putIfAbsent 方法

dynamic putIfAbsent(
  1. String key,
  2. dynamic ifAbsent(
      )
    )
    override

    查找key的值，如果不存在则添加新条目。

    如果有与key关联的值，则返回该值。如果没有，则调用ifAbsent以获取新值，将key关联到该新值，然后返回该新值。

    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}

    调用ifAbsent时不应从映射中添加或删除键。

    实现

    dynamic putIfAbsent(String key, dynamic ifAbsent()) {
  throw new UnsupportedError("Not supported");
}