Dart 简介

This page provides a brief introduction to the Dart language through samples of its main features.
本页通过 Dart 语言的主要功能示例对 Dart 语言进行了简要介绍。

To learn more about the Dart language, visit the in-depth, individual topic pages listed under Language in the left side menu.
要了解有关 Dart 语言的更多信息，请访问左侧菜单中“语言”下列出的深入的各个主题页面。

For coverage of Dart’s core libraries, check out the library tour. You can also visit the Dart cheatsheet codelab, for a more hands-on introduction.
有关 Dart 核心库的报道，请查看图书馆导览。您还可以访问 Dart 备忘单 Codelab，获取更多动手介绍。

Hello World 世界您好

Every app requires the top-level main() function, where execution starts. Functions that don’t explicitly return a value have the void return type. To display text on the console, you can use the top-level print() function:
每个应用都需要顶级函数，从该 main() 函数开始执行。不显式返回值的函数具有 void 返回类型。若要在控制台上显示文本，可以使用顶级 print() 函数：

void main() {
  print('Hello, World!');
}

Read more about the main() function in Dart, including optional parameters for command-line arguments.
阅读有关 Dart 中 main() 函数的更多信息，包括命令行参数的可选参数。

Variables 变量

Even in type-safe Dart code, you can declare most variables without explicitly specifying their type using var. Thanks to type inference, these variables’ types are determined by their initial values:
即使在类型安全的 Dart 代码中，也可以声明大多数变量，而无需使用 var 显式指定其类型。由于类型推断，这些变量的类型由其初始值决定：

var name = 'Voyager I';
var year = 1977;
var antennaDiameter = 3.7;
var flybyObjects = ['Jupiter', 'Saturn', 'Uranus', 'Neptune'];
var image = {
  'tags': ['saturn'],
  'url': '//path/to/saturn.jpg'
};

Read more about variables in Dart, including default values, the final and const keywords, and static types.
阅读有关 Dart 中的变量的更多信息，包括默认值、 final 和 const 关键字以及静态类型。

Control flow statements 控制流语句

Dart supports the usual control flow statements:
Dart 支持常用的控制流语句：

if (year >= 2001) {
  print('21st century');
} else if (year >= 1901) {
  print('20th century');
}

for (final object in flybyObjects) {
  print(object);
}

for (int month = 1; month <= 12; month++) {
  print(month);
}

while (year < 2016) {
  year += 1;
}

Read more about control flow statements in Dart, including break and continue, switch and case, and assert.
阅读有关 Dart 中的控制流语句的更多信息，包括 break 和 、 switch 和 case assert continue 。

Functions 功能

We recommend specifying the types of each function’s arguments and return value:
建议指定每个函数的参数和返回值的类型：

int fibonacci(int n) {
  if (n == 0 || n == 1) return n;
  return fibonacci(n - 1) + fibonacci(n - 2);
}

var result = fibonacci(20);

A shorthand => (arrow) syntax is handy for functions that contain a single statement. This syntax is especially useful when passing anonymous functions as arguments:
速记 => （箭头）语法对于包含单个语句的函数非常方便。当将匿名函数作为参数传递时，此语法特别有用：

flybyObjects.where((name) => name.contains('turn')).forEach(print);

Besides showing an anonymous function (the argument to where()), this code shows that you can use a function as an argument: the top-level print() function is an argument to forEach().
除了显示匿名函数（参数 的参数）之外，此代码还显示您可以使用函数作为参数：顶级 print() 函数是 的 forEach() 参数 where() 。

Read more about functions in Dart, including optional parameters, default parameter values, and lexical scope.
阅读有关 Dart 中的函数的更多信息，包括可选参数、默认参数值和词法范围。

Comments 评论

Dart comments usually start with //.
Dart 注释通常以 // .

// This is a normal, one-line comment.

/// This is a documentation comment, used to document libraries,
/// classes, and their members. Tools like IDEs and dartdoc treat
/// doc comments specially.

/* Comments like these are also supported. */

Read more about comments in Dart, including how the documentation tooling works.
阅读有关 Dart 中注释的更多信息，包括文档工具的工作原理。

Imports 进口

To access APIs defined in other libraries, use import.
要访问其他库中定义的 API，请使用 import .

// Importing core libraries
import 'dart:math';

// Importing libraries from external packages
import 'package:test/test.dart';

// Importing files
import 'path/to/my_other_file.dart';

Read more about libraries and visibility in Dart, including library prefixes, show and hide, and lazy loading through the deferred keyword.
阅读有关 Dart 中的库和可见性的更多信息，包括库前缀和 show hide ，以及通过 deferred 关键字延迟加载。

Classes 类

Here’s an example of a class with three properties, two constructors, and a method. One of the properties can’t be set directly, so it’s defined using a getter method (instead of a variable).
下面是一个具有三个属性、两个构造函数和一个方法的类的示例。其中一个属性不能直接设置，因此它是使用 getter 方法（而不是变量）定义的。
The method uses string interpolation to print variables’ string equivalents inside of string literals.
该方法使用字符串插值在字符串文本中打印变量的字符串等效项。

class Spacecraft {
  String name;
  DateTime? launchDate;

  // Read-only non-final property
  int? get launchYear => launchDate?.year;

  // Constructor, with syntactic sugar for assignment to members.
  Spacecraft(this.name, this.launchDate) {
    // Initialization code goes here.
  }

  // Named constructor that forwards to the default one.
  Spacecraft.unlaunched(String name) : this(name, null);

  // Method.
  void describe() {
    print('Spacecraft: $name');
    // Type promotion doesn't work on getters.
    var launchDate = this.launchDate;
    if (launchDate != null) {
      int years = DateTime.now().difference(launchDate).inDays ~/ 365;
      print('Launched: $launchYear ($years years ago)');
    } else {
      print('Unlaunched');
    }
  }
}

Read more about strings, including string interpolation, literals, expressions, and the toString() method.
阅读有关字符串的详细信息，包括字符串插值、文本、表达式和 toString() 方法。

You might use the Spacecraft class like this:
您可以像这样使用该 Spacecraft 类：

var voyager = Spacecraft('Voyager I', DateTime(1977, 9, 5));
voyager.describe();

var voyager3 = Spacecraft.unlaunched('Voyager III');
voyager3.describe();

Read more about classes in Dart, including initializer lists, optional new and const, redirecting constructors, factory constructors, getters, setters, and much more.
阅读有关 Dart 中的类的更多信息，包括初始值设定项列表、可选 new 和 const 重定向构造函数、构造函数、 factory getter 和 setters 等等。

Enums 枚举

Enums are a way of enumerating a predefined set of values or instances in a way which ensures that there cannot be any other instances of that type.
枚举是一种枚举一组预定义的值或实例的方法，以确保该类型的任何其他实例。

Here is an example of a simple enum that defines a simple list of predefined planet types:
下面是一个简单示例，它定义了预定义行星类型的简单 enum 列表：

enum PlanetType { terrestrial, gas, ice }

Here is an example of an enhanced enum declaration of a class describing planets, with a defined set of constant instances, namely the planets of our own solar system.
下面是一个描述行星的类的增强枚举声明示例，具有一组定义的常量实例，即我们自己太阳系的行星。

/// Enum that enumerates the different planets in our solar system
/// and some of their properties.
enum Planet {
  mercury(planetType: PlanetType.terrestrial, moons: 0, hasRings: false),
  venus(planetType: PlanetType.terrestrial, moons: 0, hasRings: false),
  // ···
  uranus(planetType: PlanetType.ice, moons: 27, hasRings: true),
  neptune(planetType: PlanetType.ice, moons: 14, hasRings: true);

  /// A constant generating constructor
  const Planet(
      {required this.planetType, required this.moons, required this.hasRings});

  /// All instance variables are final
  final PlanetType planetType;
  final int moons;
  final bool hasRings;

  /// Enhanced enums support getters and other methods
  bool get isGiant =>
      planetType == PlanetType.gas || planetType == PlanetType.ice;
}

You might use the Planet enum like this:
您可以像这样使用 Planet 枚举：

final yourPlanet = Planet.earth;

if (!yourPlanet.isGiant) {
  print('Your planet is not a "giant planet".');
}

Read more about enums in Dart, including enhanced enum requirements, automatically introduced properties, accessing enumerated value names, switch statement support, and much more.
阅读有关 Dart 中枚举的更多信息，包括增强的枚举要求、自动引入的属性、访问枚举值名称、switch 语句支持等等。

Inheritance 遗产

Dart has single inheritance.
Dart 具有单一继承权。

class Orbiter extends Spacecraft {
  double altitude;

  Orbiter(super.name, DateTime super.launchDate, this.altitude);
}

Read more about extending classes, the optional @override annotation, and more.
阅读有关扩展类、可选 @override 注释等的更多信息。

Mixins 混合

Mixins are a way of reusing code in multiple class hierarchies. The following is a mixin declaration:
Mixin 是一种在多个类层次结构中重用代码的方法。下面是一个 mixin 声明：

mixin Piloted {
  int astronauts = 1;

  void describeCrew() {
    print('Number of astronauts: $astronauts');
  }
}

To add a mixin’s capabilities to a class, just extend the class with the mixin.
要将 mixin 的功能添加到类中，只需使用 mixin 扩展该类即可。

class PilotedCraft extends Spacecraft with Piloted {
  // ···
}

PilotedCraft now has the astronauts field as well as the describeCrew() method.
PilotedCraft 现在有了 astronauts 字段和 describeCrew() 方法。

Read more about mixins.
阅读更多关于 mixins 的信息。

Interfaces and abstract classes

接口和抽象类

All classes implicitly define an interface. Therefore, you can implement any class.
所有类都隐式定义一个接口。因此，您可以实现任何类。

class MockSpaceship implements Spacecraft {
  // ···
}

Read more about implicit interfaces, or about the explicit interface keyword.
阅读有关隐式接口或显式 interface 关键字的更多信息。

You can create an abstract class to be extended (or implemented) by a concrete class. Abstract classes can contain abstract methods (with empty bodies).
您可以创建一个抽象类，由具体类扩展（或实现）。抽象类可以包含抽象方法（具有空体）。

abstract class Describable {
  void describe();

  void describeWithEmphasis() {
    print('=========');
    describe();
    print('=========');
  }
}

Any class extending Describable has the describeWithEmphasis() method, which calls the extender’s implementation of describe().
任何类扩展都具有该 describeWithEmphasis() 方法，该方法调用扩展 Describable 程序的 describe() 实现。

Read more about abstract classes and methods.
阅读有关抽象类和方法的更多信息。

Async 异步

Avoid callback hell and make your code much more readable by using async and await.
避免回调地狱，并使用 async 和 await 使您的代码更具可读性。

const oneSecond = Duration(seconds: 1);
// ···
Future<void> printWithDelay(String message) async {
  await Future.delayed(oneSecond);
  print(message);
}

The method above is equivalent to:
上述方法等价于：

Future<void> printWithDelay(String message) {
  return Future.delayed(oneSecond).then((_) {
    print(message);
  });
}

As the next example shows, async and await help make asynchronous code easy to read.
如下例所示， async 并帮助 await 使异步代码易于阅读。

Future<void> createDescriptions(Iterable<String> objects) async {
  for (final object in objects) {
    try {
      var file = File('$object.txt');
      if (await file.exists()) {
        var modified = await file.lastModified();
        print(
            'File for $object already exists. It was modified on $modified.');
        continue;
      }
      await file.create();
      await file.writeAsString('Start describing $object in this file.');
    } on IOException catch (e) {
      print('Cannot create description for $object: $e');
    }
  }
}

You can also use async*, which gives you a nice, readable way to build streams.
您还可以使用 async* ，它为您提供了一种很好的、可读的方式来构建流。

Stream<String> report(Spacecraft craft, Iterable<String> objects) async* {
  for (final object in objects) {
    await Future.delayed(oneSecond);
    yield '${craft.name} flies by $object';
  }
}

Read more about asynchrony support, including async functions, Future, Stream, and the asynchronous loop (await for).
阅读有关异步支持的更多信息，包括 async 函数、 、 Future Stream 和异步循环 （ await for ）。

Exceptions 异常

To raise an exception, use throw:
要引发异常，请使用 throw ：

if (astronauts == 0) {
  throw StateError('No astronauts.');
}

To catch an exception, use a try statement with on or catch (or both):
若要捕获异常，请使用带有 或（或 catch 两者）的 try on 语句：

Future<void> describeFlybyObjects(List<String> flybyObjects) async {
  try {
    for (final object in flybyObjects) {
      var description = await File('$object.txt').readAsString();
      print(description);
    }
  } on IOException catch (e) {
    print('Could not describe object: $e');
  } finally {
    flybyObjects.clear();
  }
}

Note that the code above is asynchronous; try works for both synchronous code and code in an async function.
请注意，上面的代码是异步的; try 适用于同步代码和 async 函数中的代码。

Read more about exceptions, including stack traces, rethrow, and the difference between Error and Exception.
阅读有关异常的更多信息，包括堆栈跟踪、 rethrow 以及 和 Exception 之间的区别 Error 。

Important concepts 重要概念

As you continue to learn about the Dart language, keep these facts and concepts in mind:
当您继续学习 Dart 语言时，请记住以下事实和概念：

• Everything you can place in a variable is an object, and every object is an instance of a class. Even numbers, functions, and null are objects. With the exception of null (if you enable sound null safety), all objects inherit from the Object class.
  可以放在变量中的所有内容都是一个对象，每个对象都是一个类的实例。偶数、函数和 null 是对象。除了 null （如果启用 sound null 安全） 之外，所有对象都继承自该 Object 类。

  merge_type Version note: Null safety was introduced in Dart 2.12. Using null safety requires a language version of at least 2.12.
  版本说明：空安全是在 Dart 2.12 中引入的。使用 null 安全要求语言版本至少为 2.12。

• Although Dart is strongly typed, type annotations are optional because Dart can infer types. In var number = 101, number is inferred to be of type int.
  虽然 Dart 是强类型的，但类型注解是可选的，因为 Dart 可以推断类型。在 中 var number = 101 ， number 被推断为 的类型 int 为 。

• If you enable null safety, variables can’t contain null unless you say they can. You can make a variable nullable by putting a question mark (?) at the end of its type. For example, a variable of type int? might be an integer, or it might be null. If you know that an expression never evaluates to null but Dart disagrees, you can add ! to assert that it isn’t null (and to throw an exception if it is). An example: int x = nullableButNotNullInt!
  如果启用 null 安全，则变量不能包含，除非您说它们可以包含 null 。您可以通过在变量类型末尾放置问号 （ ? ） 来使变量可为 null。例如，类型的 int? 变量可能是整数，也可能是 null 。如果你知道一个表达式的计算结果永远不会达到 null ，但 Dart 不同意，你可以添加 ! 以断言它不是 null（如果是，则抛出异常）。举个例子： int x = nullableButNotNullInt!

• When you want to explicitly say that any type is allowed, use the type Object? (if you’ve enabled null safety), Object, or—if you must defer type checking until runtime—the special type dynamic.
  如果要明确说明允许任何类型，请使用该类型（如果已启用 null 安全）， Object 或者（如果必须将类型检查推迟到运行时）特殊类型 dynamic Object? 。

• Dart supports generic types, like List<int> (a list of integers) or List<Object> (a list of objects of any type).
  Dart 支持泛型类型，例如 List<int> （整数列表）或 List<Object> （任何类型的对象列表）。

• Dart supports top-level functions (such as main()), as well as functions tied to a class or object (static and instance methods, respectively). You can also create functions within functions (nested or local functions).
  Dart 支持顶级函数（例如 main() ），以及绑定到类或对象的函数（分别是静态方法和实例方法）。您还可以在函数（嵌套或局部函数）中创建函数。

• Similarly, Dart supports top-level variables, as well as variables tied to a class or object (static and instance variables). Instance variables are sometimes known as fields or properties.
  类似地，Dart 支持顶级变量，以及绑定到类或对象的变量（静态变量和实例变量）。实例变量有时称为字段或属性。

• Unlike Java, Dart doesn’t have the keywords public, protected, and private. If an identifier starts with an underscore (_), it’s private to its library. For details, see Libraries and imports.
  与 Java 不同，Dart 没有关键字 public 、 protected 和 private 。如果标识符以下划线 （ _ ） 开头，则该标识符是其库的私有标识符。有关详细信息，请参阅库和导入。

• Identifiers can start with a letter or underscore (_), followed by any combination of those characters plus digits.
  标识符可以以字母或下划线 （ _ ） 开头，后跟这些字符和数字的任意组合。

• Dart has both expressions (which have runtime values) and statements (which don’t). For example, the conditional expression condition ? expr1 : expr2 has a value of expr1 or expr2. Compare that to an if-else statement, which has no value. A statement often contains one or more expressions, but an expression can’t directly contain a statement.
  Dart 既有表达式（有运行时值），也有语句（没有）。例如，条件表达式 condition ? expr1 : expr2 的值为 expr1 或 expr2 。将其与没有价值的 if-else 语句进行比较。一个语句通常包含一个或多个表达式，但一个表达式不能直接包含一个语句。

• Dart tools can report two kinds of problems: warnings and errors. Warnings are just indications that your code might not work, but they don’t prevent your program from executing. Errors can be either compile-time or run-time. A compile-time error prevents the code from executing at all; a run-time error results in an exception being raised while the code executes.
  Dart 工具可以报告两种问题：警告和错误。警告只是表明您的代码可能无法正常工作，但它们不会阻止您的程序执行。错误可以是编译时错误，也可以是运行时错误。编译时错误会阻止代码执行;运行时错误会导致在代码执行时引发异常。

Additional resources 其他资源

You can find more documentation and code samples in the library tour and the Dart API reference. This site’s code follows the conventions in the Dart style guide.
您可以在库导览和 Dart API 参考中找到更多文档和代码示例。此站点的代码遵循 Dart 风格指南中的约定。

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