1// Copyright 2014 The Flutter Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5import 'dart:collection';
6
7/// A list optimized for the observer pattern when there are small numbers of
8/// observers.
9///
10/// Consider using an [ObserverList] instead of a [List] when the number of
11/// [contains] calls dominates the number of [add] and [remove] calls.
12///
13/// This class will include in the [iterator] each added item in the order it
14/// was added, as many times as it was added.
15///
16/// If there will be a large number of observers, consider using
17/// [HashedObserverList] instead. It has slightly different iteration semantics,
18/// but serves a similar purpose, while being more efficient for large numbers
19/// of observers.
20///
21/// See also:
22///
23/// * [HashedObserverList] for a list that is optimized for larger numbers of
24/// observers.
25// TODO(ianh): Use DelegatingIterable, possibly moving it from the collection
26// package to foundation, or to dart:collection.
27class ObserverList<T> extends Iterable<T> {
28 final List<T> _list = <T>[];
29 bool _isDirty = false;
30 late final HashSet<T> _set = HashSet<T>();
31
32 /// Adds an item to the end of this list.
33 ///
34 /// This operation has constant time complexity.
35 void add(T item) {
36 _isDirty = true;
37 _list.add(item);
38 }
39
40 /// Removes an item from the list.
41 ///
42 /// This is O(N) in the number of items in the list.
43 ///
44 /// Returns whether the item was present in the list.
45 bool remove(T item) {
46 _isDirty = true;
47 _set.clear(); // Clear the set so that we don't leak items.
48 return _list.remove(item);
49 }
50
51 /// Removes all items from the list.
52 void clear() {
53 _isDirty = false;
54 _list.clear();
55 _set.clear();
56 }
57
58 @override
59 bool contains(Object? element) {
60 if (_list.length < 3) {
61 return _list.contains(element);
62 }
63
64 if (_isDirty) {
65 _set.addAll(_list);
66 _isDirty = false;
67 }
68
69 return _set.contains(element);
70 }
71
72 @override
73 Iterator<T> get iterator => _list.iterator;
74
75 @override
76 bool get isEmpty => _list.isEmpty;
77
78 @override
79 bool get isNotEmpty => _list.isNotEmpty;
80
81 @override
82 List<T> toList({bool growable = true}) {
83 return _list.toList(growable: growable);
84 }
85}
86
87/// A list optimized for the observer pattern, but for larger numbers of observers.
88///
89/// For small numbers of observers (e.g. less than 10), use [ObserverList] instead.
90///
91/// The iteration semantics of the this class are slightly different from
92/// [ObserverList]. This class will only return an item once in the [iterator],
93/// no matter how many times it was added, although it does require that an item
94/// be removed as many times as it was added for it to stop appearing in the
95/// [iterator]. It will return them in the order the first instance of an item
96/// was originally added.
97///
98/// See also:
99///
100/// * [ObserverList] for a list that is fast for small numbers of observers.
101class HashedObserverList<T> extends Iterable<T> {
102 final LinkedHashMap<T, int> _map = LinkedHashMap<T, int>();
103
104 /// Adds an item to the end of this list.
105 ///
106 /// This has constant time complexity.
107 void add(T item) {
108 _map[item] = (_map[item] ?? 0) + 1;
109 }
110
111 /// Removes an item from the list.
112 ///
113 /// This operation has constant time complexity.
114 ///
115 /// Returns whether the item was present in the list.
116 bool remove(T item) {
117 final int? value = _map[item];
118 if (value == null) {
119 return false;
120 }
121 if (value == 1) {
122 _map.remove(item);
123 } else {
124 _map[item] = value - 1;
125 }
126 return true;
127 }
128
129 @override
130 bool contains(Object? element) => _map.containsKey(element);
131
132 @override
133 Iterator<T> get iterator => _map.keys.iterator;
134
135 @override
136 bool get isEmpty => _map.isEmpty;
137
138 @override
139 bool get isNotEmpty => _map.isNotEmpty;
140}
141