Binds alpha to a Behaviour.
The Alpha object is what makes a behaviour work: for each tick of the timeline used by Alpha a new value of the alpha parameter is computed by the alpha function; the value should be used by the Behaviour to update one or more properties of the actors to which the behaviour applies.
If alpha is not null, the Behaviour will take
ownership of the Alpha instance.
| this | |
| alpha |
a Alpha or |