Invader – transitions for animations

The previous section uses the default Animation transition. This is a Linear transition, which means that the Widget instance moves from one point to another in a straight line. Invaders trajectories can be more interesting. For example, there could be accelerations, or changes of direction, as the following screenshot shows with the yellow line:

The following is the code of the invader.py:

93. # File name: invader.py
94. from kivy.core.window import Window
95. from kivy.uix.image import Image
96. from kivy.animation import Animation
97. from random import choice, randint
98. from ammo import Missile
99. 
100. class Invader(Image):
101.   pre_fix = ['in_','out_','in_out_']
102.   functions = ['back','bounce','circ','cubic',
103.     'elastic','expo','quad','quart','quint','sine']
104.   formation = True
105. 
106.   def solo_attack(self):
107.     if self.formation:
108.       self.parent.unbind_invader()
109.       animation = self.trajectory()
110.  ...