BadNote/lib/editor/input/pen_input_service.dart

// lib/editor/input/pen_input_service.dart
//
// Dart side of the native Windows pen observer (`windows/runner/pen_channel.cpp`).
//
// WHY THIS EXISTS: Flutter 3.44 on Windows delivers stylus PRESSURE but drops
// the pen's barrel button, eraser/inverted end, and tilt (it does not map
// POINTER_PEN_FLAG_* into `PointerEvent.buttons`/`invertedStylus`/`tilt`). The
// native plugin observes WM_POINTER + GetPointerPenInfo and streams the missing
// hardware state over an EventChannel; this service latches the LATEST value.
//
// CORRELATION (plan M2): we do NOT key state by Win32 pointerId joined to
// Flutter's `event.pointer` — those are different id spaces. Only one pen is
// active at a time, so a single latched "current" state is correct. The native
// observer runs at the TOP of the window proc (BEFORE Flutter synthesizes its
// pointer event, plan M1), so by the time Dart's pointer-down handler reads
// [current], the latch already reflects that exact contact — no hover required.
//
// GRACEFUL DEGRADATION: on non-Windows (or if the channel is silent) the stream
// simply never emits / errors are swallowed, and [current] stays [PenHardwareState.empty]
// so the canvas falls back to its normal Flutter-pressure drawing.

import 'dart:async';

import 'package:flutter/services.dart';

/// Latest hardware pen state delivered by the native observer.
class PenHardwareState {
  const PenHardwareState({
    this.barrel = false,
    this.inverted = false,
    this.eraser = false,
    this.tiltX = 0.0,
    this.tiltY = 0.0,
  });

  /// Side barrel button held.
  final bool barrel;

  /// Pen flipped to the inverted (eraser) end.
  final bool inverted;

  /// Hardware eraser flag set.
  final bool eraser;

  /// Tilt in degrees along X / Y ([-90, 90]); 0 = perpendicular.
  final double tiltX;
  final double tiltY;

  /// Combined tilt magnitude in degrees (for [PenPoint.tilt]).
  double get tiltMagnitude {
    final t = tiltX * tiltX + tiltY * tiltY;
    return t <= 0 ? 0.0 : _sqrt(t);
  }

  static const empty = PenHardwareState();
}

// Avoids importing dart:math for a single call.
double _sqrt(double v) {
  if (v <= 0) return 0;
  var x = v;
  var last = 0.0;
  // Newton's method; converges fast for the small (<=~127) magnitudes here.
  for (var i = 0; i < 12 && x != last; i++) {
    last = x;
    x = 0.5 * (x + v / x);
  }
  return x;
}

/// Latches the most recent [PenHardwareState] streamed by the native pen plugin.
///
/// Use the singleton [PenInputService.instance]. Call [start] once (e.g. in the
/// editor's `initState`) and [stop] on dispose.
class PenInputService {
  PenInputService._();

  /// Process-wide singleton (one physical pen).
  static final PenInputService instance = PenInputService._();

  /// Must match the native `EventChannel` name in `pen_channel.cpp`.
  static const EventChannel _channel = EventChannel('badnote/pen');

  StreamSubscription<dynamic>? _sub;
  PenHardwareState _current = PenHardwareState.empty;

  /// The latest hardware pen state (or [PenHardwareState.empty] when no native
  /// data has arrived — non-Windows, plugin absent, or channel silent).
  PenHardwareState get current => _current;

  /// Whether the native channel has delivered at least one event (i.e. the
  /// native pen plugin is present and active). Used to prefer hardware signals
  /// over the Flutter fallback only when they are actually available.
  bool get isActive => _active;
  bool _active = false;

  /// Begins listening to the native channel. Idempotent; safe on any platform
  /// (no-ops where the channel has no handler).
  void start() {
    if (_sub != null) return;
    try {
      _sub = _channel.receiveBroadcastStream().listen(
        _onEvent,
        onError: (Object _) {
          // No native handler (e.g. Linux/macOS) or transient error — ignore
          // and keep the empty fallback state.
        },
        cancelOnError: false,
      );
    } catch (_) {
      // receiveBroadcastStream can throw synchronously if the platform side is
      // unavailable; degrade silently.
    }
  }

  void _onEvent(dynamic event) {
    if (event is! Map) return;
    final flags = (event['flags'] as num?)?.toInt() ?? 0;
    _current = PenHardwareState(
      barrel: flags & 0x1 != 0,
      inverted: flags & 0x2 != 0,
      eraser: flags & 0x4 != 0,
      tiltX: (event['tiltX'] as num?)?.toDouble() ?? 0.0,
      tiltY: (event['tiltY'] as num?)?.toDouble() ?? 0.0,
    );
    _active = true;
  }

  /// Stops listening and resets state.
  void stop() {
    _sub?.cancel();
    _sub = null;
    _active = false;
    _current = PenHardwareState.empty;
  }
}