目錄
一、基本變量類型
dart 以 main 函數作為執行入口，雖然不強制類型，但是建議使用強制類型來使用，編譯器不需要去推導類型：
如果使用var聲明變量，則不會進行類型約束
void main() {
}
1、 變量聲明 & console 輸出 & 字符串拼接 & 數字轉字符串
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);
輸出結果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
hello world!
postbird , age is 122
3
5
0.0222
2、const 變量和 final 變量聲明
const 只能通過靜態數據賦值，否則會報錯
const lastName = 'postbird';
final firstName = 'bird ';
// lastName = '123'; // 報錯
// firstName = '123'; // 報錯
final time = new DateTime.now();
// const time2 = new DateTime.now(); // 報錯
輸出結果（無輸出）
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
[Done] exited with code=0 in 1.167 seconds
如果給 final 或者 const 再次賦值了，則會報錯如下：
[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
如果給 const 賦值非 const 變量，則報錯如下：
// const time2 = new DateTime.now(); // 報錯
[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 字符串換行和字符串拼接
1）換行''' '''
String content = '''
multipart
...
string
''';
print(content);
輸出結果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
multipart
...
string
2）字符串拼接
拼接除了使用加好，還可以像 js 的模板字符串直接拼接，語法差不多，只不過不需要反引號，普通引號即可，${v}，其中如果只是變量名，可以省略大括號$v：
String str1 = 'dart1';
String str2 = 'darg2';
int age = 21;
print('$str1 $str2 ${age.toString()}');
print('${str1} ${str2} ${age.toString()} ${age} ${age * age}');
輸出結果：
[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 數字 整形和浮點型
int num1 = 123;
double price = 123.452323232;
print(price * num1);
price = 12;
print(price * num1);
輸出結果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
15184.635757536
1476.0
[Done] exited with code=0 in 2.772 seconds
5、bool 類型和 if 判斷
if 判斷只能是 bool 類型的返回值，這點和 js 這衶弱類型語言完全不同：
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) { // 報錯
// print('true');
// }
輸出結果：
[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 類型判斷報錯如下：
[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 類型
List 類型是使用非常多的類型，與 js 的 Array 類似，初始賦值可以直接給一個列表，也可以通過new List()指定空的列表.
默認列表子項支持的值類型是 dynamic，不限制具體類型，如果需要限制具體類型則需要使用泛型，比如new List<String>()限制子項類型
List 作為對象提供了一些的方法和屬性： API 文檔地址：https://api.dart.dev/dev/2.4.0-dev.0.0/dart-core/List-class.html
通過add()能夠添加一個子項， 通過addAll()能夠追加另一個 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);
輸出結果:
[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 類型
與 javascript 對象類似，在 oc 中稱為字典。
可以通過字面量指定，也可以通過聲明一個new Map()的空 Map。
API 文檔地址：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']);
輸出結果：
[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、類型判斷
is操作符能夠判斷類型歸屬，比如A is B，能夠返回 bool 類型，判斷 A 是否屬於 B 類型。
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');
}
輸出結果：
[Running] dart "e:\Aprojects\DartHelloWorldDemo\demo1\demo1.dart"
123 is int
[Done] exited with code=0 in 1.567 seconds