title: Python-Kivy: on_touch_down() defined to specific children affects on all childrens
body: <p>I want to make an android-lock like thing, so I have 2 Images of the button (the button normal, the the button pressed).</p>
  
  <p>I defined a function on_touch_down on every image, so when I press it, it changes the source to the pressed button, and on_touch_up it changes it back to normal. But every time I press on any part of the screen, it changes all the buttons at once.</p>
  
  <p>How can I make it change just each button when I press it, and not change everything when I press anywhere?</p>
  
  <p>here is my kv file:</p>
  
  <pre><code>Manager:
      Principal:
  
  &lt;Principal&gt;:
      GridLayout:
          cols: 3
          Image:
              id: '1'
              size: 30,30
              source: 'button.png'
              on_touch_down: self.source = 'button_press.png'
              on_touch_up: self.source = 'button.png'
              allow_strech: True
          Image:
              id: '2'
              size: 30,30
              source: 'button.png'
              on_touch_down: self.source = 'button_press.png'
              on_touch_up: self.source = 'button.png'
              allow_strech: True
          Image:
              id: '3'
              size: 30,30
              source: 'button.png'
              on_touch_down: self.source = 'button_press.png'
              on_touch_up: self.source = 'button.png'
              allow_strech: True
          Image:
              id: '4'
              size: 30,30
              source: 'button.png'
              on_touch_down: self.source = 'button_press.png'
              on_touch_up: self.source = 'button.png'
              allow_strech: True
          Image:
              id: '5'
              size: 30,30
              source: 'button.png'
              on_touch_down: self.source = 'button_press.png'
              on_touch_up: self.source = 'button.png'
              allow_strech: True
          Image:
              id: '6'
              size: 30,30
              source: 'button.png'
              on_touch_down: self.source = 'button_press.png'
              on_touch_up: self.source = 'button.png'
              allow_strech: True
          Image:
              id: '7'
              size: 30,30
              source: 'button.png'
              on_touch_down: self.source = 'button_press.png'
              on_touch_up: self.source = 'button.png'
              allow_strech: True
          Image:
              id: '8'
              size: 30,30
              source: 'button.png'
              on_touch_down: self.source = 'button_press.png'
              on_touch_up: self.source = 'button.png'
              allow_strech: True
          Image:
              id: '9'
              size: 30,30
              source: 'button.png'
              on_touch_down: self.source = 'button_press.png'
              on_touch_up: self.source = 'button.png'
              allow_strech: True
  </code></pre>