# animation.py
# Walker M. White (wmw2)
# November 8, 2012
"""Module to show off how animation works.

To animate in Kivy, we assign a callback function to the function
Clock.schedule_interval, and instruct it the amount of time to
delay.

Clock.schedule_interval only works in Kivy applications.  You cannot
use it in general Python applications."""
from kivy.uix.floatlayout import FloatLayout
from kivy.graphics.vertex_instructions import Rectangle, Ellipse
from kivy.graphics.context_instructions import Color
from kivy.config import Config
from kivy.clock import Clock
from kivy.app import App

import random, math

# CONSTANT TO DEFINE ANIMATION STEP SIZE
ANIMATION_STEPS = 100

class AnimationPanel(FloatLayout):
    """Drawing canvas to animate the ellipse."""
    # HIDDEN FIELDS
    _vx = 1  # Velocity of x-coordinate
    _vy = 1  # Velocity of y-coordinate
    _ellipse = None  # Ellipse to animate

    # Number of steps in animation.
    # Invariant: If 0, ellipse is centered in the screen middle.
    _steps = 0
    
    def __init__(self,**kw):
        """Constructor: Makes a new panel for animation."""
        # Pass arguments to super class
        super(AnimationPanel,self).__init__(**kw)
        
        # Assign callback for animation
        Clock.schedule_interval(self.animate,1.0/60.0)
        
        # This weird command tells the panel what to draw.
        with self.canvas:
            Color(1,1,1)
            Rectangle(pos=self.pos,size=self.size)
            Color(1,0,0)
            self._ellipse = Ellipse(pos=(self.center_x-20,self.center_y-20),size=(40,40))
    
    def animate(self,dt):
        """Animate the ellipse.
        
        If _steps is 0, pick a new random velocity.
        If _steps > ANIMATION_STEPS/2, move away using velocity (vx,vy)
        If _steps <= ANIMATION_STEPS/2, but > 0, move back to the center."""
        if self._steps == 0:
            # Make velocity a unit vector
            self._vx = random.random()*random.choice([1,-1])
            self._vy = math.sqrt(1-self._vx*self._vx)*random.choice([1,-1])
            self._steps = ANIMATION_STEPS
        elif self._steps > ANIMATION_STEPS/2:
            x = self._ellipse.pos[0]
            y = self._ellipse.pos[1]
            self._ellipse.pos = (x+self._vx,y+self._vy)
            self._steps = self._steps - 1 # _steps is a loop counter!
        else:
            x = self._ellipse.pos[0]
            y = self._ellipse.pos[1]
            self._ellipse.pos = (x-self._vx,y-self._vy)
            self._steps = self._steps - 1 # _steps is a loop counter!

class AnimationApp(App):
    """Instance is an application with animation but no user interaction."""
    
    def build(self):
        """Build application with a single internal panel"""
        Config.set('graphics', 'width', '400')
        Config.set('graphics', 'height', '400')
        return AnimationPanel(size=(400,400))

# Application Code
if __name__ == '__main__':
    AnimationApp().run()