Building a custom implicitly animated widget

WHAT THIS TESTS Whether you understand how implicit animations are structured internally and can extend the right base classes instead of reinventing controller plumbing.

A GOOD ANSWER COVERS You create a widget that extends ImplicitlyAnimatedWidget, which already declares duration and curve parameters and requires createState. Your State extends AnimatedWidgetBaseState, the variant that automatically rebuilds on every animation tick. The base class creates and disposes the AnimationController and applies the curve for you, so you do not manage the controller lifecycle directly. You override forEachTween, where you call the provided visitor to create each Tween or update its end value to the new target, supplying a constructor like (value) to make the tween. When a new target arrives the base class drives the controller from the current interpolated value to the new one. In build you evaluate each tween against the animation and paint with the result.

COMMON WRONG ANSWERS Manually creating an AnimationController and addListener, which duplicates what the base class does and risks leaks. Forgetting to extend AnimatedWidgetBaseState versus ImplicitlyAnimatedWidgetState. Recreating tweens from scratch each build so they snap instead of continuing from the current value.

LIKELY FOLLOW-UPS Difference between AnimatedWidgetBaseState and ImplicitlyAnimatedWidgetState. How does forEachTween preserve smoothness across rapid target changes. Why not use TweenAnimationBuilder. How does the curve get applied.

ONE CONCRETE EXAMPLE To build an AnimatedGradientBox that smoothly transitions between two color sets, you extend ImplicitlyAnimatedWidget with begin and end colors, duration and curve. The State extends AnimatedWidgetBaseState, declares ColorTween fields, and in forEachTween updates each toward the new colors using ColorTween. build returns a DecoratedBox whose gradient uses _colorTween.evaluate(animation). Changing the color props later animates from wherever the interpolation currently is, with no controller code written by hand.