from kivy.uix.dropdown import DropDown
from kivy.app import App
from kivy.lang import Builder
from kivy.properties import NumericProperty
from kivy.animation import Animation

KV = '''
FloatLayout:
    Spinner:
        dropdown_cls: 'SlowDown'
        text: 'sing along'
        values: 'hello darkness my old friend'.split(' ')
        size_hint: None, None
        size: 100, 38
        pos_hint: {'center': (.5, .5)}
'''

class SlowDown(DropDown):
    item_delay = NumericProperty(.1)
    display_time = NumericProperty(.5)

    def open(self, widget):
        super(SlowDown, self).open(widget)

        # determine if we openned up or down
        if self.attach_to.y > self.y:
            children = reversed(self.container.children)
        else:
            children = self.container.children

        #
        for i, w in enumerate(children):
            anim = (
                Animation(duration=i * self.item_delay)
                + Animation(opacity=1, duration=self.display_time)
            )
            w.opacity = 0
            anim.start(w)

        bar_color = self.bar_color

        def restore_bar_color(*args):
            Animation(bar_color=bar_color, d=self.display_time).start(self)

        self.bar_color = (0, 0, 0, 0)

        anim.bind(on_complete=restore_bar_color)


class SlowDownApp(App):
    def build(self):
        return Builder.load_string(KV)


if __name__ == '__main__':
    SlowDownApp().run()