pieces.kv

#:kivy 1.7

<BaseSquareElement>:
    size: '20sp', '20sp'
    size_hint: (None, None)
    canvas.after:
        Rectangle:
            pos:self.pos
            size:self.size

<InvisibleElement>:
    canvas:
        Color:
            rgba: 0, 0, 0, 0

<SourceSubElement>:
    canvas:
        Color:
            rgba: .3, .9, .3, 1

<TargetSubElement>:
    canvas:
        Color:
            rgba: 1, 1, 0, 1

<TargetElement>:
    size_hint: (None, None)
    # padding: '1sp', '1sp', '1sp', '1sp'
    spacing: '1sp'#, '1sp'

       
<SourceElement>:
    size_hint: (None, None)
    # padding: '1sp', '1sp', '1sp', '1sp'
    spacing: '1sp'#, '1sp'

<PiecesGame>:
    canvas:
        Rectangle:
            pos: self.width*2/3 - sp(5), 0
            size: sp(10), self.height

  
    # add Target first(before all other elements)
    # that will keep this widget in the back end