Lære Value Equality and Comparison in Dart | Writing Clean Dart for Scalable Flutter Apps

Understanding how Dart compares objects is crucial for writing reliable and maintainable Flutter apps. Dart distinguishes between reference equality and value equality. Reference equality checks whether two variables point to the exact same object in memory, using the == operator by default if not overridden. Value equality, on the other hand, checks whether two objects contain the same data or properties, regardless of whether they are the same instance. Without explicitly overriding equality in your classes, Dart will only compare object references, not their contents.

value_equality_example.dart


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class User {
  final String name;
  final int age;

  User(this.name, this.age);

  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
      other is User &&
          runtimeType == other.runtimeType &&
          name == other.name &&
          age == other.age;

  @override
  int get hashCode => name.hashCode ^ age.hashCode;
}

void main() {
  var user1 = User('Alice', 30);
  var user2 = User('Alice', 30);
  print(user1 == user2); // true
}

In Flutter, value equality is especially important for state management and widget rebuilding. Flutter relies on object comparisons to determine if it needs to update the UI. If your data classes only use reference equality, Flutter might not recognize changes in state, leading to stale or incorrect UI. By implementing value equality, you ensure that Flutter can accurately detect changes and update widgets as needed.

flutter_list_comparison.dart


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// flutter_list_comparison.dart
class TodoItem {
  final String task;
  final bool completed;

  TodoItem(this.task, this.completed);

  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
      other is TodoItem &&
          runtimeType == other.runtimeType &&
          task == other.task &&
          completed == other.completed;

  @override
  int get hashCode => task.hashCode ^ completed.hashCode;
}

void main() {
  var list = [
    TodoItem('Buy milk', false),
    TodoItem('Walk dog', true),
  ];
  var newItem = TodoItem('Buy milk', false);

  print(list.contains(newItem)); // true, thanks to value equality
}

Note

By always implementing value equality (== and hashCode) in your custom classes, you avoid subtle bugs where collections like List, Set, or Map fail to recognize logically identical objects. This is essential for predictable behavior in Flutter apps, especially when working with state or caching.

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