Dart 语法 (Dart Syntax)¶

1. 变量与常量 (Variables and Constants)¶

1.1 变量声明 (Variable Declaration)¶

var name = 'Xihai'; // 类型自动推断为 String (Type inferred as String)
int age = 30; // 显式指定类型 (Explicitly specifying the type)
double height = 1.75;
bool isStudent = true;

1.2 常量声明 (Constant Declaration)¶

final String city = 'Beijing'; // 运行时常量 (Runtime constant)
const double pi = 3.14159; // 编译时常量 (Compile-time constant)

2. 数据类型 (Data Types)¶

int count = 10; // 整数类型 (Integer)
double price = 19.99; // 双精度浮点数 (Double)
String message = 'Hello, Dart!'; // 字符串类型 (String)
bool isActive = true; // 布尔类型 (Boolean)
List<String> fruits = ['Apple', 'Banana', 'Orange']; // 列表 (List)
Map<String, int> scores = {'Math': 95, 'English': 88}; // 映射 (Map)
Set<String> colors = {'red', 'green', 'blue'}; // 集合 (Set)
dynamic variable = 'Hello'; // 动态类型 (Dynamic type)
Object obj = 'Any type'; // 对象类型，所有类型的基类 (Object type, base class of all Dart types)

3. 函数 (Functions)¶

3.1 基本函数 (Basic Function)¶

void printMessage() {
  print('Hello, Dart!'); // 打印信息 (Print message)
}

int add(int a, int b) {
  return a + b; // 返回两个数的和 (Return the sum of two numbers)
}

3.2 箭头函数 (Arrow Functions)¶

int multiply(int a, int b) => a * b; // 乘法运算的简写形式 (Short form for multiplication)

3.3 可选命名参数 (Optional Named Parameters)¶

void greet({String? name, int? age}) {
  print('Hello $name, you are $age years old.'); // 打印问候语 (Print greeting)
}
greet(name: 'Xihai', age: 30); // 调用函数并传递参数 (Call function with arguments)

3.4 可选位置参数 (Optional Positional Parameters)¶

void printInfo(String name, [int? age]) {
  print('Name: $name, Age: $age'); // 打印姓名和年龄 (Print name and age)
}
printInfo('Xihai', 30); // 调用函数并传递位置参数 (Call function with positional arguments)

##  4. 控制流语句 (Control Flow Statements)

### 4.1 条件语句 (Conditional Statements)
```dart
if (age > 18) {
  print('Adult'); // 如果年龄大于18，打印“成人” (Print "Adult" if age is greater than 18)
} else if (age > 12) {
  print('Teenager'); // 如果年龄大于12，打印“青少年” (Print "Teenager" if age is greater than 12)
} else {
  print('Child'); // 否则打印“儿童” (Otherwise, print "Child")
}

4.2 switch 语句 (Switch Statement)¶

switch (age) {
  case 18:
    print('Just an adult'); // 如果年龄是18，打印“刚成年” (Print "Just an adult" if age is 18)
    break;
  case 30:
    print('In your prime'); // 如果年龄是30，打印“正值壮年” (Print "In your prime" if age is 30)
    break;
  default:
    print('Age is just a number'); // 默认情况下，打印“年龄只是个数字” (By default, print "Age is just a number")
}

4.3 三元运算符 (Ternary Operator)¶

String type = age > 18 ? 'Adult' : 'Not an Adult'; // 三元运算符检查年龄并返回相应的字符串 (Ternary operator checks age and returns the corresponding string)

5. 循环语句 (Loop Statements)¶

5.1 for 循环 (For Loop)¶

for (int i = 0; i < 5; i++) {
  print('i = $i'); // 打印i的值 (Print the value of i)
}

5.2 for-in 循环 (For-in Loop)¶

for (var fruit in fruits) {
  print(fruit); // 遍历列表并打印每个水果 (Iterate over the list and print each fruit)
}

5.3 while 循环 (While Loop)¶

int i = 0;
while (i < 5) {
  print('i = $i'); // 当i小于5时打印i的值 (Print the value of i while i is less than 5)
  i++; // 递增i的值 (Increment the value of i)
}

5.4 do-while 循环 (Do-while Loop)¶

int j = 0;
do {
  print('j = $j'); // 打印j的值 (Print the value of j)
  j++; // 递增j的值 (Increment the value of j)
} while (j < 5); // 当j小于5时继续循环 (Continue looping while j is less than 5)

6. 类与对象 (Classes and Objects)¶

6.1 定义类 (Defining a Class)¶

class Person {
  String name; // 属性：姓名 (Property: name)
  int age; // 属性：年龄 (Property: age)

  Person(this.name, this.age); // 构造函数 (Constructor)

  // 命名构造函数 (Named Constructor)
  Person.fromJson(Map<String, dynamic> json)
      : name = json['name'],
        age = json['age'];

  void sayHello() { // 方法：问候 (Method: greeting)
    print('Hello, my name is $name.'); // 打印问候信息 (Print greeting message)
  }
}

// 创建对象 (Creating an Object)
var person = Person('Xihai', 30);
person.sayHello(); // 调用方法 (Calling a Method)

7. 异步编程 (Asynchronous Programming)¶

7.1 使用 async/await (Using async/await)¶

Future<String> fetchData() async {
  await Future.delayed(Duration(seconds: 2)); // 模拟一个耗时操作 (Simulate a time-consuming operation)
  return 'Data fetched'; // 返回数据 (Return data)
}

void main() async {
  print('Fetching data...'); // 打印信息 (Print message)
  String data = await fetchData(); // 等待数据 (Await data)
  print(data); // 打印获取的数据 (Print fetched data)
}

7.2 使用 then (Using then)¶

fetchData().then((data) {
  print(data); // 数据获取后执行 (Execute after data is fetched)
});

7.3 Stream¶

Stream<int> countStream(int max) async* {
  for (int i = 1; i <= max; i++) {
    yield i; // 生成序列中的下一个值 (Generate the next value in the sequence)
    await Future.delayed(Duration(seconds: 1)); // 等待1秒 (Wait for 1 second)
  }
}

void main() async {
  await for (int value in countStream(5)) {
    print('value: $value'); // 逐个处理 Stream 中的每个值 (Process each value in the Stream)
  }
}

8. 异常处理 (Exception Handling)¶

try {
  int result = 10 ~/ 0; // ~/ 是整除操作符 (~/ is the integer division operator)
  print(result);
} catch (e) {
  print('Caught an exception: $e'); // 捕获异常并打印 (Catch the exception and print it)
} finally {
  print('This block is always executed'); // 最终块总是执行 (The finally block is always executed)
}

9. 泛型 (Generics)¶

9.1 泛型类 (Generic Class)¶

class Box<T> {
  T value;
  Box(this.value);
}

9.2 泛型函数 (Generic Function)¶

T getFirst<T>(List<T> items) {
  return items[0]; // 返回列表中的第一个元素 (Return the first item in the list)
}

var firstInt = getFirst<int>([1, 2, 3]); // 使用泛型函数 (Using the generic function)
var firstString = getFirst<String>(['a', 'b', 'c']);

10. 扩展方法 (Extension Methods)¶

10.1 定义扩展方法 (Defining an Extension Method)¶

extension StringExtensions on String {
  String toCapitalized() {
    return this[0].toUpperCase() + this.substring(1); // 首字母大写 (Capitalize the first letter)
  }
}

// 使用扩展方法 (Using an Extension Method)
void main() {
  String text = 'hello';
  print(text.toCapitalized()); // 输出: Hello (Output: Hello)
}

11. 常用库 (Common Libraries)¶

import 'dart:math'; // 数学函数和常用 (Math functions and constants)
import 'dart:convert'; // JSON 编码和解码 (JSON encoding and decoding)
import 'dart:io'; // 文件和网络 I/O (File and network I/O)
import 'dart:async'; // 异步编程 (Asynchronous programming)
import 'package:http/http.dart' as http; // HTTP 请求 (HTTP requests)

12. 注释 (Comments)¶

12.1 单行注释 (Single-line Comment)¶

// 这是一个单行注释 (This is a single-line comment)
var name = 'Xihai'; // 变量声明 (Variable declaration)

12.2 多行注释 (Multi-line Comment)¶

/*
  这是一个多行注释
  (This is a multi-line comment)
  它可以跨越多行 (It can span multiple lines)
*/
var age = 30;

12.3 文档注释 (Documentation Comment)¶

/// 这是一个文档注释 (This is a documentation comment)
/// 用于类、方法或变量的文档生成 (Used for generating documentation for classes, methods, or variables)
class Person {
  String name;
  int age;

  /// 构造函数 (Constructor)
  Person(this.name, this.age);

  /**
   * 这是一个多行文档注释
   * (This is a multi-line documentation comment)
   * 用于提供详细的类或方法文档 (Used for providing detailed documentation for a class or method)
   */
  void sayHello() {
    print('Hello, my name is $name.');
  }
}

13. 运算符 (Operators)¶

13.1 算术运算符 (Arithmetic Operators)¶

int a = 10;
int b = 5;

int sum = a + b; // 加法 (Addition)
int difference = a - b; // 减法 (Subtraction)
int product = a * b; // 乘法 (Multiplication)
double quotient = a / b; // 除法 (Division)
int remainder = a % b; // 取模 (Modulus)
int integerDivision = a ~/ b; // 整除 (Integer Division)

13.2 关系运算符 (Relational Operators)¶

bool isEqual = (a == b); // 等于 (Equal)
bool isNotEqual = (a != b); // 不等于 (Not equal)
bool isGreaterThan = (a > b); // 大于 (Greater than)
bool isLessThan = (a < b); // 小于 (Less than)
bool isGreaterThanOrEqual = (a >= b); // 大于等于 (Greater than or equal)
bool isLessThanOrEqual = (a <= b); // 小于等于 (Less than or equal)

13.3 逻辑运算符 (Logical Operators)¶

bool isTrue = true;
bool isFalse = false;

bool andResult = isTrue && isFalse; // 逻辑与 (Logical AND)
bool orResult = isTrue || isFalse; // 逻辑或 (Logical OR)
bool notResult = !isTrue; // 逻辑非 (Logical NOT)

13.4 位运算符 (Bitwise Operators)¶

int x = 5; // 0101 in binary
int y = 3; // 0011 in binary

int andResult = x & y; // 位与 (Bitwise AND) - 0001
int orResult = x | y; // 位或 (Bitwise OR) - 0111
int xorResult = x ^ y; // 位异或 (Bitwise XOR) - 0110
int notResult = ~x; // 位非 (Bitwise NOT) - 1010 (inverted bits)
int leftShift = x << 1; // 左移 (Left shift) - 1010 (10 in decimal)
int rightShift = x >> 1; // 右移 (Right shift) - 0010 (2 in decimal)

13.5 赋值运算符 (Assignment Operators)¶

int value = 10;
value += 5; // 相当于 value = value + 5 (Equivalent to value = value + 5)
value -= 3; // 相当于 value = value - 3 (Equivalent to value = value - 3)
value *= 2; // 相当于 value = value * 2 (Equivalent to value = value * 2)
value /= 2; // 相当于 value = value / 2 (Equivalent to value = value / 2)
value %= 3; // 相当于 value = value % 3 (Equivalent to value = value % 3)
value ~/= 2; // 相当于 value = value ~/ 2 (Equivalent to value = value ~/ 2)

13.6 条件运算符 (Conditional Operators)¶

String result = (age > 18) ? 'Adult' : 'Not an Adult'; // 三元运算符 (Ternary Operator)

String? name;
String displayName = name ?? 'Guest'; // 如果 name 为 null，则返回 'Guest' (Return 'Guest' if name is null)

13.7 类型测试运算符 (Type Test Operators)¶

if (name is String) {
  print('Name is a string'); // 检查 name 是否为 String 类型 (Check if name is of type String)
}

if (age is! int) {
  print('Age is not an integer'); // 检查 age 是否不是 int 类型 (Check if age is not of type int)
}

14. Null 安全 (Null Safety)¶

14.1 可空类型 (Nullable Types)¶

String? name; // name 可能为 null (name can be null)

14.2 非空断言操作符 (Null Assertion Operator)¶

String name = getName()!; // 确定 getName() 返回的值不为 null (Assert that the result of getName() is not null)

14.3 空条件操作符 (Null-aware Operators)¶

String? name;
String displayName = name ?? 'Guest'; // 如果 name 为 null，则返回 'Guest' (Return 'Guest' if name is null)

name?.toLowerCase(); // 如果 name 不为 null，则调用 toLowerCase() (Call toLowerCase() if name is not null)

15. Dart 与集合 (Dart and Collections)¶

15.1 列表 (Lists)¶

List<String> fruits = ['Apple', 'Banana', 'Orange']; // 定义一个字符串列表 (Define a list of strings)

fruits.add('Mango'); // 添加元素 (Add an element)
fruits.remove('Banana'); // 删除元素 (Remove an element)
fruits[0] = 'Grapes'; // 更新元素 (Update an element)
String firstFruit = fruits[0]; // 访问元素 (Access an element)

15.2 映射 (Maps)¶

Map<String, int> scores = {'Math': 95, 'English': 88}; // 定义一个映射 (Define a map)

scores['Science'] = 90; // 添加键值对 (Add a key-value pair)
scores.remove('English'); // 删除键值对 (Remove a key-value pair)
int mathScore = scores['Math']!; // 访问值 (Access a value)

15.3 集合 (Sets)¶

Set<String> colors = {'red', 'green', 'blue'}; // 定义一个集合 (Define a set)

colors.add('yellow'); // 添加元素 (Add an element)
colors.remove('green'); // 删除元素 (Remove an element)
bool containsRed = colors.contains('red'); // 检查集合是否包含某元素 (Check if the set contains an element)

16. 导入库 (Importing Libraries)¶

import 'dart:math'; // 导入 Dart 内置数学库 (Import Dart's built-in math library)
import 'package:http/http.dart' as http; // 导入第三方 HTTP 库 (Import a third-party HTTP library)

17. `late` 关键字 (The `late` Keyword)¶

late 是 Dart 中的一个关键字，用于声明一个延迟初始化的变量。使用 late 声明的变量会在它首次被访问时才会进行初始化，而不是在声明时立即初始化。

late is a keyword in Dart used to declare a lazily initialized variable. A variable declared with late will only be initialized the first time it is accessed, rather than at the time of declaration.