One common confusion in Kivy is the correct distinction between to_window
, to_widget
, to_local
and to_parent
methods. In this post I will focus in to_window
and to_widget
methods.
Kivy usually works with absolute coordinates, i.e. no matter where you are trying to position a Widget
, the origin coordinate (0,0) will always be at the bottom-left corner of your window.
Please notice that this post applies specifically to the absolute position properties (pos
, x
, y
, top
, bottom
), not to the proportional position property (pos_hint
property) .
Apart from the pos_hint
property, there are some exceptions where Kivy stops using absolute coordinates. For example, when we are directly inside a RelativeLayout
or Scatter
. In this cases, we might need to convert coordinates relative to the Widget
to absolute Window
coordinates, or vice-versa.
A common situation in which we need to do this is when we are receiving touch events. Let's study an example. Here is the Kivy code:
<RelativeLayout>: pos: 100,100 size_hint: .6,.6 canvas: Line: rectangle: 0,0, self.width, self.height <Relatives>: RelativeLayout: Button: pos: 0,0 size_hint: .4,.3 text: "Press Me!" on_press: text = root.test(*args)
What we have is a Button
inside a RelativeLayout
inside Relatives
. A Relatives
instance will be the the main Widget
of the Window
, and therefore it will share its absolute coordinates. Here is a screenshot of the example:
The following is the Python code which is going to print the coordinates of to_window
and to_widget
in the test
method:
import kivy kivy.require('1.7.0') from kivy.app import App from kivy.uix.relativelayout import RelativeLayout class Relatives (RelativeLayout): def test(self, inst): print "inst.pos = " + str(inst.pos) to_window = inst.to_window(*inst.pos) print "to_window = " + str(to_window) to_widget = inst.to_widget(*to_window) print "to_widget = " + str(to_widget) class RelativesApp(App): def build(self): return Relatives() RelativesApp().run()
This is the output of the program:
inst.pos = [0, 0] to_window = (200, 200) to_widget = (0, 0)
The initial position of the Button
is (0,0), relative to the RelativeLayout
. When to_window
is called on, the relative coordinate is transformed into an absolute one (200,200). The Button
is inside two RelativeLayouts
instances. Each of one is at the position (100,100). Relatives
is at (100,100) of the Window
and the insider RelativeLayout
is at (100,100) of the Relatives
instance. Therefore the Button
is at (200,200) of the Window
. The coordinate is restored calling the to_widget
method.
The to_window
method will always return absolute coordinates relative to the Window
, no matter how many RelativeLayout
(or Scatter
) instances are in the middle. In other words, if there are several RelativeLayout
instances embedded into each other, the method will traverse them until it gets the coordinates in the Window
.
I will be posting about to_parent
and to_local
methods very soon.