Tweening

Tweening in FlatRedBall2

Built on the FlatRedBall.InterpolationCore NuGet. FRB2 ships two extension methods that return a running Tweener:

using FlatRedBall2.Tweening;              // Entity.Tween / Screen.Tween
using FlatRedBall.Glue.StateInterpolation; // InterpolationType, Easing, Tweener

// Entity-scoped (primary) — tween dies with the entity. Target a child shape directly:
var t = this.Tween(v => _circle.Radius = v, from: 0f, to: 100f, duration: TimeSpan.FromSeconds(0.5),
                   InterpolationType.Cubic, Easing.Out);

// Screen-scoped (for tweens with no natural entity owner).
this.Tween(v => Camera.Zoom = v, Camera.Zoom, 1.5f, TimeSpan.FromSeconds(0.3));

Two using directives — why

InterpolationType, Easing, and Tweener come from the upstream FlatRedBall.Glue.StateInterpolation namespace and are not re-exported under FlatRedBall2.Tweening. Gum already depends transitively on FlatRedBall.InterpolationCore (via FlatRedBall.GumCommon), so the upstream namespace is present in every FRB2 project whether you use tweens or not. Declaring FRB2 wrappers with the same names would create IntelliSense auto-import ambiguity — picking the wrong one silently compiles but produces the wrong types. One set of types everywhere beats two similar ones, even if the upstream namespace is ugly.

Choosing the scope

Use Entity.Tween whenever the setter writes to an entity or its children. Destroying the entity clears its tweens automatically, so there is no use-after-destroy risk when the entity dies mid-tween.

Use Screen.Tween only when there is no entity owner — screen fades, global UI transitions, camera-level state.

If in doubt, pick Entity.

Pause behavior

Tweens freeze while Screen.IsPaused is true, alongside the rest of the Activity pipeline. For finer-grained control, override ShouldAdvanceTweens on the Entity or Screen subclass — e.g., freeze tweens on a single stunned entity without pausing the whole screen.

Gotchas

• Setter fires twice on the completing frame. The upstream Tweener fires PositionChanged with a near-to value, then the wrapper invokes the setter once more with exactly to so the final value is precise. Fine for plain assignment (v => X = v). If your setter has side effects, guard them.
• PositionChanged is assigned, not subscribed. The wrapper wires it with =. Additional listeners must use += after Tween(...) returns.
• Ended is an event. Use +=. Fires once, after the final setter call, only if the tween completes (not on Stop()).
• Vector2 and Color have native overloads. No need to compose multiple float tweens. Color defaults to ColorTweenMode.Rgb (channel-wise lerp, matches Color.Lerp); pass ColorTweenMode.Hsv for shortest-arc hue interpolation when you want a vivid sweep (e.g. red→blue passes through magenta, not gray).
• TweenAsync for chaining. Each Tween overload has a TweenAsync twin returning Task that completes after the terminal-snap setter call. Use await this.TweenAsync(...) to sequence tweens without duplicating the duration in Engine.Time.Delay. Cancellation: entity/screen destroy throws TaskCanceledException; fire-and-forget (_ = TweenAsync(...)) silently swallows it.

Bump — wiggle from rest

Tween interpolates between two values — calling Tween(0 → 100, Elastic, Out) on a property already at 100 first snaps to 0 and then plays the curve. That snap is wrong for reaction effects (a button at rest that should "bump" when clicked, a hit-pulse on a sprite, a UI panel that wiggles on validation error).

Use Bump instead. It starts at restValue, peaks at exactly restValue + amplitude, and settles back to restValue — no snap.

// Button at scale 1.0 bumps to 1.2 and settles back, with a classic elastic wiggle.
this.Bump(v => button.Scale = v, restValue: 1.0f, amplitude: 0.2f,
          duration: TimeSpan.FromSeconds(0.4));

// "Kick" feel — single overshoot, no oscillation.
this.Bump(v => sprite.X = v, restValue: 100f, amplitude: 8f,
          duration: TimeSpan.FromSeconds(0.25), curve: BumpCurve.Back);

amplitude is in the setter's units (pixels, scale, alpha) — independent of restValue.

Curves

All three curves share the same high-level shape — start at restValue, peak at exactly restValue + amplitude, settle back to restValue. Only the tail differs:

• BumpCurve.Elastic (default) — oscillates above AND below rest with decreasing amplitude. Classic wiggle.
• BumpCurve.Back — single overshoot, then a smooth settle. No oscillation.
• BumpCurve.Bounce — oscillates between rest and progressively smaller peaks above rest. Never crosses below rest. Ball-settling feel.

Negative amplitude works for all three — the bump is mirrored: peak is below rest at restValue + amplitude, settles at rest.

Gotchas

• Stacking is not coordinated. Calling Bump while another Tween/Bump is driving the same property results in both setters firing each frame and fighting. Stop the previous tween first if needed. Same engine-wide stacking limitation as Tween.
• Terminal snap precision. Like Tween, the final setter call is exactly restValue.