Returns a list of the sides of the sprite
                which are touching the edge of the canvas.

            0 = Bottom
            1 = Left
            2 = Top
            3 = Right

def onEdge(self, canvas):
        """
            Returns a list of the sides of the sprite
                which are touching the edge of the canvas.

            0 = Bottom
            1 = Left
            2 = Top
            3 = Right
        """
        sides = []
        if int(self.position[0]) <= 0:
            sides.append(1)

        if (int(self.position[0]) + self.image.width) >= canvas.width:
            sides.append(3)

        if int(self.position[1]) <= 0:
            sides.append(2)

        if (int(self.position[1]) + self.image.height) >= canvas.height:
            sides.append(0)

        return sides