一组如何确定两个对象在dart中是否相等？

Question

我不明白一个集合如何确定两个对象何时相等.更具体地说,add一个集合的方法何时真正添加一个新对象,何时它不是一个新对象,因为该对象已经在集合中？

例如,我有以下类中的对象:

class Action {
  final Function function;
  final String description;

  Action(this.function, this.description);

  call() => function();

  toString() => description;
}

现在我认为以下集合将包含2个元素,因为其中2个是相等的:

void main() {
  Set<Action> actions = new Set()
    ..add(new Action(() => print("a"), "print a"))  
    ..add(new Action(() => print("a"), "print a"))
    ..add(new Action(() => print("b"), "print b"));
}

但相反,这个集合包含3个Action对象.看演示.如何确保在集合中看到相等的对象相等？

Answer 1

有关operator==Dart 的全面介绍,请参阅http://work.j832.com/2014/05/equality-and-dart.html

它只检查它们是否相等a == b您可以覆盖==运算符以自定义此行为.请记住,hashCode在覆盖==运算符时也应该重写.

class Action {
  @override
  bool operator==(other) {
    // Dart ensures that operator== isn't called with null
    // if(other == null) {
    //   return false;
    // }
    if(other is! Action) {
      return false;
    }
    return description == (other as Action).description;
  }

  // hashCode must never change otherwise the value can't
  // be found anymore for example when used as key 
  // in hashMaps therefore we cache it after first creation.
  // If you want to combine more values in hashCode creation
  // see http://stackoverflow.com/a/26648915/217408
  // This is just one attempt, depending on your requirements
  // different handling might be more appropriate.
  // As far as I am aware there is no correct answer for
  // objects where the members taking part of hashCode and
  // equality calculation are mutable.
  // See also http://stackoverflow.com/a/27609/217408
  int _hashCode;
  @override
  int get hashCode {
    if(_hashCode == null) {
      _hashCode = description.hashCode
    }
    return _hashCode;
  }
  // when the key (description) is immutable and the only
  // member of the key you can just use
  // int get hashCode => description.hashCode
}

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