Dart 基本数据类型与类型归属判断【转】
目录
一、基本变量类型
dart 以 main 函数作为执行入口，虽然不强制类型，但是建议使用强制类型来使用，编译器不需要去推导类型：
如果使用var声明变量，则不会进行类型约束
void main() {
}
1、 变量声明 & console 输出 & 字符串拼接 & 数字转字符串
print('hello world!');
var firstName = 'post';
String lastName = 'bird';
var age = 122;
print(firstName + lastName + ' , age is ' + age.toString());
var name = 'ptbird';
name = 123.toString();
int num = 3;
double num2 = 0.111;
print( num ++ );
print( ++ num );
print(num2 / num);
输出结果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
hello world!
postbird , age is 122
3
5
0.0222
2、const 变量和 final 变量声明
const 只能通过静态数据赋值，否则会报错
const lastName = 'postbird';
final firstName = 'bird ';
// lastName = '123'; // 报错
// firstName = '123'; // 报错
final time = new DateTime.now();
// const time2 = new DateTime.now(); // 报错
输出结果（无输出）
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
[Done] exited with code=0 in 1.167 seconds
如果给 final 或者 const 再次赋值了，则会报错如下：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
demo1/demo1.dart:20:3: Error: Setter not found: 'lastName'.
lastName = '123';
^^^^^^^^
[Done] exited with code=254 in 2.262 seconds
如果给 const 赋值非 const 变量，则报错如下：
// const time2 = new DateTime.now(); // 报错
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
demo1/demo1.dart:23:17: Error: New expression is not a constant expression.
const time2 = new DateTime.now();
^^^
[Done] exited with code=254 in 2.992 seconds
3、String 字符串换行和字符串拼接
1）换行''' '''
String content = '''
multipart
...
string
''';
print(content);
输出结果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
multipart
...
string
2）字符串拼接
拼接除了使用加好，还可以像 js 的模板字符串直接拼接，语法差不多，只不过不需要反引号，普通引号即可，${v}，其中如果只是变量名，可以省略大括号$v：
String str1 = 'dart1';
String str2 = 'darg2';
int age = 21;
print('$str1 $str2 ${age.toString()}');
print('${str1} ${str2} ${age.toString()} ${age} ${age * age}');
输出结果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
dart1 darg2 21
dart1 darg2 21 21 441
[Done] exited with code=0 in 2.336 seconds
4、int 和 double 数字 整形和浮点型
int num1 = 123;
double price = 123.452323232;
print(price * num1);
price = 12;
print(price * num1);
输出结果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
15184.635757536
1476.0
[Done] exited with code=0 in 2.772 seconds
5、bool 类型和 if 判断
if 判断只能是 bool 类型的返回值，这点和 js 这衶弱类型语言完全不同：
bool flag = true;
if(flag) {
print('--- true');
}
int num1 = 1;
double num2 = 1.0;
String num3 = '1';
if(num1 == num2) {
print('num1 == num2');
} else {
print('num1 != num2');
}
if(num1 == num3) {
print('num1 == num3');
} else {
print('num1 != num3');
}
// int a = 1;
// if(a) { // 报错
// print('true');
// }
输出结果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
--- true
num1 == num2
num1 != num3
[Done] exited with code=0 in 1.452 seconds
如果 if 使用了非 bool 类型判断报错如下：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
demo1/demo1.dart:65:6: Error: A value of type 'int' can't be assigned to a variable of type 'bool'.
Try changing the type of the left hand side, or casting the right hand side to 'bool'.
if(a) {
^
[Done] exited with code=254 in 1.974 seconds
6、List 类型
List 类型是使用非常多的类型，与 js 的 Array 类似，初始赋值可以直接给一个列表，也可以通过new List()指定空的列表.
默认列表子项支持的值类型是 dynamic，不限制具体类型，如果需要限制具体类型则需要使用泛型，比如new List<String>()限制子项类型
List 作为对象提供了一些的方法和属性： API 文档地址：https://api.dart.dev/dev/2.4.0-dev.0.0/dart-core/List-class.html
通过add()能够添加一个子项， 通过addAll()能够追加另一个 List
List l1 = [123, '123', 'postbird'];
print(l1);
List l2 = new List();
l2.add('abc');
l2.add(123);
l2.addAll(['iterable', '222', '333', 123]);
print(l2);
List l3 = new List<String>();
l3.add('abc');
// l3.add(123);
print(l3);
List l4 = new List<int>();
l4.add(123);
// l4.add(123.12);
print(l4);
List l5 = new List<int>();
l5.add(1);
l5.add(3);
l5.add(2);
l5.add(4);
l5.add(6);
l5.add(2);
print(l5);
l5.sort();
print(l5);
输出结果:
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
[123, 123, postbird]
[abc, 123, iterable, 222, 333, 123]
[abc]
[123]
[1, 3, 2, 4, 6, 2]
[1, 2, 2, 3, 4, 6]
[Done] exited with code=0 in 2.079 seconds
7、Map 类型
与 javascript 对象类似，在 oc 中称为字典。
可以通过字面量指定，也可以通过声明一个new Map()的空 Map。
API 文档地址：https://api.dart.dev/dev/2.4.0-dev.0.0/dart-core/Map-class.html
var person = {
'name': 'ptbird',
'age': 24,
'work': ['it1', 'it2']
};
print(person);
print(person.toString());
print(person['name']);
print(person['age']);
print(person['work']);
Map person2 = new Map();
person2['name'] = 'name2';
person2['age'] = 24;
person2['work'] = ['it1', 'it2'];
print(person2);
print(person2['work']);
输出结果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
{name: ptbird, age: 24, work: [it1, it2]}
{name: ptbird, age: 24, work: [it1, it2]}
ptbird
24
[it1, it2]
{name: name2, age: 24, work: [it1, it2]}
[it1, it2]
[Done] exited with code=0 in 1.584 seconds
8、类型判断
is操作符能够判断类型归属，比如A is B，能够返回 bool 类型，判断 A 是否属于 B 类型。
var value = 123;
if(value is String) {
print('${value} is string');
} else if (value is int) {
print('${value} is int');
} else if (value is double) {
print('${value} is double');
} else {
print('${value} is other type');
}
输出结果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
123 is int
[Done] exited with code=0 in 1.567 seconds
