Dart 语言核心库一览

本页介绍如何使用 Dart 核心库中的主要功能。这只是一个概览，并不全面。当你需要有关类的更多详细信息时，请参阅Dart API 参考。

dart:core
内置类型，集合和其他核心功能。该库会被自动导入到所有的 Dart 程序。

dart:async
支持异步编程，包括Future和Stream等类。

dart:math
数学常数和函数，以及随机数生成器。

dart:convert
用于在不同数据表示之间进行转换的编码器和解码器，包括 JSON 和 UTF-8。

dart:html
用于基于浏览器应用的 DOM 和其他 API。

dart:io
服务器和命令行应用程序的 I/O 操作，包括 Flutter 应用，服务端应用，以及命令行脚本。

本章只是一个概述；只涵盖了几个 dart:* 库，不包括第三方库。

更多库信息可以在 Pub site 和 Dart web developer library guide. 查找。所有 dart:* 库的 API 文档可以在 Dart API reference 查找，如果使用的是 Flutter 可以在 Flutter API reference. 查找。

dart:core - 数字，集合，字符串等

dart:core 库 (API reference) 提供了一个少量但是重要的内置功能集合。该库会被自动导入每个 Dart 程序。

控制台打印

顶级 print() 方法接受一个参数任意对象）并输出显示这个对象的字符串值(由 toString() 返回) 到控制台。

print(anObject);
print('I drink $tea.');

有关基本字符串和 toString() 的更多信息，参考 Strings in the language tour.

数字

dart:core 库定义了 num ，int 以及 double 类，这些类拥有一定的工具方法来处理数字。

使用 int 和 double 的 parse() 方法将字符串转换为整型或双浮点型对象：

assert(int.parse('42') == 42);
assert(int.parse('0x42') == 66);
assert(double.parse('0.50') == 0.5);

或者使用 num 的 parse() 方法，该方法可能会创建一个整型，否则为浮点型对象：

assert(num.parse('42') is int);
assert(num.parse('0x42') is int);
assert(num.parse('0.50') is double);

通过添加 radix 参数，指定整数的进制基数：

assert(int.parse('42', radix: 16) == 66);

使用 toString() 方法将整型或双精度浮点类型转换为字符串类型。使用 toStringAsFixed(). 指定小数点右边的位数，使用 toStringAsPrecision(): 指定字符串中的有效数字的位数。

// Convert an int to a string.
assert(42.toString() == '42');

// Convert a double to a string.
assert(123.456.toString() == '123.456');

// Specify the number of digits after the decimal.
assert(123.456.toStringAsFixed(2) == '123.46');

// Specify the number of significant figures.
assert(123.456.toStringAsPrecision(2) == '1.2e+2');
assert(double.parse('1.2e+2') == 120.0);

For more information, see the API documentation for int, double, and num. Also see the dart:math section.

字符和正则表达式

在 Dart 中一个字符串是一个固定不变的 UTF-16 编码单元序列。语言概览中有更多关于 strings 的内容。使用正则表达式 (RegExp 对象) 可以在字符串内搜索和替换部分字符串。

String 定义了例如 split()， contains()， startsWith()， endsWith() 等方法。

在字符串中搜索

可以在字符串内查找特定字符串的位置，以及检查字符串是否以特定字符串作为开头或结尾。例如：

// Check whether a string contains another string.
assert('Never odd or even'.contains('odd'));

// Does a string start with another string?
assert('Never odd or even'.startsWith('Never'));

// Does a string end with another string?
assert('Never odd or even'.endsWith('even'));

// Find the location of a string inside a string.
assert('Never odd or even'.indexOf('odd') == 6);

从字符串中提取数据

可以获取字符串中的单个字符，将其作为字符串或者整数。确切地说，实际上获取的是单独的UTF-16编码单元; 诸如高音谱号符号 (‘\u{1D11E}’) 之类的高编号字符分别为两个编码单元。

你也可以获取字符串中的子字符串或者将一个字符串分割为子字符串列表：

// Grab a substring.
assert('Never odd or even'.substring(6, 9) == 'odd');

// Split a string using a string pattern.
var parts = 'progressive web apps'.split(' ');
assert(parts.length == 3);
assert(parts[0] == 'progressive');

// Get a UTF-16 code unit (as a string) by index.
assert('Never odd or even'[0] == 'N');

// Use split() with an empty string parameter to get
// a list of all characters (as Strings); good for
// iterating.
for (final char in 'hello'.split('')) {
  print(char);
}

// Get all the UTF-16 code units in the string.
var codeUnitList = 'Never odd or even'.codeUnits.toList();
assert(codeUnitList[0] == 78);

首字母大小写转换

可以轻松的对字符串的首字母大小写进行转换：

// Convert to uppercase.
assert('web apps'.toUpperCase() == 'WEB APPS');

// Convert to lowercase.
assert('WEB APPS'.toLowerCase() == 'web apps');

Trimming 和空字符串

使用 trim() 移除首尾空格。使用 isEmpty 检查一个字符串是否为空（长度为 0）。

// Trim a string.
assert('  hello  '.trim() == 'hello');

// Check whether a string is empty.
assert(''.isEmpty);

// Strings with only white space are not empty.
assert('  '.isNotEmpty);

替换部分字符串

字符串是不可变的对象，也就是说字符串可以创建但是不能被修改。如果仔细阅读了 String API docs, 你会注意到，没有一个方法实际的改变了字符串的状态。例如，方法 replaceAll() 返回一个新字符串，并没有改变原始字符串：

var greetingTemplate = 'Hello, NAME!';
var greeting = greetingTemplate.replaceAll(RegExp('NAME'), 'Bob');

// greetingTemplate didn't change.
assert(greeting != greetingTemplate);

构建一个字符串

要以代码方式生成字符串，可以使用 StringBuffer 。在调用 toString() 之前， StringBuffer 不会生成新字符串对象。 writeAll() 的第二个参数为可选参数，用来指定分隔符，本例中使用空格作为分隔符。

var sb = StringBuffer();
sb
  ..write('Use a StringBuffer for ')
  ..writeAll(['efficient', 'string', 'creation'], ' ')
  ..write('.');

var fullString = sb.toString();

assert(fullString == 'Use a StringBuffer for efficient string creation.');

正则表达式

RegExp 类提供与 JavaScript 正则表达式相同的功能。使用正则表达式可以对字符串进行高效搜索和模式匹配。

// Here's a regular expression for one or more digits.
var numbers = RegExp(r'\d+');

var allCharacters = 'llamas live fifteen to twenty years';
var someDigits = 'llamas live 15 to 20 years';

// contains() can use a regular expression.
assert(!allCharacters.contains(numbers));
assert(someDigits.contains(numbers));

// Replace every match with another string.
var exedOut = someDigits.replaceAll(numbers, 'XX');
assert(exedOut == 'llamas live XX to XX years');

You can work directly with the RegExp class, too. The Match class provides access to a regular expression match.

var numbers = RegExp(r'\d+');
var someDigits = 'llamas live 15 to 20 years';

// Check whether the reg exp has a match in a string.
assert(numbers.hasMatch(someDigits));

// Loop through all matches.
for (final match in numbers.allMatches(someDigits)) {
  print(match.group(0)); // 15, then 20
}

更多信息

有关完整的方法列表，请参考 String API docs。另请参考 StringBuffer， Pattern， RegExp， 和 Match 的 API 文档。

集合

Dart 附带了核心集合 API ，其中包括 list、set 和 map 类。

Lists

如语言概览中介绍，lists 可以通过字面量来创建和初始化。另外，也可以使用 List 的构造函数。 List 类还定义了若干方法，用于向列表添加或删除项目。

// Create an empty list of strings.
var grains = <String>[];
assert(grains.isEmpty);

// Create a list using a list literal.
var fruits = ['apples', 'oranges'];

// Add to a list.
fruits.add('kiwis');

// Add multiple items to a list.
fruits.addAll(['grapes', 'bananas']);

// Get the list length.
assert(fruits.length == 5);

// Remove a single item.
var appleIndex = fruits.indexOf('apples');
fruits.removeAt(appleIndex);
assert(fruits.length == 4);

// Remove all elements from a list.
fruits.clear();
assert(fruits.isEmpty);

// You can also create a List using one of the constructors.
var vegetables = List.filled(99, 'broccoli');
assert(vegetables.every((v) => v == 'broccoli'));

使用 indexOf() 方法查找一个对象在 list 中的下标值。

var fruits = ['apples', 'oranges'];

// Access a list item by index.
assert(fruits[0] == 'apples');

// Find an item in a list.
assert(fruits.indexOf('apples') == 0);

使用 sort() 方法排序一个 list 。你可以提供一个排序函数用于比较两个对象。比较函数在 小于 时返回 \ <0，相等 时返回 0，bigger 时返回 > 0 。下面示例中使用 compareTo() 函数，该函数在 Comparable 中定义，并被 String 类实现。

var fruits = ['bananas', 'apples', 'oranges'];

// Sort a list.
fruits.sort((a, b) => a.compareTo(b));
assert(fruits[0] == 'apples');

列表是参数化类型（泛型），因此可以指定 list 应该包含的元素类型：

// This list should contain only strings.
var fruits = <String>[];

fruits.add('apples');
var fruit = fruits[0];
assert(fruit is String);

fruits.add(5); // Error: 'int' can't be assigned to 'String'

全部的方法介绍，请参考 List API docs。

Sets

在 Dart 中，set 是一个无序的，元素唯一的集合。因为一个 set 是无序的，所以无法通过下标（位置）获取 set 中的元素。

// Create an empty set of strings.
var ingredients = <String>{};

// Add new items to it.
ingredients.addAll(['gold', 'titanium', 'xenon']);
assert(ingredients.length == 3);

// Adding a duplicate item has no effect.
ingredients.add('gold');
assert(ingredients.length == 3);

// Remove an item from a set.
ingredients.remove('gold');
assert(ingredients.length == 2);

// You can also create sets using
// one of the constructors.
var atomicNumbers = Set.from([79, 22, 54]);

使用 contains() 和 containsAll() 来检查一个或多个元素是否在 set 中：

var ingredients = Set<String>();
ingredients.addAll(['gold', 'titanium', 'xenon']);

// Check whether an item is in the set.
assert(ingredients.contains('titanium'));

// Check whether all the items are in the set.
assert(ingredients.containsAll(['titanium', 'xenon']));

交集是另外两个 set 中的公共元素组成的 set。

var ingredients = Set<String>();
ingredients.addAll(['gold', 'titanium', 'xenon']);

// Create the intersection of two sets.
var nobleGases = Set.from(['xenon', 'argon']);
var intersection = ingredients.intersection(nobleGases);
assert(intersection.length == 1);
assert(intersection.contains('xenon'));

全部的方法介绍，请参考 Set API docs。

Maps

map 是一个无序的 key-value （键值对）集合，就是大家熟知的 dictionary 或者 hash。 map 将 kay 与 value 关联，以便于检索。和 JavaScript 不同，Dart 对象不是 map。

声明 map 可以使用简洁的字面量语法，也可以使用传统构造函数：

// Maps often use strings as keys.
var hawaiianBeaches = {
  'Oahu': ['Waikiki', 'Kailua', 'Waimanalo'],
  'Big Island': ['Wailea Bay', 'Pololu Beach'],
  'Kauai': ['Hanalei', 'Poipu']
};

// Maps can be built from a constructor.
var searchTerms = Map();

// Maps are parameterized types; you can specify what
// types the key and value should be.
var nobleGases = Map<int, String>();

通过大括号语法可以为 map 添加，获取，设置元素。使用 remove() 方法从 map 中移除键值对。

var nobleGases = {54: 'xenon'};

// Retrieve a value with a key.
assert(nobleGases[54] == 'xenon');

// Check whether a map contains a key.
assert(nobleGases.containsKey(54));

// Remove a key and its value.
nobleGases.remove(54);
assert(!nobleGases.containsKey(54));

可以从一个 map 中检索出所有的 key 或所有的 value：

var hawaiianBeaches = {
  'Oahu': ['Waikiki', 'Kailua', 'Waimanalo'],
  'Big Island': ['Wailea Bay', 'Pololu Beach'],
  'Kauai': ['Hanalei', 'Poipu']
};

// Get all the keys as an unordered collection
// (an Iterable).
var keys = hawaiianBeaches.keys;

assert(keys.length == 3);
assert(Set.from(keys).contains('Oahu'));

// Get all the values as an unordered collection
// (an Iterable of Lists).
var values = hawaiianBeaches.values;
assert(values.length == 3);
assert(values.any((v) => v.contains('Waikiki')));

使用 containsKey() 方法检查一个 map 中是否包含某个key 。因为 map 中的 value 可能会是 null ，所有通过 key 获取 value，并通过判断 value 是否为 null 来判断 key 是否存在是不可靠的。

var hawaiianBeaches = {
  'Oahu': ['Waikiki', 'Kailua', 'Waimanalo'],
  'Big Island': ['Wailea Bay', 'Pololu Beach'],
  'Kauai': ['Hanalei', 'Poipu']
};

assert(hawaiianBeaches.containsKey('Oahu'));
assert(!hawaiianBeaches.containsKey('Florida'));

如果当且仅当该 key 不存在于 map 中，且要为这个 key 赋值，可使用 putIfAbsent() 方法。该方法需要一个方法返回这个 value。

var teamAssignments = <String, String>{};
teamAssignments.putIfAbsent('Catcher', () => pickToughestKid());
assert(teamAssignments['Catcher'] != null);

全部的方法介绍，请参考 Map API docs。

公共集合方法

List, Set, 和 Map 共享许多集合中的常用功能。其中一些常见功能由 Iterable 类定义，这些函数由 List 和 Set 实现。

使用 isEmpty 和 isNotEmpty 方法可以检查 list， set 或 map 对象中是否包含元素：

var coffees = <String>[];
var teas = ['green', 'black', 'chamomile', 'earl grey'];
assert(coffees.isEmpty);
assert(teas.isNotEmpty);

使用 forEach() 可以让 list， set 或 map 对象中的每个元素都使用一个方法。

var teas = ['green', 'black', 'chamomile', 'earl grey'];

teas.forEach((tea) => print('I drink $tea'));

当在 map 对象上调用 `forEach() 方法时，函数必须带两个参数（key 和 value）：

hawaiianBeaches.forEach((k, v) {
  print('I want to visit $k and swim at $v');
  // I want to visit Oahu and swim at
  // [Waikiki, Kailua, Waimanalo], etc.
});

Iterable 提供 map() 方法，这个方法将所有结果返回到一个对象中。

var teas = ['green', 'black', 'chamomile', 'earl grey'];

var loudTeas = teas.map((tea) => tea.toUpperCase());
loudTeas.forEach(print);

使用 map().toList() 或 map().toSet() ，可以强制在每个项目上立即调用函数。

var loudTeas = teas.map((tea) => tea.toUpperCase()).toList();

使用 Iterable 的 where() 方法可以获取所有匹配条件的元素。使用 Iterable 的 any() 和 every() 方法可以检查部分或者所有元素是否匹配某个条件。

var teas = ['green', 'black', 'chamomile', 'earl grey'];

// Chamomile is not caffeinated.
bool isDecaffeinated(String teaName) => teaName == 'chamomile';

// Use where() to find only the items that return true
// from the provided function.
var decaffeinatedTeas = teas.where((tea) => isDecaffeinated(tea));
// or teas.where(isDecaffeinated)

// Use any() to check whether at least one item in the
// collection satisfies a condition.
assert(teas.any(isDecaffeinated));

// Use every() to check whether all the items in a
// collection satisfy a condition.
assert(!teas.every(isDecaffeinated));

有关方法的完整列表，请参考 Iterable API docs, 以及 List, Set, and Map.

URIs

在使用 URI（可能你会称它为 URLs）时，Uri 类 提供对字符串的编解码操作。这些函数用来处理 URI 特有的字符，例如 ＆ 和 = 。 Uri 类还可以解析和处理 URI—host，port，scheme等组件。

编码和解码完整合法的URI

使用 encodeFull() 和 decodeFull() 方法，对 URI 中除了特殊字符（例如 /， :， &， #）以外的字符进行编解码，这些方法非常适合编解码完整合法的 URI，并保留 URI 中的特殊字符。

var uri = 'https://example.org/api?foo=some message';

var encoded = Uri.encodeFull(uri);
assert(encoded == 'https://example.org/api?foo=some%20message');

var decoded = Uri.decodeFull(encoded);
assert(uri == decoded);

注意上面代码只编码了 some 和 message 之间的空格。

编码和解码 URI 组件

使用 encodeComponent() 和 decodeComponent() 方法，对 URI 中具有特殊含义的所有字符串字符，特殊字符包括（但不限于）/， &，和 :。

var uri = 'https://example.org/api?foo=some message';

var encoded = Uri.encodeComponent(uri);
assert(
    encoded == 'https%3A%2F%2Fexample.org%2Fapi%3Ffoo%3Dsome%20message');

var decoded = Uri.decodeComponent(encoded);
assert(uri == decoded);

注意上面代码编码了所有的字符。例如 / 被编码为 %2F。

解析 URI

使用 Uri 对象的字段（例如 path），来获取一个 Uri 对象或者 URI 字符串的一部分。使用 parse() 静态方法，可以使用字符串创建 Uri 对象。

var uri = Uri.parse('https://example.org:8080/foo/bar#frag');

assert(uri.scheme == 'https');
assert(uri.host == 'example.org');
assert(uri.path == '/foo/bar');
assert(uri.fragment == 'frag');
assert(uri.origin == 'https://example.org:8080');

有关 URI 组件的更多内容，参考 Uri API docs。

构建 URI

使用 Uri() 构造函数，可以将各组件部分构建成 URI 。

var uri = Uri(
    scheme: 'https',
    host: 'example.org',
    path: '/foo/bar',
    fragment: 'frag',
    queryParameters: {'lang': 'dart'});
assert(uri.toString() == 'https://example.org/foo/bar?lang=dart#frag');

If you don’t need to specify a fragment, to create a URI with a http or https scheme, you can instead use the Uri.http or Uri.https factory constructors:

var httpUri = Uri.http('example.org', '/foo/bar', {'lang': 'dart'});
var httpsUri = Uri.https('example.org', '/foo/bar', {'lang': 'dart'});

assert(httpUri.toString() == 'http://example.org/foo/bar?lang=dart');
assert(httpsUri.toString() == 'https://example.org/foo/bar?lang=dart');

日期和时间

DateTime 对象代表某个时刻，时区可以是 UTC 或者本地时区。

DateTime 对象可以通过若干构造函数和方法创建：

// Get the current date and time.
var now = DateTime.now();

// Create a new DateTime with the local time zone.
var y2k = DateTime(2000); // January 1, 2000

// Specify the month and day.
y2k = DateTime(2000, 1, 2); // January 2, 2000

// Specify the date as a UTC time.
y2k = DateTime.utc(2000); // 1/1/2000, UTC

// Specify a date and time in ms since the Unix epoch.
y2k = DateTime.fromMillisecondsSinceEpoch(946684800000, isUtc: true);

// Parse an ISO 8601 date in the UTC time zone.
y2k = DateTime.parse('2000-01-01T00:00:00Z');

// Create a new DateTime from an existing one, adjusting just some properties:
var sameTimeLastYear = now.copyWith(year: now.year - 1);

日期中 millisecondsSinceEpoch 属性返回自 “Unix 纪元（January 1, 1970, UTC）”以来的毫秒数：

// 1/1/2000, UTC
var y2k = DateTime.utc(2000);
assert(y2k.millisecondsSinceEpoch == 946684800000);

// 1/1/1970, UTC
var unixEpoch = DateTime.utc(1970);
assert(unixEpoch.millisecondsSinceEpoch == 0);

Use the Duration class to calculate the difference between two dates and to shift a date forward or backward:

var y2k = DateTime.utc(2000);

// Add one year.
var y2001 = y2k.add(const Duration(days: 366));
assert(y2001.year == 2001);

// Subtract 30 days.
var december2000 = y2001.subtract(const Duration(days: 30));
assert(december2000.year == 2000);
assert(december2000.month == 12);

// Calculate the difference between two dates.
// Returns a Duration object.
var duration = y2001.difference(y2k);
assert(duration.inDays == 366); // y2k was a leap year.

参考 DateTime 和 Duration API 文档了解全部方法列表。

工具类

核心库包含各种工具类，可用于排序，映射值以及迭代。

比较对象

如果实现了 Comparable 接口，也就是说可以将该对象与另一个对象进行比较，通常用于排序。 compareTo() 方法在 小于 时返回 < 0，在 相等 时返回 0，在 大于 时返回 > 0。

class Line implements Comparable<Line> {
  final int length;
  const Line(this.length);

  @override
  int compareTo(Line other) => length - other.length;
}

void main() {
  var short = const Line(1);
  var long = const Line(100);
  assert(short.compareTo(long) < 0);
}

Implementing map keys

在 Dart 中每个对象会默认提供一个整数的哈希值，因此在 map 中可以作为 key 来使用，重写 hashCode 的 getter 方法来生成自定义哈希值。如果重写 hashCode 的 getter 方法，那么可能还需要重写 == 运算符。相等的（通过 == ）对象必须拥有相同的哈希值。哈希值并不要求是唯一的，但是应该具有良好的分布形态。

class Person {
  final String firstName, lastName;

  Person(this.firstName, this.lastName);

  // Override hashCode using the static hashing methods
  // provided by the `Object` class.
  @override
  int get hashCode => Object.hash(firstName, lastName);

  // You should generally implement operator `==` if you
  // override `hashCode`.
  @override
  bool operator ==(Object other) {
    return other is Person &&
        other.firstName == firstName &&
        other.lastName == lastName;
  }
}

void main() {
  var p1 = Person('Bob', 'Smith');
  var p2 = Person('Bob', 'Smith');
  var p3 = 'not a person';
  assert(p1.hashCode == p2.hashCode);
  assert(p1 == p2);
  assert(p1 != p3);
}

迭代

Iterable 和 Iterator 类支持 for-in 循环。当创建一个类的时候，继承或者实现 Iterable，可以为该类提供用于 for-in 循环的 Iterators。实现 Iterator 来定义实际的遍历操作。

如果你在 for-in 循环里要创建一个可以提供 Iterator 的类，如果可以，请选择 extend 或者 implement Iterable 的方式。 Implement Iterator 来定义一个实际的迭代能力。

class Process {
  // Represents a process...
}

class ProcessIterator implements Iterator<Process> {
  @override
  Process get current => ...
  @override
  bool moveNext() => ...
}

// A mythical class that lets you iterate through all
// processes. Extends a subclass of [Iterable].
class Processes extends IterableBase<Process> {
  @override
  final Iterator<Process> iterator = ProcessIterator();
}

void main() {
  // Iterable objects can be used with for-in.
  for (final process in Processes()) {
    // Do something with the process.
  }
}

异常

Dart 核心库定义了很多公共的异常和错误类。异常通常是一些可以预见和预知的情况。错误是无法预见或者预防的情况。

两个最常见的错误：

NoSuchMethodError

当方法的接受对象（可能为null）没有实现该方法时抛出。

ArgumentError

当方法在接受到一个不合法参数时抛出。

通常通过抛出一个应用特定的异常，来表示应用发生了错误。通过实现 Exception 接口来自定义异常：

class FooException implements Exception {
  final String? msg;

  const FooException([this.msg]);

  @override
  String toString() => msg ?? 'FooException';
}

更多内容，参考 Exceptions 以及 Exception API 文档。

弱引用和终结器 (finalizers)

Dart 语言支持 垃圾回收 (GC)，即所有未被引用的 Dart 对象最终都会被垃圾回收并销毁。某些涉及到原生资源和目标对象无法修改的场景， GC 的行为可能不会符合预期。

WeakReference 会保存目标对象的引用，并且不会影响目标对象被 GC。另一种方案是使用 Expando 对对象添加一些属性。

终结器 (Finalizer) 可以在对象已不再被引用时执行一个回调函数。然而，终结器的回调并不保证一定会执行。

NativeFinalizer 为使用 dart:ffi 与原生交互的代码提供了更加强力的回调保证。它的回调会在对象不再引用后至少调用一次。同时，它也可以用来关闭原生资源，例如数据库链接和打开的文件。

想要确保一个对象不会过早地被回收，其对应的类可以实现 Finalizable 接口。当一个方法内的变量是 Finalizable，直到代码执行完毕后它才会被回收。

dart:async - 异步编程

异步编程通常使用回调方法来实现，但是 Dart 提供了其他方案：Future 和 Stream 对象。 Future 类似与 JavaScript 中的 Promise ，代表在将来某个时刻会返回一个结果。 Stream 类可以用来获取一系列的值，比如，一系列事件。 Future， Stream，以及更多内容，参考 dart:async library (API reference)。

dart:async 库可以工作在 web 应用及 command-line 应用。通过 import dart:async 来使用。

import 'dart:async';

Future

在 Dart 库中随处可见 Future 对象，通常异步函数返回的对象就是一个 Future。当一个 future 完成执行后，future 中的值就已经可以使用了。

使用 await

在直接使用 Future API 前，首先应该考虑 await 来替代。代码中使用 await 表达式会比直接使用 Future API 更容易理解。

阅读思考下面代码。代码使用 Future 的 then() 方法在同一行执行了三个异步函数，要等待上一个执行完成，再执行下一个任务。

void runUsingFuture() {
  // ...
  findEntryPoint().then((entryPoint) {
    return runExecutable(entryPoint, args);
  }).then(flushThenExit);
}

通过 await 表达式实现等价的代码，看起来非常像同步代码：

Future<void> runUsingAsyncAwait() async {
  // ...
  var entryPoint = await findEntryPoint();
  var exitCode = await runExecutable(entryPoint, args);
  await flushThenExit(exitCode);
}

async 函数能够捕获来自 Future 的异常。例如：

var entryPoint = await findEntryPoint();
try {
  var exitCode = await runExecutable(entryPoint, args);
  await flushThenExit(exitCode);
} catch (e) {
  // Handle the error...
}

更多关于 await 的使用及相关的 Dart 语言特征，参考 Asynchrony support。

基本用法

当 future 执行完成后，then() 中的代码会被执行。 then() 中的代码会在 future 完成后被执行。例如，HttpRequest.getString() 返回一个 future 对象，因为 HTTP 请求可能需要一段时间。当 Future 完成并且保证字符串值有效后，使用 then() 来执行你需要的代码：

HttpRequest.getString(url).then((String result) {
  print(result);
});

使用 catchError() 来处理一些 Future 对象可能抛出的错误或者异常。

HttpRequest.getString(url).then((String result) {
  print(result);
}).catchError((e) {
  // Handle or ignore the error.
});

then().catchError() 组合是 try-catch 的异步版本。

链式异步编程

then() 方法返回一个 Future 对象，这样就提供了一个非常好的方式让多个异步方法按顺序依次执行。如果用 then() 注册的回调返回一个 Future ，那么 then() 返回一个等价的 Future 。如果回调返回任何其他类型的值，那么 then() 会创建一个以该值完成的新 Future 。

Future result = costlyQuery(url);
result
    .then((value) => expensiveWork(value))
    .then((_) => lengthyComputation())
    .then((_) => print('Done!'))
    .catchError((exception) {
  /* Handle exception... */
});

在上面的示例中，方法按下面顺序执行：

1. costlyQuery()
2. expensiveWork()
3. lengthyComputation()

这是使用 await 编写的等效代码：

try {
  final value = await costlyQuery(url);
  await expensiveWork(value);
  await lengthyComputation();
  print('Done!');
} catch (e) {
  /* Handle exception... */
}

等待多个 Future

有时代码逻辑需要调用多个异步函数，并等待它们全部完成后再继续执行。使用 Future.wait() 静态方法管理多个 Future 以及等待它们完成：

Future<void> deleteLotsOfFiles() async =>  ...
Future<void> copyLotsOfFiles() async =>  ...
Future<void> checksumLotsOfOtherFiles() async =>  ...

await Future.wait([
  deleteLotsOfFiles(),
  copyLotsOfFiles(),
  checksumLotsOfOtherFiles(),
]);
print('Done with all the long steps!');

Future.wait() returns a future which completes once all the provided futures have completed. It completes either with their results, or with an error if any of the provided futures fail.

Handling errors for multiple futures

You can also wait for parallel operations on an iterable or record of futures.

These extensions return a future with the resulting values of all provided futures. Unlike Future.wait, they also let you handle errors.

If any future in the collection completes with an error, wait completes with a ParallelWaitError. This allows the caller to handle individual errors and dispose successful results if necessary.

When you don’t need the result values from each individual future, use wait on an iterable of futures:

void main() async {
  Future<void> delete() async =>  ...
  Future<void> copy() async =>  ...
  Future<void> errorResult() async =>  ...
  
  try {
    // Wait for each future in a list, returns a list of futures:
    var results = await [delete(), copy(), errorResult()].wait;

    } on ParallelWaitError<List<bool?>, List<AsyncError?>> catch (e) {

    print(e.values[0]);    // Prints successful future
    print(e.values[1]);    // Prints successful future
    print(e.values[2]);    // Prints null when the result is an error

    print(e.errors[0]);    // Prints null when the result is successful
    print(e.errors[1]);    // Prints null when the result is successful
    print(e.errors[2]);    // Prints error
  }

}

When you do need the individual result values from each future, use wait on a record of futures. This provides the additional benefit that the futures can be of different types:

void main() async {
  Future<int> delete() async =>  ...
  Future<String> copy() async =>  ...
  Future<bool> errorResult() async =>  ...

  try {    
    // Wait for each future in a record, returns a record of futures:
    (int, String, bool) result = await (delete(), copy(), errorResult()).wait;
  
  } on ParallelWaitError<(int?, String?, bool?),
      (AsyncError?, AsyncError?, AsyncError?)> catch (e) {
    // ...
    }

  // Do something with the results:
  var deleteInt  = result.$1;
  var copyString = result.$2;
  var errorBool  = result.$3;
}

Stream

在 Dart API 中 Stream 对象随处可见，Stream 用来表示一系列数据。例如，HTML 中的按钮点击就是通过 stream 传递的。同样也可以将文件作为数据流来读取。

异步循环

有时，可以使用异步 for 循环 await for ，来替代 Stream API 。

思考下面示例函数。它使用 Stream 的 listen() 方法来订阅文件列表，传入一个搜索文件或目录的函数

void main(List<String> arguments) {
  // ...
  FileSystemEntity.isDirectory(searchPath).then((isDir) {
    if (isDir) {
      final startingDir = Directory(searchPath);
      startingDir.list().listen((entity) {
        if (entity is File) {
          searchFile(entity, searchTerms);
        }
      });
    } else {
      searchFile(File(searchPath), searchTerms);
    }
  });
}

下面是使用 await 表达式和异步 for 循环 (await for) 实现的等价的代码，看起来更像是同步代码：

void main(List<String> arguments) async {
  // ...
  if (await FileSystemEntity.isDirectory(searchPath)) {
    final startingDir = Directory(searchPath);
    await for (final entity in startingDir.list()) {
      if (entity is File) {
        searchFile(entity, searchTerms);
      }
    }
  } else {
    searchFile(File(searchPath), searchTerms);
  }
}

有关 await 的使用及 Dart 语言的相关信息，参考 Asynchrony support。

监听流数据 (stream data)

使用 await for 或者使用 listen() 方法监听 stream，来获取每个到达的数据流值：

// Add an event handler to a button.
submitButton.onClick.listen((e) {
  // When the button is clicked, it runs this code.
  submitData();
});

下面示例中，ID 为 “submitInfo” button 提供的 onClick 属性是一个 Stream 对象。

如果只关心其中一个事件，可以使用，例如，first， last，或 single 属性来获取。要在处理时间前对事件进行测试，可以使用，例如 firstWhere()， lastWhere()，或 singleWhere() 方法。

如果只关心事件中的一个子集，可以使用，例如，skip()， skipWhile()，take()，takeWhile()，和 where()。

传递流数据 (stream data)

常常，在使用流数据前需要改变数据的格式。使用 transform() 方法生成具有不同类型数据的流：

var lines =
    inputStream.transform(utf8.decoder).transform(const LineSplitter());

上面例子中使用了两个 transformer 。第一个使用 utf8.decoder 将整型流转换为字符串流。接着，使用了 LineSplitter 将字符串流转换为多行字符串流。这些 transformer 来自 dart:convert 库（参考dart:convert section）。

处理错误和完成

处理错误和完成代码方式，取决于使用的是异步 for 循环（await for）还是 Stream API 。

如果使用的是异步 for 循环，那么通过 try-catch 来处理错误。代码位于异步 for 循环之后，会在 stream 被关闭后执行。

Future<void> readFileAwaitFor() async {
  var config = File('config.txt');
  Stream<List<int>> inputStream = config.openRead();

  var lines =
      inputStream.transform(utf8.decoder).transform(const LineSplitter());
  try {
    await for (final line in lines) {
      print('Got ${line.length} characters from stream');
    }
    print('file is now closed');
  } catch (e) {
    print(e);
  }
}

如果使用的是 Stream API，那么通过注册 onError 监听来处理错误。代码位于注册的 onDone 中，会在 stream 被关闭后执行。

var config = File('config.txt');
Stream<List<int>> inputStream = config.openRead();

inputStream.transform(utf8.decoder).transform(const LineSplitter()).listen(
    (String line) {
  print('Got ${line.length} characters from stream');
}, onDone: () {
  print('file is now closed');
}, onError: (e) {
  print(e);
});

更多内容

更多在 command-line 应用中使用 Future 和 Stream 的实例，参考 dart:io 概览 也可以参考下列文章和教程：

• Asynchronous programming: futures, async, await
• Futures and error handling
• Asynchronous programming: streams
• Creating streams in Dart
• Dart asynchronous programming: Isolates and event loops

dart:math - 数学和随机数

dart:math 库（API reference）提供通用的功能，例如，正弦和余弦，最大值和最小值，以及数学常数，例如 pi 和 e。大多数在 Math 库中的功能是作为顶级函数实现的。

通过导入 dart:math 来引入使用该库。

import 'dart:math';

三角函数

Math 库提供基本的三角函数：

// Cosine
assert(cos(pi) == -1.0);

// Sine
var degrees = 30;
var radians = degrees * (pi / 180);
// radians is now 0.52359.
var sinOf30degrees = sin(radians);
// sin 30° = 0.5
assert((sinOf30degrees - 0.5).abs() < 0.01);

最大值和最小值

Math 库提供 max() 和 min() 方法：

assert(max(1, 1000) == 1000);
assert(min(1, -1000) == -1000);

数学常数

在 Math 库中可以找到你需要的数学常数，例如，pi， e 等等：

// See the Math library for additional constants.
print(e); // 2.718281828459045
print(pi); // 3.141592653589793
print(sqrt2); // 1.4142135623730951

随机数

使用 Random 类产生随机数。可以为 Random 构造函数提供一个可选的种子参数。

var random = Random();
random.nextDouble(); // Between 0.0 and 1.0: [0, 1)
random.nextInt(10); // Between 0 and 9.

也可以产生随机布尔值序列：

var random = Random();
random.nextBool(); // true or false

更多内容

完整方法列表参考 Math API docs。在 API 文档中参考 num, int, 和 double。

dart:convert - 编解码JSON，UTF-8等

dart:convert 库（API reference）提供 JSON 和 UTF-8 转换器，以及创建其他转换器。 JSON 是一种用于表示结构化对象和集合的简单文本格式。 UTF-8 是一种常见的可变宽度编码，可以表示Unicode字符集中的每个字符。

使用时，通过 import dart:convert 引入。

import 'dart:convert';

编解码JSON

使用 jsonDecode() 解码 JSON 编码的字符串为 Dart 对象：

// NOTE: Be sure to use double quotes ("),
// not single quotes ('), inside the JSON string.
// This string is JSON, not Dart.
var jsonString = '''
  [
    {"score": 40},
    {"score": 80}
  ]
''';

var scores = jsonDecode(jsonString);
assert(scores is List);

var firstScore = scores[0];
assert(firstScore is Map);
assert(firstScore['score'] == 40);

使用 jsonEncode() 编码 Dart 对象为 JSON 格式的字符串：

var scores = [
  {'score': 40},
  {'score': 80},
  {'score': 100, 'overtime': true, 'special_guest': null}
];

var jsonText = jsonEncode(scores);
assert(jsonText ==
    '[{"score":40},{"score":80},'
        '{"score":100,"overtime":true,'
        '"special_guest":null}]');

只有 int， double， String， bool, null, List, 或者 Map 类型对象可以直接编码成 JSON。 List 和 Map 对象进行递归编码。

不能直接编码的对象有两种方式对其编码。第一种方式是调用 jsonEncode() 时赋值第二个参数，这个参数是一个函数，该函数返回一个能够直接编码的对象第二种方式是省略第二个参数，着这种情况下编码器调用对象的 toJson() 方法。

更多示例及 JSON 包相关链接，参考 JSON Support 。

编解码 UTF-8 字符

使用 utf8.decode() 解码 UTF8 编码的字符创为 Dart 字符串：

List<int> utf8Bytes = [
  0xc3, 0x8e, 0xc3, 0xb1, 0xc5, 0xa3, 0xc3, 0xa9,
  0x72, 0xc3, 0xb1, 0xc3, 0xa5, 0xc5, 0xa3, 0xc3,
  0xae, 0xc3, 0xb6, 0xc3, 0xb1, 0xc3, 0xa5, 0xc4,
  0xbc, 0xc3, 0xae, 0xc5, 0xbe, 0xc3, 0xa5, 0xc5,
  0xa3, 0xc3, 0xae, 0xe1, 0xbb, 0x9d, 0xc3, 0xb1
];

var funnyWord = utf8.decode(utf8Bytes);

assert(funnyWord == 'Îñţérñåţîöñåļîžåţîờñ');

将 UTF-8 字符串流转换为 Dart 字符串，为 Stream 的 transform() 方法上指定 utf8.decoder：

var lines = utf8.decoder.bind(inputStream).transform(const LineSplitter());
try {
  await for (final line in lines) {
    print('Got ${line.length} characters from stream');
  }
  print('file is now closed');
} catch (e) {
  print(e);
}

使用 utf8.encode() 将 Dart 字符串编码为一个 UTF8 编码的字节流：

Uint8List encoded = utf8.encode('Îñţérñåţîöñåļîžåţîờñ');

assert(encoded.length == utf8Bytes.length);
for (int i = 0; i < encoded.length; i++) {
  assert(encoded[i] == utf8Bytes[i]);
}

其他功能

dart:convert 库同样包含 ASCII 和 ISO-8859-1 (Latin1) 转换器。更多详情，参考 API docs for the dart:convert library。

dart:html - 基于浏览器应用

Use the dart:html library to program the browser, manipulate objects and elements in the DOM, and access HTML5 APIs. DOM stands for Document Object Model, which describes the hierarchy of an HTML page.

Other common uses of dart:html are manipulating styles (CSS), getting data using HTTP requests, and exchanging data using WebSockets. HTML5 (and dart:html) has many additional APIs that this section doesn’t cover. Only web apps can use dart:html, not command-line apps.

To use the HTML library in your web app, import dart:html:

import 'dart:html';

Manipulating the DOM

To use the DOM, you need to know about windows, documents, elements, and nodes.

A Window object represents the actual window of the web browser. Each Window has a Document object, which points to the document that’s currently loaded. The Window object also has accessors to various APIs such as IndexedDB (for storing data), requestAnimationFrame (for animations), and more. In tabbed browsers, each tab has its own Window object.

With the Document object, you can create and manipulate Element objects within the document. Note that the document itself is an element and can be manipulated.

The DOM models a tree of Nodes. These nodes are often elements, but they can also be attributes, text, comments, and other DOM types. Except for the root node, which has no parent, each node in the DOM has one parent and might have many children.

Finding elements

To manipulate an element, you first need an object that represents it. You can get this object using a query.

Find one or more elements using the top-level functions querySelector() and querySelectorAll(). You can query by ID, class, tag, name, or any combination of these. The CSS Selector Specification guide defines the formats of the selectors such as using a # prefix to specify IDs and a period (.) for classes.

The querySelector() function returns the first element that matches the selector, while querySelectorAll()returns a collection of elements that match the selector.

// Find an element by id (an-id).
Element idElement = querySelector('#an-id')!;

// Find an element by class (a-class).
Element classElement = querySelector('.a-class')!;

// Find all elements by tag (<div>).
List<Element> divElements = querySelectorAll('div');

// Find all text inputs.
List<Element> textInputElements = querySelectorAll(
  'input[type="text"]',
);

// Find all elements with the CSS class 'class'
// inside of a <p> that is inside an element with
// the ID 'id'.
List<Element> specialParagraphElements = querySelectorAll('#id p.class');

Manipulating elements

You can use properties to change the state of an element. Node and its subtype Element define the properties that all elements have. For example, all elements have classes, hidden, id, style, and title properties that you can use to set state. Subclasses of Element define additional properties, such as the href property of AnchorElement.

Consider this example of specifying an anchor element in HTML:

<a id="example" href="/another/example">link text</a>

This <a> tag specifies an element with an href attribute and a text node (accessible via a text property) that contains the string “link text”. To change the URL that the link goes to, you can use AnchorElement’s href property:

var anchor = querySelector('#example') as AnchorElement;
anchor.href = 'https://dart.dev';

Often you need to set properties on multiple elements. For example, the following code sets the hidden property of all elements that have a class of “mac”, “win”, or “linux”. Setting the hidden property to true has the same effect as adding display: none to the CSS.

<!-- In HTML: -->
<p>
  <span class="linux">Words for Linux</span>
  <span class="macos">Words for Mac</span>
  <span class="windows">Words for Windows</span>
</p>

// In Dart:
const osList = ['macos', 'windows', 'linux'];
final userOs = determineUserOs();

// For each possible OS...
for (final os in osList) {
  // Matches user OS?
  bool shouldShow = (os == userOs);

  // Find all elements with class=os. For example, if
  // os == 'windows', call querySelectorAll('.windows')
  // to find all elements with the class "windows".
  // Note that '.$os' uses string interpolation.
  for (final elem in querySelectorAll('.$os')) {
    elem.hidden = !shouldShow; // Show or hide.
  }
}

When the right property isn’t available or convenient, you can use Element’s attributes property. This property is a Map<String, String>, where the keys are attribute names. For a list of attribute names and their meanings, see the MDN Attributes page. Here’s an example of setting an attribute’s value:

elem.attributes['someAttribute'] = 'someValue';

Creating elements

You can add to existing HTML pages by creating new elements and attaching them to the DOM. Here’s an example of creating a paragraph (<p>) element:

var elem = ParagraphElement();
elem.text = 'Creating is easy!';

You can also create an element by parsing HTML text. Any child elements are also parsed and created.

var elem2 = Element.html(
  '<p>Creating <em>is</em> easy!</p>',
);

Note that elem2 is a ParagraphElement in the preceding example.

Attach the newly created element to the document by assigning a parent to the element. You can add an element to any existing element’s children. In the following example, body is an element, and its child elements are accessible (as a List<Element>) from the children property.

document.body!.children.add(elem2);

Adding, replacing, and removing nodes

Recall that elements are just a kind of node. You can find all the children of a node using the nodes property of Node, which returns a List<Node> (as opposed to children, which omits non-Element nodes). Once you have this list, you can use the usual List methods and operators to manipulate the children of the node.

To add a node as the last child of its parent, use the List add() method:

querySelector('#inputs')!.nodes.add(elem);

To replace a node, use the Node replaceWith() method:

querySelector('#status')!.replaceWith(elem);

To remove a node, use the Node remove() method:

// Find a node by ID, and remove it from the DOM if it is found.
querySelector('#expendable')?.remove();

Manipulating CSS styles

CSS, or cascading style sheets, defines the presentation styles of DOM elements. You can change the appearance of an element by attaching ID and class attributes to it.

Each element has a classes field, which is a list. Add and remove CSS classes simply by adding and removing strings from this collection. For example, the following sample adds the warning class to an element:

var elem = querySelector('#message')!;
elem.classes.add('warning');

It’s often very efficient to find an element by ID. You can dynamically set an element ID with the id property:

var message = DivElement();
message.id = 'message2';
message.text = 'Please subscribe to the Dart mailing list.';

You can reduce the redundant text in this example by using method cascades:

var message = DivElement()
  ..id = 'message2'
  ..text = 'Please subscribe to the Dart mailing list.';

While using IDs and classes to associate an element with a set of styles is best practice, sometimes you want to attach a specific style directly to the element:

message.style
  ..fontWeight = 'bold'
  ..fontSize = '3em';

Handling events

To respond to external events such as clicks, changes of focus, and selections, add an event listener. You can add an event listener to any element on the page. Event dispatch and propagation is a complicated subject; research the details if you’re new to web programming.

Add an event handler using element.onEvent.listen(function), where Event is the event name and function is the event handler.

For example, here’s how you can handle clicks on a button:

// Find a button by ID and add an event handler.
querySelector('#submitInfo')!.onClick.listen((e) {
  // When the button is clicked, it runs this code.
  submitData();
});

Events can propagate up and down through the DOM tree. To discover which element originally fired the event, use e.target:

document.body!.onClick.listen((e) {
  final clickedElem = e.target;
  // ...
});

To see all the events for which you can register an event listener, look for “onEventType” properties in the API docs for Element and its subclasses. Some common events include:

• change
• blur
• keyDown
• keyUp
• mouseDown
• mouseUp

Using HTTP resources with HttpRequest

Formerly known as XMLHttpRequest, the HttpRequest class gives you access to HTTP resources from within your browser-based app. Traditionally, AJAX-style apps make heavy use of HttpRequest. Use HttpRequest to dynamically load JSON data or any other resource from a web server. You can also dynamically send data to a web server.

Getting data from the server

The HttpRequest static method getString() is an easy way to get data from a web server. Use await with the getString() call to ensure that you have the data before continuing execution.

void main() async {
  String pageHtml = await HttpRequest.getString(url);
  // Do something with pageHtml...
}

Use try-catch to specify an error handler:

try {
  var data = await HttpRequest.getString(jsonUri);
  // Process data...
} catch (e) {
  // Handle exception...
}

If you need access to the HttpRequest, not just the text data it retrieves, you can use the request() static method instead of getString(). Here’s an example of reading XML data:

void main() async {
  HttpRequest req = await HttpRequest.request(
    url,
    method: 'HEAD',
  );
  if (req.status == 200) {
    // Successful URL access...
  }
  // ···
}

You can also use the full API to handle more interesting cases. For example, you can set arbitrary headers.

The general flow for using the full API of HttpRequest is as follows:

1. Create the HttpRequest object.
2. Open the URL with either GET or POST.
3. Attach event handlers.
4. Send the request.

For example:

var request = HttpRequest();
request
  ..open('POST', url)
  ..onLoadEnd.listen((e) => requestComplete(request))
  ..send(encodedData);

Sending data to the server

HttpRequest can send data to the server using the HTTP method POST. For example, you might want to dynamically submit data to a form handler. Sending JSON data to a RESTful web service is another common example.

Submitting data to a form handler requires you to provide name-value pairs as URI-encoded strings. (Information about the URI class is in the URIs section of the Dart Library Tour.) You must also set the Content-type header to application/x-www-form-urlencoded if you wish to send data to a form handler.

String encodeMap(Map<String, String> data) => data.entries
    .map((e) =>
        '${Uri.encodeComponent(e.key)}=${Uri.encodeComponent(e.value)}')
    .join('&');

void main() async {
  const data = {'dart': 'fun', 'angular': 'productive'};

  var request = HttpRequest();
  request
    ..open('POST', url)
    ..setRequestHeader(
      'Content-type',
      'application/x-www-form-urlencoded',
    )
    ..send(encodeMap(data));

  await request.onLoadEnd.first;

  if (request.status == 200) {
    // Successful URL access...
  }
  // ···
}

Sending and receiving real-time data with WebSockets

A WebSocket allows your web app to exchange data with a server interactively—no polling necessary. A server creates the WebSocket and listens for requests on a URL that starts with ws://—for example, ws://127.0.0.1:1337/ws. The data transmitted over a WebSocket can be a string or a blob. Often, the data is a JSON-formatted string.

To use a WebSocket in your web app, first create a WebSocket object, passing the WebSocket URL as an argument:

var ws = WebSocket('ws://echo.websocket.org');

Sending data

To send string data on the WebSocket, use the send() method:

ws.send('Hello from Dart!');

Receiving data

To receive data on the WebSocket, register a listener for message events:

ws.onMessage.listen((MessageEvent e) {
  print('Received message: ${e.data}');
});

The message event handler receives a MessageEvent object. This object’s data field has the data from the server.

Handling WebSocket events

Your app can handle the following WebSocket events: open, close, error, and (as shown earlier) message. Here’s an example of a method that creates a WebSocket object and registers handlers for open, close, error, and message events:

void initWebSocket([int retrySeconds = 1]) {
  var reconnectScheduled = false;

  print('Connecting to websocket');

  void scheduleReconnect() {
    if (!reconnectScheduled) {
      Timer(Duration(seconds: retrySeconds),
          () => initWebSocket(retrySeconds * 2));
    }
    reconnectScheduled = true;
  }

  ws.onOpen.listen((e) {
    print('Connected');
    ws.send('Hello from Dart!');
  });

  ws.onClose.listen((e) {
    print('Websocket closed, retrying in $retrySeconds seconds');
    scheduleReconnect();
  });

  ws.onError.listen((e) {
    print('Error connecting to ws');
    scheduleReconnect();
  });

  ws.onMessage.listen((MessageEvent e) {
    print('Received message: ${e.data}');
  });
}

More information

This section barely scratched the surface of using the dart:html library. For more information, see the documentation for dart:html. Dart has additional libraries for more specialized web APIs, such as web audio, IndexedDB, and WebGL.

For more information about Dart web libraries, see the web library overview.

dart:io - 服务器和命令行应用程序的 I/O 。

The dart:io library provides APIs to deal with files, directories, processes, sockets, WebSockets, and HTTP clients and servers.

In general, the dart:io library implements and promotes an asynchronous API. Synchronous methods can easily block an application, making it difficult to scale. Therefore, most operations return results via Future or Stream objects, a pattern common with modern server platforms such as Node.js.

The few synchronous methods in the dart:io library are clearly marked with a Sync suffix on the method name. Synchronous methods aren’t covered here.

To use the dart:io library you must import it:

import 'dart:io';

Files and directories

The I/O library enables command-line apps to read and write files and browse directories. You have two choices for reading the contents of a file: all at once, or streaming. Reading a file all at once requires enough memory to store all the contents of the file. If the file is very large or you want to process it while reading it, you should use a Stream, as described in Streaming file contents.

Reading a file as text

When reading a text file encoded using UTF-8, you can read the entire file contents with readAsString(). When the individual lines are important, you can use readAsLines(). In both cases, a Future object is returned that provides the contents of the file as one or more strings.

void main() async {
  var config = File('config.txt');

  // Put the whole file in a single string.
  var stringContents = await config.readAsString();
  print('The file is ${stringContents.length} characters long.');

  // Put each line of the file into its own string.
  var lines = await config.readAsLines();
  print('The file is ${lines.length} lines long.');
}

Reading a file as binary

The following code reads an entire file as bytes into a list of ints. The call to readAsBytes() returns a Future, which provides the result when it’s available.

void main() async {
  var config = File('config.txt');

  var contents = await config.readAsBytes();
  print('The file is ${contents.length} bytes long.');
}

Handling errors

To capture errors so they don’t result in uncaught exceptions, you can register a catchError handler on the Future, or (in an async function) use try-catch:

void main() async {
  var config = File('config.txt');
  try {
    var contents = await config.readAsString();
    print(contents);
  } catch (e) {
    print(e);
  }
}

Streaming file contents

Use a Stream to read a file, a little at a time. You can use either the Stream API or await for, part of Dart’s asynchrony support.

import 'dart:io';
import 'dart:convert';

void main() async {
  var config = File('config.txt');
  Stream<List<int>> inputStream = config.openRead();

  var lines = utf8.decoder.bind(inputStream).transform(const LineSplitter());
  try {
    await for (final line in lines) {
      print('Got ${line.length} characters from stream');
    }
    print('file is now closed');
  } catch (e) {
    print(e);
  }
}

Writing file contents

You can use an IOSink to write data to a file. Use the File openWrite() method to get an IOSink that you can write to. The default mode, FileMode.write, completely overwrites existing data in the file.

var logFile = File('log.txt');
var sink = logFile.openWrite();
sink.write('FILE ACCESSED ${DateTime.now()}\n');
await sink.flush();
await sink.close();

To add to the end of the file, use the optional mode parameter to specify FileMode.append:

var sink = logFile.openWrite(mode: FileMode.append);

To write binary data, use add(List<int> data).

Listing files in a directory

Finding all files and subdirectories for a directory is an asynchronous operation. The list() method returns a Stream that emits an object when a file or directory is encountered.

void main() async {
  var dir = Directory('tmp');

  try {
    var dirList = dir.list();
    await for (final FileSystemEntity f in dirList) {
      if (f is File) {
        print('Found file ${f.path}');
      } else if (f is Directory) {
        print('Found dir ${f.path}');
      }
    }
  } catch (e) {
    print(e.toString());
  }
}

Other common functionality

The File and Directory classes contain other functionality, including but not limited to:

• Creating a file or directory: create() in File and Directory
• Deleting a file or directory: delete() in File and Directory
• Getting the length of a file: length() in File
• Getting random access to a file: open() in File

Refer to the API docs for File and Directory for a full list of methods.

HTTP clients and servers

The dart:io library provides classes that command-line apps can use for accessing HTTP resources, as well as running HTTP servers.

HTTP server

The HttpServer class provides the low-level functionality for building web servers. You can match request handlers, set headers, stream data, and more.

The following sample web server returns simple text information. This server listens on port 8888 and address 127.0.0.1 (localhost), responding to requests for the path /dart. For any other path, the response is status code 404 (page not found).

void main() async {
  final requests = await HttpServer.bind('localhost', 8888);
  await for (final request in requests) {
    processRequest(request);
  }
}

void processRequest(HttpRequest request) {
  print('Got request for ${request.uri.path}');
  final response = request.response;
  if (request.uri.path == '/dart') {
    response
      ..headers.contentType = ContentType(
        'text',
        'plain',
      )
      ..write('Hello from the server');
  } else {
    response.statusCode = HttpStatus.notFound;
  }
  response.close();
}

HTTP client

The HttpClient class helps you connect to HTTP resources from your Dart command-line or server-side application. You can set headers, use HTTP methods, and read and write data. The HttpClient class does not work in browser-based apps. When programming in the browser, use the dart:html HttpRequest class. Here’s an example of using HttpClient:

void main() async {
  var url = Uri.parse('http://localhost:8888/dart');
  var httpClient = HttpClient();
  var request = await httpClient.getUrl(url);
  var response = await request.close();
  var data = await utf8.decoder.bind(response).toList();
  print('Response ${response.statusCode}: $data');
  httpClient.close();
}

More information

This page showed how to use the major features of the dart:io library. Besides the APIs discussed in this section, the dart:io library also provides APIs for processes, sockets, and web sockets. For more information about server-side and command-line app development, see the server-side Dart overview.

总结

本页向您介绍了 Dart 内置库中最常用的功能。但是，并没有涵盖所有内置库。您可能想要查看的其他内容包括 dart:collection 和 dart:typed_data, ，以及特定于平台的库，如 Dart web development libraries 和 Flutter libraries. 。

您可以使用 pub 包管理 工具获得更多库。 collection, crypto, http, intl, 以及 test 以上只是简单的列举了一些可以通过 pub 安装的库。

要了解有关 Dart 语言的更多信息，请参考 语言概览。