相当于 Swift “扩展”的 dart 是什么？

Question

Keyword/reserved word extension allow code to be much cleaner by using an existing class in the standard frameworks without subclassing them. Is there a keyword in Dart that allows the same behavior?

extension Double {
    var mm: Double { return self / 1_000.0 }
}

let oneInch = 25.4.mm
print("One inch is \(oneInch) meters")

Answer 1

They are finally here 🥳

From dart version 2.6 (not 100% sure).

class Foo {
  printFoo()  {
    print("Foo");
  }
}

extension Bar on Foo { 
  printBar()  {
    print("Bar");
  }
}

void main() {
  final foo = Foo();
  foo.printFoo();
  foo.printBar();
}

prints:

Foo

Bar

You can give it a try in dart pad.

Answer 2

Extension methods are currently being implemented into the language.

https://github.com/dart-lang/language/projects/1#card-18758939

Specifications document

https://github.com/dart-lang/language/blob/master/accepted/future-releases/static-extension-methods/feature-specification.md

Answer 3

TL;DR: Sadly, Dart doesn't support extension methods.

In the Dart repository, there's an issue about how to add new functionality to existing types for that. More specifically, there are issues for extension methods and extension types .

Of course, you can always use new class es that extend existing classes introduce new functionality on top of them. However, that only works for non-primitive classes, so let's hope for the best. Your best shot probably is to upvote the issues linked above.