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Extension 方法

Extension methods, introduced in Dart 2.7, are a way to add functionality to existing libraries. You might use extension methods without even knowing it. For example, when you use code completion in an IDE, it suggests extension methods alongside regular methods.

If you like to learn by watching videos, here’s a good overview of extension methods.

Overview

When you’re using someone else’s API or when you implement a library that’s widely used, it’s often impractical or impossible to change the API. But you might still want to add some functionality.

For example, consider the following code that parses a string into an integer:

int.parse('42')

It might be nice — shorter and easier to use with tools — to have that functionality be on String instead:

'42'.parseInt()

To enable that code, you can import a library that contains an extension of the String class:

import 'string_apis.dart';
// ···
print('42'.parseInt()); // Use an extension method.

Extensions can define not just methods, but also other members such as getter, setters, and operators. Also, extensions have names, which can be helpful if an API conflict arises. Here’s how you might implement the extension method parseInt(), using an extension (named NumberParsing) that operates on strings:

extension NumberParsing on String {
  int parseInt() {
    return int.parse(this);
  }
  // ···
}
lib/string_apis.dart

The next section describes how to use extension methods. After that are sections about implementing extension methods.

Using extension methods

Like all Dart code, extension methods are in libraries. You’ve already seen how to use an extension method — just import the library it’s in, and use it like an ordinary method:

// Import a library that contains an extension on String.
import 'string_apis.dart';
// ···
print('42'.padLeft(5)); // Use a String method.
print('42'.parseInt()); // Use an extension method.

That’s all you usually need to know to use extension methods. As you write your code, you might also need to know how extension methods depend on static types (as opposed to dynamic) and how to resolve API conflicts.

Static types and dynamic

You can’t invoke extension methods on variables of type dynamic. For example, the following code results in a runtime exception:

dynamic d = '2';
print(d.parseInt()); // Runtime exception: NoSuchMethodError

Extension methods do work with Dart’s type inference. The following code is fine because the variable v is inferred to have type String:

var v = '2';
print(v.parseInt()); // Output: 2

The reason that dynamic doesn’t work is that extension methods are resolved against the static type of the receiver. Because extension methods are resolved statically, they’re as fast as calling a static function.

For more information about static types and dynamic, see The Dart type system.

API conflicts

If an extension member conflicts with an interface or with another extension member, then you have a few options.

One option is changing how you import the conflicting extension, using show or hide to limit the exposed API:

// Defines the String extension method parseInt().
import 'string_apis.dart';

// Also defines parseInt(), but hiding NumberParsing2
// hides that extension method.
import 'string_apis_2.dart' hide NumberParsing2;

// ···
// Uses the parseInt() defined in 'string_apis.dart'.
print('42'.parseInt());

Another option is applying the extension explicitly, which results in code that looks as if the extension is a wrapper class:

// Both libraries define extensions on String that contain parseInt(),
// and the extensions have different names.
import 'string_apis.dart'; // Contains NumberParsing extension.
import 'string_apis_2.dart'; // Contains NumberParsing2 extension.

// ···
// print('42'.parseInt()); // Doesn't work.
print(NumberParsing('42').parseInt());
print(NumberParsing2('42').parseInt());

If both extensions have the same name, then you might need to import using a prefix:

// Both libraries define extensions named NumberParsing
// that contain the extension method parseInt(). One NumberParsing
// extension (in 'string_apis_3.dart') also defines parseNum().
import 'string_apis.dart';
import 'string_apis_3.dart' as rad;

// ···
// print('42'.parseInt()); // Doesn't work.

// Use the ParseNumbers extension from string_apis.dart.
print(NumberParsing('42').parseInt());

// Use the ParseNumbers extension from string_apis_3.dart.
print(rad.NumberParsing('42').parseInt());

// Only string_apis_3.dart has parseNum().
print('42'.parseNum());

As the example shows, you can invoke extension methods implicitly even if you import using a prefix. The only time you need to use the prefix is to avoid a name conflict when invoking an extension explicitly.

Implementing extension methods

Use the following syntax to create an extension:

extension <extension name> on <type> {
  (<member definition>)*
}

For example, here’s how you might implement an extension on the String class:

extension NumberParsing on String {
  int parseInt() {
    return int.parse(this);
  }

  double parseDouble() {
    return double.parse(this);
  }
}
lib/string_apis.dart

To create a local extension that’s visible only in the library where it’s declared, either omit the extension name or give it a name that starts with an underscore (_).

The members of the extension can be methods, getters, setters, operators. Extensions can also have static fields and static helper methods.

Implementing generic extensions

Extensions can have generic type parameters. For example, here’s some code that extends the built-in List<T> type with a getter, an operator, and a method:

extension MyFancyList<T> on List<T> {
  int get doubleLength => length * 2;
  List<T> operator -() => reversed.toList();
  List<List<T>> split(int at) => <List<T>>[sublist(0, at), sublist(at)];
}

The type T is bound based on the static type of the list that the methods are called on.

Resources

For more information about extension methods, see the following: