更改检查地图项是否相等的方式 [英] Change how map keys are checked for equality

问题描述

我有以下代码:

class KeyClass {
  int property;

  KeyClass(this.property);
}

void main() {
  KeyClass kc1 = KeyClass(1);
  KeyClass kc2 = KeyClass(2);

  Map<KeyClass, String> map = Map();
  map[kc1] = 'hello';
  map[kc2] = 'world';
  ...
}

我的目标是使以下两行从我的 map 中获得相同的值:

My goal is to for the following two lines to get the same value from my map:

print(map[kc1]);          // prints 'hello'
print(map[KeyClass(1)]);  // prints 'null', should print 'hello' too!

Dart语言有可能吗?

Is this possible in Dart language?

推荐答案

默认的 Map 实现是 LinkedHashMap ，因此它依赖于计算键的哈希码.有几种方法可以使您的键比较相等:

The default Map implementation is a LinkedHashMap, so it relies on computing hash codes for the keys. There are a few ways you could make your keys compare equal:

1. 实施 KeyClass.operator == 和 KeyCode.hashCode :

class KeyClass {
  int property;

  KeyClass(this.property);

  bool operator ==(dynamic other) {
    return runtimeType == other.runtimeType && property == other.property;
  }

  int get hashCode => property.hashCode;
}

直接使用 LinkedHashMap . LinkedHashMap 的构造函数允许提供自定义回调以计算相等性和哈希码:

Use LinkedHashMap directly. LinkedHashMap's constructor allows providing custom callbacks for computing equality and hash codes:

bool keyEquals(KeyClass k1, KeyClass k2) => k1.property == k2.property;
int keyHashCode(KeyClass k) => k.property.hashCode;

Map<KeyClass, String> map = LinkedHashMap<KeyClass, String>(
  equals: keyEquals,
  hashCode: keyHashCode,
);

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