| // Copyright (c) 2017, the Dart project authors. Please see the AUTHORS file |
| // for details. All rights reserved. Use of this source code is governed by a |
| // BSD-style license that can be found in the LICENSE file. |
| |
| import 'dart:collection'; |
| |
| import 'combined_iterable.dart'; |
| |
| /// Returns a new map that represents maps flattened into a single map. |
| /// |
| /// All methods and accessors treat the new map as-if it were a single |
| /// concatenated map, but the underlying implementation is based on lazily |
| /// accessing individual map instances. In the occasion where a key occurs in |
| /// multiple maps the first value is returned. |
| /// |
| /// The resulting map has an index operator (`[]`) and `length` property that |
| /// are both `O(maps)`, rather than `O(1)`, and the map is unmodifiable - but |
| /// underlying changes to these maps are still accessible from the resulting |
| /// map. |
| class CombinedMapView<K, V> extends UnmodifiableMapBase<K, V> { |
| final Iterable<Map<K, V>> _maps; |
| |
| /// Create a new combined view into multiple maps. |
| /// |
| /// The iterable is accessed lazily so it should be collection type like |
| /// [List] or [Set] rather than a lazy iterable produced by `map()` et al. |
| CombinedMapView(this._maps); |
| |
| V operator [](Object key) { |
| for (var map in _maps) { |
| // Avoid two hash lookups on a positive hit. |
| var value = map[key]; |
| if (value != null || map.containsKey(value)) { |
| return value; |
| } |
| } |
| return null; |
| } |
| |
| /// The keys of [this]. |
| /// |
| /// The returned iterable has efficient `length` and `contains` operations, |
| /// based on [length] and [containsKey] of the individual maps. |
| /// |
| /// The order of iteration is defined by the individual `Map` implementations, |
| /// but must be consistent between changes to the maps. |
| /// |
| /// Unlike most [Map] implementations, modifying an individual map while |
| /// iterating the keys will _sometimes_ throw. This behavior may change in |
| /// the future. |
| Iterable<K> get keys => CombinedIterableView<K>(_maps.map((m) => m.keys)); |
| } |