UI: Add Transitions to arcade GUI [after 3.0 release]

[eruvanos]

Adds a complete GUI animation stack to Arcade, shipped under arcade.gui.experimental so the novel per-property takeover semantics, cached overdraw and hover-zone behavior can be iterated on without deprecations. Stabilization later is purely additive (e.g. promoting animate() onto UIWidget once proven).

High-level API

UIAnimatedGroup is the main entry point. It wraps a single child (or a whole subtree), renders it into a cached surface, and tweens transform properties that do not affect layouting:

from arcade.gui.experimental import UIAnimatedGroup

group = UIAnimatedGroup(child=UIFlatButton(text="Play"))
layout.add(group)

# scale to 108% within 0.15s, with a "back out" overshoot
group.animate(scale=1.08, duration=0.15, ease=Easing.BACK_OUT)

Animatable transform properties: scale, angle, alpha, offset_x, offset_y, tint.

animate() supports:

• multiple properties per call
• sequencing via .then(...), completion callbacks via .on_finish(...), cancellation via .stop()
• repetition (repeat, yoyo), delays, easing (arcade.anim.Easing)
• relative targets via rel() (e.g. angle=rel(-360))
• per-property takeover: starting a new animation takes over just its properties from running ones — so hover in/out is a one-liner with no cleanup.

UIAnimatedGroup is interactive: hovered / pressed / on_click are hit-tested against the group's untransformed rect, so the trigger zone stays stable while the visuals scale or rotate. The child subtree still receives mouse events transformed into its local coordinate system.

def on_hover_change():
    if group.hovered:
        group.animate(scale=1.08, duration=0.15, ease=Easing.BACK_OUT)
    else:
        group.animate(scale=1.0, duration=0.15)

bind(group, "hovered", on_hover_change)

UIRenderGroup is the non-interactive caching primitive underneath, available for use on its own.

Low-level transitions

animate() is built on a small set of composable Transition* primitives (also exported from arcade.gui.experimental), which can be combined with + (parallel) and | (sequential):

• TransitionAttr — tween a value over time (start → end)
• TransitionAttrIncr — increment a value over time
• TransitionAttrSet — set a value after a delay
• TransitionParallel — run transitions in parallel
• TransitionChain — run transitions sequentially
• TransitionDelay — pause within a chain

Animation implements the TransitionBase protocol, so animations and raw transitions compose freely.

Examples

• arcade.examples.gui.exp_animations — an animated main menu: staggered pop-in entrances, pulsing/swaying idle animations, hover pops, spinning tiles, fade-out on quit.
• arcade.examples.gui.exp_animations_2 — hoverable cards that lift and enlarge.
• arcade.examples.gui.transitions — a minimal low-level showcase animating offset_x/offset_y.

Docs & tests

• New programming guide page doc/programming_guide/gui/animations.rst plus updates to concepts.rst; the experimental API is indexed under GUI Experimental Features.
• Full unit coverage in tests/unit/gui/ for animate, transition, and the render/animated groups (parent-space hover, transform-invariant hit-testing, ticking, takeover semantics).

Notes

• Supporting changes to the GUI render path (surface.py + the surface_vs/fs.glsl shaders), the reactive property.py system (adds AliasProperty), and UIScrollArea came along on this branch.
• Everything animation-related lives in arcade.gui.experimental — the stable arcade.gui namespace does not export any of it yet.

[eruvanos]

Talked to @einarf :

• missing feature: cancel transition
• for Sprites, this might be a performance problem

[eruvanos]

Will be added after 3.0 release. It is more important to release!