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

// Copyright 2015 The Chromium 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 'events.dart';

/// An object that can hit-test pointers.
abstract class HitTestable { // ignore: one_member_abstracts
  // This class is intended to be used as an interface with the implements
  // keyword, and should not be extended directly.
  factory HitTestable._() => null;

  /// Check whether the given position hits this object.
  ///
  /// If this given position hits this object, consider adding a [HitTestEntry]
  /// to the given hit test result.
  void hitTest(HitTestResult result, Offset position);
}

/// An object that can dispatch events.
abstract class HitTestDispatcher { // ignore: one_member_abstracts
  // This class is intended to be used as an interface with the implements
  // keyword, and should not be extended directly.
  factory HitTestDispatcher._() => null;

  /// Override this method to dispatch events.
  void dispatchEvent(PointerEvent event, HitTestResult result);
}

/// An object that can handle events.
abstract class HitTestTarget { // ignore: one_member_abstracts
  // This class is intended to be used as an interface with the implements
  // keyword, and should not be extended directly.
  factory HitTestTarget._() => null;

  /// Override this method to receive events.
  void handleEvent(PointerEvent event, HitTestEntry entry);
}

/// Data collected during a hit test about a specific [HitTestTarget].
///
/// Subclass this object to pass additional information from the hit test phase
/// to the event propagation phase.
class HitTestEntry {
  /// Creates a hit test entry.
  const HitTestEntry(this.target);

  /// The [HitTestTarget] encountered during the hit test.
  final HitTestTarget target;

  @override
  String toString() => '$target';
}

/// The result of performing a hit test.
class HitTestResult {
  /// Creates a hit test result.
  ///
  /// If the [path] argument is null, the [path] field will be initialized with
  /// and empty list.
  HitTestResult({ List<HitTestEntry> path })
    : _path = path ?? <HitTestEntry>[];

  /// An unmodifiable list of [HitTestEntry] objects recorded during the hit test.
  ///
  /// The first entry in the path is the most specific, typically the one at
  /// the leaf of tree being hit tested. Event propagation starts with the most
  /// specific (i.e., first) entry and proceeds in order through the path.
  List<HitTestEntry> get path => new List<HitTestEntry>.unmodifiable(_path);
  final List<HitTestEntry> _path;

  /// Add a [HitTestEntry] to the path.
  ///
  /// The new entry is added at the end of the path, which means entries should
  /// be added in order from most specific to least specific, typically during an
  /// upward walk of the tree being hit tested.
  void add(HitTestEntry entry) {
    _path.add(entry);
  }

  @override
  String toString() => 'HitTestResult(${_path.isEmpty ? "<empty path>" : _path.join(", ")})';
}