.mCanvasExtents.keys():
            self.mCanvasExtents[canvas] = None
            canvas.extentsChanged.connect(lambda : self.onExtentsChanged(canvas))
            canvas.destroyed.connect(lambda : self.disconnectCanvas(canvas))
            self.onExtentsChanged(canvas)

    def disconnectCanvas(self, canvas):
            self.mCanvasExtents.pop(canvas)

    def onExtentsChanged(self, canvas):
        assert isinstance(canvas, QgsMapCanvas)

        ext = SpatialExtent.fromMapCanvas(canvas)
        ext = ext.toCrs(self.canvas.mapSettings().destinationCrs())

        geom = QgsPolygon()
        assert geom.fromWkt(ext.asWktPolygon())

        self.mCanvasExtents[canvas] = (ext, geom)
        self.refreshExtents()

    def refreshExtents(self):
        multi = QgsPolygon()
        if self.mShow:
            for canvas, t in self.mCanvasExtents.items():
                ext, geom = t
                multi.addGeometry(geom.clone())
        self.setGeometry(multi)

    def paint(self, painter, QStyleOptionGraphicsItem=None, QWidget_widget=None):
        super(CanvasBoundingBoxItem, self).paint(painter)

        if self.mShowTitles and self.mShow:
            painter.setPen(Qt.blue);
            painter.setFont(QFont("Arial", 30))

            for canvas, t in self.mCanvasExtents.items():
                ext, geom = t
                ULpx = self.toCanvasCoordinates(ext.center())
                txt = canvas.windowTitle()
                painter.drawLine(0, 0, 200, 200);
                painter.drawText(ULpx,  txt)


    def setShow(self, b):
        assert isinstance(b, bool)
        self.mShow = b

    def setShowTitles(self, b):
        assert isinstance(b, bool)
        self.mShowTitles = b