# Objective
Animating component fields requires too much boilerplate at the moment:
```rust
#[derive(Reflect)]
struct FontSizeProperty;
impl AnimatableProperty for FontSizeProperty {
type Component = TextFont;
type Property = f32;
fn get_mut(component: &mut Self::Component) -> Option<&mut Self::Property> {
Some(&mut component.font_size)
}
}
animation_clip.add_curve_to_target(
animation_target_id,
AnimatableKeyframeCurve::new(
[0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0]
.into_iter()
.zip([24.0, 80.0, 24.0, 80.0, 24.0, 80.0, 24.0]),
)
.map(AnimatableCurve::<FontSizeProperty, _>::from_curve)
.expect("should be able to build translation curve because we pass in valid samples"),
);
```
## Solution
This adds `AnimatedField` and an `animated_field!` macro, enabling the
following:
```rust
animation_clip.add_curve_to_target(
animation_target_id,
AnimatableCurve::new(
animated_field!(TextFont::font_size),
AnimatableKeyframeCurve::new(
[0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0]
.into_iter()
.zip([24.0, 80.0, 24.0, 80.0, 24.0, 80.0, 24.0]),
)
.expect(
"should be able to build translation curve because we pass in valid samples",
),
),
);
```
This required reworking the internals a bit, namely stripping out a lot
of the `Reflect` usage, as that implementation was fundamentally
incompatible with the `AnimatedField` pattern. `Reflect` was being used
in this context just to downcast traits. But we can get downcasting
behavior without the `Reflect` requirement by implementing `Downcast`
for `AnimationCurveEvaluator`.
This also reworks "evaluator identity" to support either a (Component /
Field) pair, or a TypeId. This allows properties to reuse evaluators,
even if they have different accessor methods. The "contract" here is
that for a given (Component / Field) pair, the accessor will return the
same value. Fields are identified by their Reflect-ed field index. The
(TypeId, usize) is prehashed and cached to optimize for lookup speed.
This removes the built-in hard-coded TranslationCurve / RotationCurve /
ScaleCurve in favor of AnimatableField.
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
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