Function to execute to animate.
Call it when the animation is starts (considering `delay`).
Call it when the animation is ready.
Call it when the animation is deleted.
Get the current value in relative mode.
Describe the path (curve) of animations.
Current time in animation. Set to negative to make delay.
Duration of playback animation.
Repeat count for the animation.
1: Apply start value immediately even is there is `delay`.
Play back is in progress.
Indicates the animation has run in this round.
Indicates that the `start_cb` was already called.