import 'package:flutter/material.dart';

/// Shared motion primitives.
///
/// Both of these are deliberately drop-in: [Entrance.stagger] takes the list a
/// screen already builds and hands back the same list, and [PressScale] lives
/// inside the shared cards rather than in any screen. Adding motion to a
/// screen should not mean rewiring it.

/// Fades and lifts its child into place, once, when it first appears.
///
/// A screen that arrives fully drawn tells you nothing about how it is put
/// together. Letting the rows land in order — a short distance, quickly —
/// shows the reading order before the worker has to find it, which matters
/// most on the screens that are a long column of similar cards.
class Entrance extends StatefulWidget {
  const Entrance({super.key, required this.child, this.delay = Duration.zero});

  final Widget child;

  /// How long to wait before starting. [stagger] fills this in.
  final Duration delay;

  /// Wraps each child with an increasing delay.
  ///
  /// The stagger stops counting after [_maxSteps] so a long list does not end
  /// up with a row that waits a second and a half to appear — past the first
  /// handful the effect has already done its job.
  static List<Widget> stagger(List<Widget> children, {Duration step = _step}) {
    return [
      for (final (i, child) in children.indexed)
        Entrance(delay: step * (i > _maxSteps ? _maxSteps : i), child: child),
    ];
  }

  static const _step = Duration(milliseconds: 55);
  static const _maxSteps = 6;

  @override
  State<Entrance> createState() => _EntranceState();
}

class _EntranceState extends State<Entrance>
    with SingleTickerProviderStateMixin {
  late final AnimationController _controller;
  late final Animation<double> _curve;

  static const _travel = Duration(milliseconds: 380);

  @override
  void initState() {
    super.initState();

    // The delay is folded into the controller as a leading Interval rather
    // than run as a Future.delayed. A standalone timer outlives the widget it
    // was armed for — a `mounted` guard protects the callback but not the
    // timer, which then shows up as a pending timer at teardown and keeps a
    // disposed screen's work alive. The controller goes away with the state.
    final total = widget.delay + _travel;
    _controller = AnimationController(vsync: this, duration: total);
    _curve = CurvedAnimation(
      parent: _controller,
      curve: Interval(
        widget.delay.inMicroseconds / total.inMicroseconds,
        1,
        curve: Curves.easeOutCubic,
      ),
    );
    _controller.forward();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: _curve,
      // The child is passed through untouched, so it is built once rather
      // than rebuilt on every frame of the entrance.
      child: widget.child,
      builder: (context, child) => Opacity(
        opacity: _curve.value,
        child: Transform.translate(
          offset: Offset(0, (1 - _curve.value) * 14),
          child: child,
        ),
      ),
    );
  }
}

/// Shrinks its child slightly while a finger is down on it.
///
/// Material's ink ripple answers "the app heard you" after the fact, and on a
/// white card on a white page it is nearly invisible. A card that gives under
/// the thumb answers during the press instead, which is the half-second that
/// actually feels unresponsive.
class PressScale extends StatefulWidget {
  const PressScale({super.key, required this.child, this.enabled = true});

  final Widget child;

  /// False for a card with no tap handler, which should not pretend to react.
  final bool enabled;

  @override
  State<PressScale> createState() => _PressScaleState();
}

class _PressScaleState extends State<PressScale> {
  bool _down = false;

  void _set(bool down) {
    if (!widget.enabled || _down == down) return;
    setState(() => _down = down);
  }

  @override
  Widget build(BuildContext context) {
    if (!widget.enabled) return widget.child;

    return Listener(
      // Listener rather than GestureDetector: this has to read the press
      // without competing for the gesture, so the InkWell underneath still
      // gets its tap and a scroll that starts on a card still scrolls.
      onPointerDown: (_) => _set(true),
      onPointerUp: (_) => _set(false),
      onPointerCancel: (_) => _set(false),
      child: AnimatedScale(
        scale: _down ? 0.975 : 1,
        duration: const Duration(milliseconds: 130),
        curve: Curves.easeOut,
        child: widget.child,
      ),
    );
  }
}