一个集合如何确定两个对象在飞镖中相等？答案

【问题标题】：How does a set determine that two objects are equal in dart?一个集合如何确定两个对象在飞镖中相等？
【发布时间】：2015-06-16 12:29:10
【问题描述】：

我不明白集合如何确定两个对象何时相等。更具体地说，集合的add方法什么时候真正添加了一个新对象，什么时候不作为一个新对象，因为该对象已经在集合中？

例如，我有以下类的对象：

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"));
}

但是，这个集合包含 3 个Action 对象。请参阅demo。如何确保在集合中相等的对象被视为相等？

【问题讨论】：

标签： dart

【解决方案1】：

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

它只是检查它们是否相等 a == b 您可以覆盖 == 运算符来自定义此行为。请记住，当== 运算符被覆盖时，hashCode 也应该被覆盖。

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
}

试试DartPad

【讨论】：

operator== 的参数永远不会是 null，因为在调用 operator== 之前会明确检查 null。您可以删除other == null 检查。
太好了，非常感谢！这不是我记得的，但它更全面。

【解决方案2】：

这是一个测试顶级函数、静态方法和实例方法是否相等的示例：

void foo() {} // A top-level function

class A {
  static void bar() {} // A static method
  void baz() {} // An instance method
}

void main() {
  var x;

  // Comparing top-level functions.
  x = foo;
  assert(foo == x);

  // Comparing static methods.
  x = A.bar;
  assert(A.bar == x);

  // Comparing instance methods.
  var v = A(); // Instance #1 of A
  var w = A(); // Instance #2 of A
  var y = w;
  x = w.baz;

  // These closures refer to the same instance (#2),
  // so they're equal.
  assert(y.baz == x);

  // These closures refer to different instances,
  // so they're unequal.
  assert(v.baz != w.baz);
}

【讨论】：