ux/lib/src/view_padding.dart

import 'dart:ui' show FlutterView;

import 'package:flutter/scheduler.dart';
import 'package:flutter/widgets.dart';

/// Smoothly-animated view padding (safe-area insets).
///
/// Replaces a raw `view.viewPadding` read with a value that lerps toward the
/// system inset over [duration] when it changes — so when the OS bars toggle
/// visibility (e.g. switching out of `SystemUiMode.immersiveSticky` back to
/// `edgeToEdge`) bottom- and top-anchored UI slides into place instead of
/// snapping by the nav-bar / status-bar height.
///
/// Use the singleton [instance] and listen via [addListener]; or wrap a
/// subtree in [XAnimatedInsets] so descendant `MediaQuery.viewPaddingOf`
/// reads pick up the animated value.
///
/// Tracks only `viewPadding` — keyboard insets (`viewInsets`) are handled by
/// [XKeyboard] and intentionally bypass this smoothing.
class XInsets with ChangeNotifier, WidgetsBindingObserver {
  XInsets._() {
    WidgetsBinding.instance.addObserver(this);
    final view = WidgetsBinding.instance.platformDispatcher.implicitView;
    _system = _read(view);
    _current = _system;
  }

  /// Singleton instance. Constructed lazily on first access; requires
  /// `WidgetsFlutterBinding.ensureInitialized()` to have been called.
  static final XInsets instance = XInsets._();

  EdgeInsets _system = EdgeInsets.zero;
  EdgeInsets _current = EdgeInsets.zero;
  EdgeInsets _from = EdgeInsets.zero;
  Duration _start = Duration.zero;
  bool _ticking = false;

  /// Lerp duration. Set to [Duration.zero] in tests for deterministic
  /// frame-by-frame goldens.
  Duration duration = const Duration(milliseconds: 220);

  /// Lerp curve.
  Curve curve = Curves.easeOut;

  /// Smoothed view-padding in logical pixels.
  EdgeInsets get viewPadding => _current;

  /// Raw system view-padding (the un-lerped target). Use sparingly — most
  /// callers want [viewPadding].
  EdgeInsets get systemViewPadding => _system;

  @override
  void didChangeMetrics() {
    final next = _read(WidgetsBinding.instance.platformDispatcher.implicitView);
    if (next == _system) return;
    _system = next;
    if (duration == Duration.zero) {
      _current = next;
      _ticking = false;
      notifyListeners();
      return;
    }
    _from = _current;
    _start = SchedulerBinding.instance.currentSystemFrameTimeStamp;
    if (!_ticking) {
      _ticking = true;
      SchedulerBinding.instance.scheduleFrameCallback(_tick);
      SchedulerBinding.instance.scheduleFrame();
    }
  }

  void _tick(Duration ts) {
    if (!_ticking) return;
    final dt = (ts - _start).inMicroseconds / duration.inMicroseconds;
    final t = curve.transform(dt.clamp(0.0, 1.0));
    _current = EdgeInsets.lerp(_from, _system, t)!;
    notifyListeners();
    if (dt < 1.0) {
      SchedulerBinding.instance.scheduleFrameCallback(_tick);
      SchedulerBinding.instance.scheduleFrame();
    } else {
      _ticking = false;
    }
  }

  static EdgeInsets _read(FlutterView? v) {
    if (v == null) return EdgeInsets.zero;
    final r = v.devicePixelRatio;
    final p = v.viewPadding;
    return EdgeInsets.fromLTRB(p.left / r, p.top / r, p.right / r, p.bottom / r);
  }
}

/// Wraps [child] in a [MediaQuery] whose `viewPadding` is sourced from
/// [XInsets.instance] (smoothly animated). Use once at the app root so every
/// descendant `MediaQuery.viewPaddingOf(context)` read returns the animated
/// value.
class XAnimatedInsets extends StatefulWidget {
  const XAnimatedInsets({super.key, required this.child});

  final Widget child;

  @override
  State<XAnimatedInsets> createState() => _XAnimatedInsetsState();
}

class _XAnimatedInsetsState extends State<XAnimatedInsets> {
  @override
  void initState() {
    super.initState();
    XInsets.instance.addListener(_onTick);
  }

  @override
  void dispose() {
    XInsets.instance.removeListener(_onTick);
    super.dispose();
  }

  void _onTick() {
    if (mounted) setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    final mq = MediaQuery.of(context);
    return MediaQuery(
      data: mq.copyWith(viewPadding: XInsets.instance.viewPadding),
      child: widget.child,
    );
  }
}