Dart语言基础教程：从入门到精通

Dart

Flutte

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"这篇资源是关于Dart编程语言的入门指南，主要介绍如何使用Dart的各种主要特性，包括变量、运算符、类和库。它假设读者已经具备其他语言的编程基础。此外，还提到了Dart的核心库巡览以及Dart语言规范，供深入学习时参考。" 在Dart语言中，程序的基本构建块包括： 1. **注释(Comments)**：Dart支持单行注释（以`//`开头）和多行注释（以`/*`和`*/`包围）。文档注释（以`///`开头）用于生成API文档。 2. **类型(Type)**：Dart是一种静态类型的语言，`int`表示整型，例如`42`是一个整数字面量。其他内置类型包括`String`（字符串）、`List`（列表）和`bool`（布尔值）。 3. **数字字面量(Number Literals)**：`42`是一个数字字面量，它们在编译时即被确定。 4. **打印函数(print())**：`print()`函数用于输出到控制台，是调试和显示程序状态的常用工具。 5. **字符串(Strings)**：Dart支持单引号`' '`和双引号`" "`包围的字符串字面量。`''`或`""`中的`$variableName`或`${expression}`是字符串插值，允许将变量或表达式的值插入到字符串中。 6. **主函数(main())**：`main()`是Dart程序的入口点，每个Dart应用程序必须包含一个顶级的`main()`函数，执行程序的起点。除了这些基本元素，Dart还有许多其他重要的特性： - **变量(Variables)**：Dart支持声明变量，如`var`、`final`和`const`。`var`声明的变量可以根据赋值自动推断类型，而`final`和`const`声明的变量一旦初始化就不能改变。 - **运算符(Operators)**：Dart提供常见的算术、比较和逻辑运算符，例如`+`、`-`、`*`、`/`、`==`、`&&`、`||`等。 - **类(Classes)**：Dart是面向对象的语言，支持类的定义、继承、封装和多态。 - **库(Libraries)**：Dart通过导入库（`import`语句）来组织和共享代码，库之间可以有依赖关系。 - **异常处理(Exception Handling)**：Dart使用`try`、`catch`和`finally`来处理运行时错误。 - **异步编程(Asynchronous Programming)**：Dart提供了`async`和`await`关键字进行异步操作，便于处理回调函数和Promise。 - **集合(Collections)**：Dart的`List`、`Set`和`Map`提供了丰富的集合操作，支持泛型。 - **闭包(Closures)**：Dart的函数是第一类公民，可以作为参数传递和作为返回值，因此支持闭包。 - **dart:io库**：对于命令行应用，dart:io库提供了读写文件、网络通信等功能。 - **dart:ui库**：主要用于Flutter框架，提供图形用户界面相关的API。为了更深入地了解Dart的各个特性，可以查阅《Dart语言规范》和《Dart核心库巡览》。这两份资源将详细介绍Dart的语法、类库和最佳实践，帮助开发者全面掌握Dart编程。

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Use

.length

to get the number of key-value pairs in the map:

To create a map that

’

s a compile-time constant, add

const

before the map literal:

For more information about maps, see Generics and Maps.

Runes

In Dart, runes are the UTF-32 code points of a string.

Unicode de

ﬁ

nes a unique numeric value for each letter, digit, and symbol used in all of the world

’

s writing systems. Because a Dart string is a

sequence of UTF-16 code units, expressing 32-bit Unicode values within a string requires special syntax.

The usual way to express a Unicode code point is

\uXXXX

, where XXXX is a 4-digit hexadecimal value. For example, the heart character (

♥

) is

\u2665

. To specify more or less than 4 hex digits, place the value in curly brackets. For example, the laughing emoji (



) is

\u{1f600}

The String class has several properties you can use to extract rune information. The

codeUnitAt

and

codeUnit

properties return 16-bit code units.

Use the

runes

property to get the runes of a string.

The following example illustrates the relationship between runes, 16-bit code units, and 32-bit code points. Click the run button to see runes in

action.

Symbols

A Symbol object represents an operator or identi

ﬁ

er declared in a Dart program. You might never need to use symbols, but they

’

re invaluable for

APIs that refer to identi

ﬁ

ers by name, because mini

ﬁ

cation changes identi

ﬁ

er names but not identi

ﬁ

er symbols.

To get the symbol for an identi

ﬁ

er, use a symbol literal, which is just

followed by the identi

ﬁ

er:

Symbol literals are compile-time constants.

xxxxxxxxxx



main() {

var clapping = '\u{1f44f}';

print(clapping);

print(clapping.codeUnits);

print(clapping.runes.toList());

Runes input = new Runes(

  '\u2665 \u{1f605} \u{1f60e} \u{1f47b} \u{1f596} \u{1f44d}');

print(new String.fromCharCodes(input));

}

Console output

var gifts = {'first': 'partridge'};

assert(gifts['fifth'] == null);

var gifts = {'first': 'partridge'};

gifts['fourth'] = 'calling birds';

assert(gifts.length == 2);

final constantMap = const {

2: 'helium',

10: 'neon',

18: 'argon',

};

// constantMap[2] = 'Helium'; // Uncommenting this causes an error.

Note:

Be careful when manipulating runes using list operations. This approach can easily break down, depending on the particular language,

character set, and operation. For more information, see How do I reverse a String in Dart? on Stack Over

ﬂ

ow.

#radix

#bar

Functions
Dart is a true object-oriented language, so even functions are objects and have a type, Function. This means that functions can be assigned to
variables or passed as arguments to other functions. You can also call an instance of a Dart class as if it were a function. For details, see Callable
classes.
Here
’
s an example of implementing a function:
Although E
ﬀ
ective Dart recommends type annotations for public APIs, the function still works if you omit the types:
For functions that contain just one expression, you can use a shorthand syntax:
The 
=> expr
 syntax is a shorthand for 
{ return expr; }
. The 
=>
 notation is sometimes referred to as 
arrow
 syntax.
A function can have two types of parameters: required and optional. The required parameters are listed 
ﬁ
rst, followed by any optional parameters.
Named optional parameters can also be marked as 
@required
. See the next section for details.
Optional parameters
Optional parameters can be either positional or named, but not both.
Optional named parameters
When calling a function, you can specify named parameters using 
paramName: value
. For example:
When de
ﬁ
ning a function, use 
{param1, param2, …}
 to specify named parameters:
Flutter instance creation expressions can get complex, so widget constructors use named parameters exclusively. This makes instance creation
expressions easier to read.
You can annotate a named parameter in any Dart code (not just Flutter) with @required to indicate that it is a 
required
 parameter. For example:
When a 
Scrollbar
 is constructed, the analyzer reports an issue when the 
child
 argument is absent.
Required is de
ﬁ
ned in the meta package. Either import 
package:meta/meta.dart
 directly, or import another package that exports 
meta
, such as
Flutter
’
s 
package:flutter/material.dart
.
Optional positional parameters
Wrapping a set of function parameters in 
[]
 marks them as optional positional parameters:
bool isNoble(int atomicNumber) { 
  return _nobleGases[atomicNumber] != null; 
}
isNoble(atomicNumber) { 
  return _nobleGases[atomicNumber] != null; 
}
bool isNoble(int atomicNumber) => _nobleGases[atomicNumber] != null;
Note:
 Only an 
expression
—
not a 
statement
—
can appear between the arrow (=>) and the semicolon (;). For example, you can
’
t put an if
statement there, but you can use a conditional expression.
enableFlags(bold: true, hidden: false);
/// Sets the [bold] and [hidden] flags ... 
void enableFlags({bool bold, bool hidden}) {...}
const Scrollbar({Key key,  @required  Widget child})