The CSS3 Transitions specification from the W3C defines the possibility
of using a parametrized step() timing function, with the following
prototype:
steps(n_steps, [ start | end ])
where @n_steps represents the number of steps used to divide an interval
between 0 and 1; the 'start' and 'end' tokens describe whether the value
change should happen at the start of the transition, or at the end.
For instance, the "steps(3, start)" timing function has the following
profile:
1 | x
| |
| x---|
| ' |
| x---' |
| ' |
0 |---' |
Whereas the "steps(3, end)" timing function has the following profile:
1 | x---|
| ' |
| x---' |
| ' |
x---' |
| |
0 | |
Since ClutterTimeline uses an enumeration for controlling the progress
mode, we need additional API to define the parameters of the steps()
progress; for this reason, we need a CLUTTER_STEPS enumeration value,
and a method for setting the number of steps and the value transition
policy.
The CSS3 Transitions spec helpfully also defines a step-start and a
step-end shorthands, which expand to step(1, start) and step(1, end)
respectively; we can provide a CLUTTER_STEP_START and CLUTTER_STEP_END
enumeration values for those.
Instead of having the easing functions be dependent of ClutterAlpha, and
static to the clutter-alpha.c source file, we should make them generic
and move them to their own internal header and source files. This will
allow to re-use them in the near future.
