如何正确覆盖 class 以便在 Flutter [Dart] 中将其唯一添加到地图中

Question

我想在 Flutter 中创建一个 Map 并将 activityProject 唯一地存储在地图中。您可以在下方看到截取的代码。

void main() {
  Map<ActivityProject, int> timesheet = Map.identity();

  timesheet.putIfAbsent(ActivityProject('activity', 'project'), () => 1);
  timesheet.putIfAbsent(ActivityProject('activity', 'project'), () => 2);
  timesheet.putIfAbsent(ActivityProject('activity', 'project'), () => 3);

    
  timesheet.entries.forEach((element) {
    print("Hashcode: ${element.key.hashCode}" );
    print(
        "Key: ${element.key.activity}-${element.key.project}, Value: ${element.value}");
  });
}

ActivityProjectclass定义如下

class ActivityProject {
  String activity;
  String project;

  ActivityProject(this.activity, this.project);

  @override
  bool operator ==(Object other) =>
      other is ActivityProject &&
      other.activity == activity &&
      other.project == project;

  @override
  int get hashCode => hashValues(activity, project);
}

我已经覆盖了“==”和hashCode，但即使哈希码相同，值也会放在里面地图。

输出：

Hashcode: 176486800
Key: activity-project, Value: 1
Hashcode: 176486800
Key: activity-project, Value: 2
Hashcode: 176486800
Key: activity-project, Value: 3

如有任何建议，我们将不胜感激。

Answer 1

这里的问题是您正在使用 Map.identity()。在 Map<K, V>.identity constructor.

上阅读此处的文档

An identity map uses identical for equality and identityHashCode for hash codes of keys instead of the intrinsic Object.== and Object.hashCode of the keys.

根据文档，使用 Map.identity 构建的地图不使用 == 和 hashCode。

您应该这样定义您的地图：

Map<ActivityProject, int> timesheet = {};