CODESAMPLE

Specification - Dart

Specification Dart

The Specification pattern is a functional approach to defining complex data validation or filtering rules. It encapsulates a condition as an object, allowing for reusable and composable logic. Instead of embedding validation directly within a class, Specifications define what criteria data must meet, separate from how the data is handled.

This Dart implementation uses a generic Specification class with a satisfies method that takes an object and returns a boolean. Concrete specifications are created by extending this class and overriding satisfies. The and, or, and not methods allow for combining specifications, promoting code reuse and readability. This approach aligns with Dart’s support for functional programming and encourages a declarative style.

// specification.dart

abstract class Specification<T> {
  bool satisfies(T item);

  Specification<T> and(Specification<T> other) {
    return _AndSpecification(this, other);
  }

  Specification<T> or(Specification<T> other) {
    return _OrSpecification(this, other);
  }

  Specification<T> not() {
    return _NotSpecification(this);
  }
}

class _AndSpecification<T> implements Specification<T> {
  final Specification<T> first;
  final Specification<T> second;

  _AndSpecification(this.first, this.second);

  @override
  bool satisfies(T item) => first.satisfies(item) && second.satisfies(item);
}

class _OrSpecification<T> implements Specification<T> {
  final Specification<T> first;
  final Specification<T> second;

  _OrSpecification(this.first, this.second);

  @override
  bool satisfies(T item) => first.satisfies(item) || second.satisfies(item);
}

class _NotSpecification<T> implements Specification<T> {
  final Specification<T> spec;

  _NotSpecification(this.spec);

  @override
  bool satisfies(T item) => !spec.satisfies(item);
}

class IsPositive extends Specification<int> {
  @override
  bool satisfies(int number) => number > 0;
}

class IsEven extends Specification<int> {
  @override
  bool satisfies(int number) => number % 2 == 0;
}

void main() {
  final isPositiveAndEven = IsPositive().and(IsEven());

  print(isPositiveAndEven.satisfies(4));   // true
  print(isPositiveAndEven.satisfies(5));   // false
  print(isPositiveAndEven.satisfies(-2));  // false

  final isPositiveOrNegative = IsPositive().or(IsEven().not());

  print(isPositiveOrNegative.satisfies(3)); // true
  print(isPositiveOrNegative.satisfies(2)); // false
  print(isPositiveOrNegative.satisfies(-1)); // true
}