Flutter学习一: Dart数据类型

系统内建类型

• numbers
• strings
• booleans
• lists (数组)
• maps (字典)
• runes (用于字符串中表示Unicode字符)
• symbols (标识符, 编译时常量)

1. Numbers

有两种类型: int 和 double

1.1 int

int类型的值分两种:
(1). Dart VM上, 取值区间: -2^63 to 2^63 - 1
(2). 被编译成JavaScript, 取值区间: -2^53 to 2^53 - 1

   var x = 1;
   var hex = 0xDEADBEEF;

1.2 double

64位（双精度）浮点型数据

   var y = 1.1;
   var exponents = 1.42e5;

int double都是num的子类, num的子类还包括一些操作符(= - * /)和abs(), ceil(), floor()等, 位操作符(>>)被定义再int类中,
其他的参考: https://api.dartlang.org/stable/2.1.0/dart-math/dart-math-library.html

note: Dart2.1之后, int可以自动转成double, 2.1之前会报错
double z = 1; // z = 1.0.

类型间的转换:

  // String -> int
  var one = int.parse('1');
  assert(one == 1);

  // String -> double
  var onePointOne = double.parse('1.1');
  assert(onePointOne == 1.1);

  // int -> String
  String oneAsString = 1.toString();
  assert(oneAsString == '1');

  // double -> String
  String piAsString = 3.14159.toStringAsFixed(2);
  assert(piAsString == '3.14');

int类型位操作:

  assert((3 << 1) == 6); // 0011 << 1 == 0110
  assert((3 >> 1) == 1); // 0011 >> 1 == 0001
  assert((3 | 4) == 7); // 0011 | 0100 == 0111

2. Strings

Dart字符串是UTF-16 code序列, 可以使用双引号""或单引号’'创建

  var s1 = 'Single quotes work well for string literals.';
  var s2 = "Double quotes work just as well.";

2.1. 合并字符串可以使用 + 操作符

  var s1 = 'a1'
  var s2 = 'a2'
  assert(s1 + s2 == 'a1a2');

2.2. 字符串镶嵌

你可以使用 ${expression} 镶嵌一个表达式或者变量, 如果是个变量还可以省略{}

  var s = 'string interpolation';
  assert('Dart has $s, which is very handy.' == 'Dart has string interpolation, ' + 'which is very handy.');

注意: 如果两个字符串有相同的UTF-16 code序列, 则两个字符串==

2.3. 创建多行字符串

使用 """..""" '''..'''

  var s1 = '''
  You can create
  multi-line strings like this one.
  ''';
  var s2 = """This is also a
  multi-line string.""";

2.4. “raw” string

  var s = r'In a raw string, not even \n gets special treatment.';

\n不会被转义, 即字符串不会换行

更多: https://www.dartlang.org/guides/libraries/library-tour#strings-and-regular-expressions

3. Booleans

bool 只有两个值: true false
Dart 是类型安全的, 所以除非值或者表达式为true, 否则都是false

4. Lists

数组, 这个不管是学过什么语言的都应该相当熟悉了
Dart创建的数组默认都是可变的

4.1 创建数组

var vegetables = List();
var fruits = [];

4.2 添加数据

fruits.add('kiwis');
// 增加多个
fruits.addAll(['grapes', 'bananas']);

assert(fruits.length == 5);

4.2 移除数据

// 移除单个
var appleIndex = fruits.indexOf('apples');
fruits.removeAt(appleIndex);
assert(fruits.length == 4);
// 移除一组
fruits.clear();
assert(fruits.length == 0);

4.3 使用indexOf()获取object在数组中的索引

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

assert(fruits[0] == 'apples');
assert(fruits.indexOf('apples') == 0);

4.4 使用sort() 对list排序

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

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

4.5 泛型

Lists are parameterized types, so you can specify the type that a list should contain:

var fruits = List<String>();

fruits.add('apples');
var fruit = fruits[0];
assert(fruit is String);
fruits.add(5); // Error: 'int' can't be assigned to 'String'

Generics: https://www.dartlang.org/guides/language/language-tour#generics
Collections: https://www.dartlang.org/guides/libraries/library-tour#collections

5. Maps

字典

    Map<String, String>  gifts = {'first': 'partridge'};

5.1. Map默认创建的都是可变的

 gifts['fourth'] = 'calling birds'; // Add a key-value pair

5.2. 用不存在的key从map中取值value = null

  var gifts = {'first': 'partridge'};
  assert(gifts['fifth'] == null);

5.3. 创建不可变的map, 使用const

  final constantMap = const {
    2: 'helium',
    10: 'neon',
    18: 'argon',
  }; // constantMap[2] = 'Helium'; // Uncommenting this causes an error.

https://api.dartlang.org/stable/2.1.0/dart-core/Map-class.html
https://www.dartlang.org/guides/language/language-tour#generics

6. Runes

在Dart中, Runs是UTF-32 code的字符串

  var clapping = '\u{1f44f}';
  print(clapping); // ?
  print(clapping.codeUnits); // [55357, 56399]
  print(clapping.runes.toList()); // [128079]

  Runes input = new Runes(
      '\u2665  \u{1f605}  \u{1f60e}  \u{1f47b}  \u{1f596}  \u{1f44d}');
  print(new String.fromCharCodes(input)); // ♥  ?  ?  ?  ?  ?

7. Symbols

Symbol对象表示Dart程序中声明的运算符或标识符。您可能永远不需要使用符号，但它们对于按名称引用标识符的API非常有用，因为缩小会更改标识符名称而不会更改标识符符号。

https://api.dartlang.org/stable/2.1.0/dart-core/Symbol-class.html

tips:

1. 官网地址: https://www.dartlang.org/guides/language/language-tour#built-in-types
2. Dartpad代码运行工具: https://dartpad.dartlang.org