Flutter——最详细(Map)使用教程

Map简介

键值对的集合，您可以使用其关联的键从中检索值。

普通的 HashMap 是无序的（不保证顺序），LinkedHashMap 按键插入顺序迭代，而像 SplayTreeMap 这样的排序映射按排序顺序迭代键。

1，添加元素 addEntries()

dart 复制代码

final planets = <int, String>{1: 'Mercury', 2: 'Venus',
  3: 'Earth', 4: 'Mars'};
final gasGiants = <int, String>{5: 'Jupiter', 6: 'Saturn'};
final iceGiants = <int, String>{7: 'Uranus', 8: 'Neptune'};
planets.addEntries(gasGiants.entries);
planets.addEntries(iceGiants.entries);
print(planets);
// {1: Mercury, 2: Venus, 3: Earth, 4: Mars, 5: Jupiter, 6: Saturn,
//  7: Uranus, 8: Neptune}

2，更新组元素 update()

dart 复制代码

final planetsFromSun = <int, String>{1: 'Mercury', 2: 'unknown',
  3: 'Earth'};
// Update value for known key value 2.
planetsFromSun.update(2, (value) => 'Venus');
print(planetsFromSun); // {1: Mercury, 2: Venus, 3: Earth}

final largestPlanets = <int, String>{1: 'Jupiter', 2: 'Saturn',
  3: 'Neptune'};
// Key value 8 is missing from list, add it using [ifAbsent].
largestPlanets.update(8, (value) => 'New', ifAbsent: () => 'Mercury');
print(largestPlanets); // {1: Jupiter, 2: Saturn, 3: Neptune, 8: Mercury}

3，更新所有元素的值 updateAll()

dart 复制代码

final terrestrial = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
terrestrial.updateAll((key, value) => value.toUpperCase());
print(terrestrial); // {1: MERCURY, 2: VENUS, 3: EARTH}

4，删除指定的元素 removeWhere

dart 复制代码

final terrestrial = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
terrestrial.removeWhere((key, value) => value.startsWith('E'));
print(terrestrial); // {1: Mercury, 2: Venus}

5，查找 $key$ 的值，如果不存在，则添加新条目。 putIfAbsent

dart 复制代码

final diameters = <num, String>{1.0: 'Earth'};
final otherDiameters = <double, String>{0.383: 'Mercury', 0.949: 'Venus'};

for (final item in otherDiameters.entries) {
  diameters.putIfAbsent(item.key, () => item.value);
}
print(diameters); // {1.0: Earth, 0.383: Mercury, 0.949: Venus}

// If the key already exists, the current value is returned.
final result = diameters.putIfAbsent(0.383, () => 'Random');
print(result); // Mercury
print(diameters); // {1.0: Earth, 0.383: Mercury, 0.949: Venus}

6，将所有键值对添加到此映射中。 addAll

dart 复制代码

final planets = <int, String>{1: 'Mercury', 2: 'Earth'};
planets.addAll({5: 'Jupiter', 6: 'Saturn'});
print(planets); // {1: Mercury, 2: Earth, 5: Jupiter, 6: Saturn}

7，从映射中删除 $key$ 及其关联值（如果存在）。 remove

dart 复制代码

final terrestrial = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
final removedValue = terrestrial.remove(2); // Venus
print(terrestrial); // {1: Mercury, 3: Earth}

8，从映射中移除所有条目。 clear

dart 复制代码

final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
planets.clear(); // {}

9，循环应用于映射的每个键值对。 forEach

dart 复制代码

final planetsByMass = <num, String>{0.81: 'Venus', 1: 'Earth',
  0.11: 'Mars', 17.15: 'Neptune'};

planetsByMass.forEach((key, value) {
  print('$key: $value');
  // 0.81: Venus
  // 1: Earth
  // 0.11: Mars
  // 17.15: Neptune
});

10，此映射是否包含给定的 $key$ 。 containsKey

dart 复制代码

final moonCount = <String, int>{'Mercury': 0, 'Venus': 0, 'Earth': 1,
  'Mars': 2, 'Jupiter': 79, 'Saturn': 82, 'Uranus': 27, 'Neptune': 14 };
final containsUranus = moonCount.containsKey('Uranus'); // true
final containsPluto = moonCount.containsKey('Pluto'); // false

11，此映射是否包含给定的 $value$ 。 containsValue

dart 复制代码

final moonCount = <String, int>{'Mercury': 0, 'Venus': 0, 'Earth': 1,
  'Mars': 2, 'Jupiter': 79, 'Saturn': 82, 'Uranus': 27, 'Neptune': 14 };
final moons3 = moonCount.containsValue(3); // false
final moons82 = moonCount.containsValue(82); // true

12，创建一个 Map 实例，其中键和值是从 $iterable$ 计算的。 fromIterable

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final numbers = <int>[1, 2, 3];
final map = Map<String, int>.fromIterable(numbers,
    key: (item) => item.toString(),
    value: (item) => item * item);
print(map); // {1: 1, 2: 4, 3: 9}

final numbers = <int>[1, 2, 3];
final map = Map.fromIterable(numbers);
print(map); // {1: 1, 2: 2, 3: 3}

13，创建一个映射，将给定的 $键$ 与给定的 $值$ 相关联。 fromIterables

dart 复制代码

final rings = <bool>[false, false, true, true];
final planets = <String>{'Earth', 'Mars', 'Jupiter', 'Saturn'};
final map = Map<String, bool>.fromIterables(planets, rings);
print(map); // {Earth: false, Mars: false, Jupiter: true, Saturn: true}

14，创建一个映射，将给定的 $键$ 与给定的 $值$ 相关联。 unmodifiable

dart 复制代码

final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
final unmodifiableMap = Map.unmodifiable(planets);
unmodifiableMap[4] = 'Mars'; // Throws