Question 167811 in Flutter Question posted in
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Flutter – How to use Comparable correctly with Dart 3?

With Dart SDK 2.x.x. it was okay to use any class as mixin. But with Dart 3 this is no longer allowed as stated in the documentation: https://dart.dev/resources/dart-3-migration#mixin

Now I struggle with the migration. For example, I used the Comparable interface as mixin like this:

mixin Compare<T> on Comparable<T> {
  bool operator <=(T other) => compareTo(other) <= 0;
  bool operator >=(T other) => compareTo(other) >= 0;
  bool operator <(T other) => compareTo(other) < 0;
  bool operator >(T other) => compareTo(other) > 0;
}


class Foo with Comparable<Foo>, Compare<Foo> {
  final int value;

  const Foo({required this.value});
  
  @override
  int compareTo(Foo other) => value.compareTo(other.value);
}

But this gives me the error error: The class 'Comparable' can't be used as a mixin because it's neither a mixin class nor a mixin.

But if I try to use it as interface like this

mixin Compare<T> on Comparable<T> {
  bool operator <=(T other) => compareTo(other) <= 0;
  bool operator >=(T other) => compareTo(other) >= 0;
  bool operator <(T other) => compareTo(other) < 0;
  bool operator >(T other) => compareTo(other) > 0;
}


class Foo extends Comparable<Foo> with Compare<Foo> {
  final int value;

  const Foo({required this.value});

  @override
  int compareTo(Foo other) => value.compareTo(other.value);
}

I get another error The class 'Comparable' can't be extended outside of its library because it's an interface class.

So what is the best way to migrate my code to Dart 3?

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    The comment of @jamesdlin answers my question:

    mixin Compare<T> implements Comparable<T> {
  bool operator <=(T other) => compareTo(other) <= 0;
  bool operator >=(T other) => compareTo(other) >= 0;
  bool operator <(T other) => compareTo(other) < 0;
  bool operator >(T other) => compareTo(other) > 0;
}


class Foo with Compare<Foo> {
  final int value;

  const Foo({required this.value});
  
  @override
  int compareTo(Foo other) => value.compareTo(other.value);
}

    That why, Compare can be a mixin.


  2. 0

    What you were trying to get can be achieved by extensions.

    void main() {
  final a = Foo(value: 20);
  final b = Foo(value: 40);

  print(a > b);
}

extension Compare<T> on Comparable<T> {
  bool operator <=(T other) => compareTo(other) <= 0;
  bool operator >=(T other) => compareTo(other) >= 0;
  bool operator <(T other) => compareTo(other) < 0;
  bool operator >(T other) => compareTo(other) > 0;
}

class Foo implements Comparable<Foo> {
  final int value;

  const Foo({required this.value});

  @override
  int compareTo(Foo other) => value.compareTo(other.value);
}

    If you comment extension code, you will get compile time error for print(a >b); hence this is valid code as per Dart3

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