Why does this flicker?

[Webmaster4o]

I've written some incredibly basic code for a custom view class that can be dragged/dropped.

class DragDrop(ui.View):
	def touch_moved(self, touch):
		tx, ty = touch.location
		self.x,self.y = tx-self.frame[2]/2, ty-self.frame[3]/2

v=ui.View()
dnd = DragDrop(frame=(100,100,100,100))
dnd.background_color=(0,0,0)
v.add_subview(dnd)
v.present(hide_title_bar=1)

However when I drag an object, its position flickers between two positions. Why is this, and how can I fix it?

[Cethric]

Touch position changes in the touch_moved method between calls (yeah bear with me that ones kinda obvious) but what I mean is that moving your finger from top left to bottom right on each move there is two changes for some reason. (0,0) and (0,-1) simultaneously hence the flicker.
This is shown simply by printing to touch coordinats.

print tx, ty

Solution: find delta x and delta y (ie the change from the current position and the last position)

class DragDrop(ui.View):
    def touch_moved(self, touch):
        cx, cy = touch.location
        ox, oy = touch.prev_location
        tx, ty = ox-cx, oy-cy
        print tx, ty
        self.x -= tx
        self.y -= ty