def destroy_existing(self, destroyed_blocks):
     all_anims = []
     overlay_objs = []
     for id1, id2 in destroyed_blocks:
         # Generate a square containing both arrow blocks, and fade colour.
         if self.DESTRUCTION_RUNTIME > 0:
             anims = []
             if self.DESTRUCTION_ARROW_ANIM is not None:
                 anims.append(self.DESTRUCTION_ARROW_ANIM(self.arrow_blocks[id1]))
                 anims.append(self.DESTRUCTION_ARROW_ANIM(self.arrow_blocks[id2]))
             else:
                 # Instantly remove on animate.
                 anims.append(FadeOut(self.arrow_blocks[id1], rate_func=lambda t: 1))
                 anims.append(FadeOut(self.arrow_blocks[id2], rate_func=lambda t: 1))
             if self.DESTRUCTION_OVERLAY_COLOUR is not None:
                 top = max(self.arrow_blocks[id1].get_top()[1], self.arrow_blocks[id2].get_top()[1])
                 right = max(self.arrow_blocks[id1].get_right()[0], self.arrow_blocks[id2].get_right()[0])
                 fade_square = Square(side_length=self.scale * 2, color=self.DESTRUCTION_OVERLAY_COLOUR)
                 fade_square.set_opacity(self.DESTRUCTION_OVERLAY_STARTING_ALPHA)
                 fade_square.move_to([right - self.scale, top - self.scale, 0])
                 overlay_objs.append(fade_square)
                 anims.append(FadeOut(fade_square))
             all_anims.append(AnimationGroup(*anims))
         else:
             self.remove(self.arrow_blocks[id1], self.arrow_blocks[id2])
         del self.arrow_blocks[id1]
         del self.arrow_blocks[id2]
     if self.DESTRUCTION_RUNTIME > 0 and len(all_anims) > 0:
         return overlay_objs, LaggedStart(*all_anims, lag_ratio=self.DESTRUCTION_LAG_RATIO)
     return None, None
 def create_arrows(self, created):
     all_anims = []
     overlay_objs = []
     for (id1, (p11, p12), (d1x, d1y)), (id2, (p21, p22), (d2x, d2y)) in created:
         self.arrow_blocks[id1] = self._create_arrow(
             [((p11[0] + p12[0])/2 + 0.5) * self.scale, ((p11[1] + p12[1])/2 + 0.5) * self.scale, 0],
             [d1x, d1y],
         )
         self.arrow_blocks[id2] = self._create_arrow(
             [((p21[0] + p22[0])/2 + 0.5) * self.scale, ((p21[1] + p22[1])/2 + 0.5) * self.scale, 0],
             [d2x, d2y],
         )
         overlay_objs.extend([self.arrow_blocks[id1], self.arrow_blocks[id2]])
         if self.ARROW_OVERLAY_COLOUR is not None:
             top = max(self.arrow_blocks[id1].get_top()[1], self.arrow_blocks[id2].get_top()[1])
             right = max(self.arrow_blocks[id1].get_right()[0], self.arrow_blocks[id2].get_right()[0])
             fade_square = Square(side_length=self.scale * 2, color=self.ARROW_OVERLAY_COLOUR)
             fade_square.set_opacity(self.ARROW_OVERLAY_STARTING_ALPHA)
             fade_square.move_to([right - self.scale, top - self.scale, 0])
             overlay_objs.append(fade_square)
         if self.ARROW_CREATE_RUNTIME > 0:
             anims = []
             if self.ARROW_OVERLAY_COLOUR is not None:
                 anims.append(FadeOut(fade_square))
             if self.ARROW_CREATE_ANIM is not None:
                 if self.ARROW_CREATE_ANIM == FadeInFrom:
                     anims.append(FadeInFrom(self.arrow_blocks[id1], direction=[-d1x * self.scale, -d1y * self.scale, 0]))
                     anims.append(FadeInFrom(self.arrow_blocks[id2], direction=[-d2x * self.scale, -d2y * self.scale, 0]))
                 else:
                     anims.append(self.ARROW_CREATE_ANIM(self.arrow_blocks[id1]))
                     anims.append(self.ARROW_CREATE_ANIM(self.arrow_blocks[id2]))
             else:
                 # This adds them to the scene.
                 anims.append(Transform(self.arrow_blocks[id1], self.arrow_blocks[id1]))
                 anims.append(Transform(self.arrow_blocks[id2], self.arrow_blocks[id2]))
             all_anims.append(AnimationGroup(*anims))
     if self.ARROW_CREATE_RUNTIME > 0 and len(all_anims) > 0:
         return overlay_objs, LaggedStart(*all_anims, lag_ratio=self.ARROW_LAG_RATIO)
     return overlay_objs, None