# Property Sharing¶

Many attributes of a Shady.World or Shady.Stimulus instance are what we call “managed properties”. These are stored in arrays (even if they only contain a single value and otherwise behave like scalars). Many of them are transferred to the rendering engine on every frame for drawing. By default, every new Stimulus object you create has a fresh new set of managed property arrays created for it, initialized to their default values. However, it is possible to force multiple stimuli to share the memory space for their properties, linking their behavior together at with no extra runtime computational cost. Sharing properties in this way will cause those stimuli to be linked until you explicitly unlink them. This allows your program to be less complex and more CPU-efficient, since you will only have to change the property value of one stimulus and the change will affect all the others.

The simplest way to share a property between two stimuli is to use the shorthand convention of assigning an actual Stimulus instance to the relevant property value of another stimulus:

import Shady
world = Shady.World( 700, top=100, frame=True )
stim1 = world.Patch( x=-200, rotation=30 )
stim2 = world.Stimulus( x=+200, rotation=stim1 )
# now, any change to either stimulus's rotation will affect both
stim1.rotation = 45
print( stim2.rotation )
# --> 45


This technique is in fact a syntactic shorthand for the ShareProperties method of the first stimulus:

stim1.ShareProperties( stim2, 'rotation' )
# same effect as stim1.rotation=stim2


This more powerful method can be used to share multiple properties at a time between multiple stimuli:

import Shady
world = Shady.World( 700, top=100, frame=True )
master = world.Stimulus( size=50, y=100 )
followers = [ world.Stimulus( size=50, x=x ) for x in range( -400, 400, 100 ) ]
master.ShareProperties( followers, 'rotation', 'scale' )


Note that the name master here may be slightly misleading, because sharing is fully symmetric: any change to the rotation, or scale of any of the Stimulus instances in followers will affect the remaining contents of followers and our master. That said, it may be useful to designate one stimulus as the master to indicate a convention that this stimulus should be used to control the others, especially if you are going to use dynamic property assignment:

master.rotation = lambda t: t * 20


ShareProperties() also allows you to set property values at the same time as sharing them:

import Shady
world = Shady.World( 700, top=100, frame=True )
stim1 = world.Patch( x=-200 )
stim2 = world.Patch( x=+200 )
# share color and set that shared color to red
stim1.ShareProperties( stim2, color=( 1, 0, 0 ) )


If you want a stimulus to stop sharing properties, you can again use the shorthand of Stimulus-instance assignment:

stim2.color = stim2      # tell stim2 to "be yourself"


…which is really a shortcut for stim2.MakePropertiesIndependent( 'color' ). The MakePropertiesIndependent method can also simultaneously change the value(s) of one or more properties as it unlinks them - here’s another example:

# ...
stim1.ShareProperties( stim2, position=( -100, 200 ), alpha=0.5, scale=2.5 )

stim1.Set( position=600 )
print( stim2.position )
# --> [ 600, 600 ]

print( ( stim1.scale, stim2.scale ) )
# --> ( 2.5, 2.5 )
stim2.MakePropertiesIndependent( scale=7 )
print( ( stim1.scale, stim2.scale ) )
# --> ( 2.5, 7.0 )

stim2.alpha = 0.3     # still linked
print( ( stim1.alpha, stim2.alpha ) )
# --> ( 0.3, 0.3 )


One final warning: property sharing does not work with property index shortcuts, as two stimuli cannot share just part of a full property array. If you want to share specific property dimensions such as x or red, but not the other dimensions of that property, you should use a dynamic function instead to ensure it is continually updated:

### WRONG ###
# ...
stim2.x = stim1
# --> ValueError: x is the name of a shortcut, not a fully-fledged
#     property - cannot link it across objects

### RIGHT ###
# ...
stim2.x = lambda t: stim1.x   # (although it comes at a small CPU cost)


See the demo script examples/sharing.py for more.