packages/flutter/lib/src/gestures/pointer_signal_resolver.dart

// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'package:flutter/foundation.dart';

import 'events.dart';

/// The callback to register with a [PointerSignalResolver] to express
/// interest in a pointer signal event.
typedef PointerSignalResolvedCallback = void Function(PointerSignalEvent event);

bool _isSameEvent(PointerSignalEvent event1, PointerSignalEvent event2) {
  return (event1.original ?? event1) == (event2.original ?? event2);
}

/// An resolver for pointer signal events.
///
/// Objects interested in a [PointerSignalEvent] should register a callback to
/// be called if they should handle the event. The resolver's purpose is to
/// ensure that the same pointer signal is not handled by multiple objects in
/// a hierarchy.
///
/// Pointer signals are immediate, so unlike a gesture arena it always resolves
/// at the end of event dispatch. The first callback registered will be the one
/// that is called.
class PointerSignalResolver {
  PointerSignalResolvedCallback _firstRegisteredCallback;

  PointerSignalEvent _currentEvent;

  /// Registers interest in handling [event].
  void register(PointerSignalEvent event, PointerSignalResolvedCallback callback) {
    assert(event != null);
    assert(callback != null);
    assert(_currentEvent == null || _isSameEvent(_currentEvent, event));
    if (_firstRegisteredCallback != null) {
      return;
    }
    _currentEvent = event;
    _firstRegisteredCallback = callback;
  }

  /// Resolves the event, calling the first registered callback if there was
  /// one.
  ///
  /// Called after the framework has finished dispatching the pointer signal
  /// event.
  void resolve(PointerSignalEvent event) {
    if (_firstRegisteredCallback == null) {
      assert(_currentEvent == null);
      return;
    }
    assert(_isSameEvent(_currentEvent, event));
    try {
      _firstRegisteredCallback(_currentEvent);
    } catch (exception, stack) {
      InformationCollector collector;
      assert(() {
        collector = () sync* {
          yield DiagnosticsProperty<PointerSignalEvent>('Event', event, style: DiagnosticsTreeStyle.errorProperty);
        };
        return true;
      }());
      FlutterError.reportError(FlutterErrorDetails(
        exception: exception,
        stack: stack,
        library: 'gesture library',
        context: ErrorDescription('while resolving a PointerSignalEvent'),
        informationCollector: collector
      ));
    }
    _firstRegisteredCallback = null;
    _currentEvent = null;
  }
}