Dart基础

1. Hello World

dart

void main() {
  print('Hello, World!');
}

2. Variables and Constants

变量和常量定义

dart

//  自动推断类型
var name = 'Bob';
int age = 20;
double height = 1.80;
bool isStudent = true;
// const 常量 不能修改 在编译时就确定
const String ID = "123456";

// final 只能赋值一次 但是可以在运行时确定
final DateTime now = DateTime.now();

print('name: $name, age: $age, height: $height, isStudent: $isStudent');
// name: Bob, age: 20, height: 1.8, isStudent: true

print("ID: ${ID}");
// ID: 123456

print("now: ${now}");
// now: 2021-08-25 10:37:44.387928

变量空值

dart

// ? is used to declare a variable as nullable variable  
// ? 可以用来声明一个变量可为空变量
int? age;

print(age); // null

age = 20;

print(age); // 20

3. 内置类型

Number

dart

int age = 20;
double height = 1.80;

print(age); // 20
print(height); // 1.8

// int can be assigned to double but not vice versa
height = 1; 

print(height); // 1.0

// int to double
double newHeight = age.toDouble();
print(newHeight); // 20.0

// double to int
int newAge = height.toInt();
print(newAge); // 1

const _NaN = 0 / 0;

print(_NaN.isNaN); // true

print(_NaN is double); // true
print(_NaN is int); // false

Boolean

dart

bool isStudent = true;

print(isStudent); // true

const count = 0;
// Error: Conditions must have a static type of 'bool'. No implicit casts are allowed.
if (count) {
  print('count is true');
}

String

dart

String name = 'Bob';
String message = 'Hello, $name';

print(message); // Hello, Bob

String multiLine = '''
  Hello, 
  $name
''';

print(multiLine);

// Hello,
// Bob

List

dart

List<int> numbers = [1, 2, 3, 4, 5];

print(numbers); // [1, 2, 3, 4, 5]

List<String> names = ['Bob', 'Alice', 'John'];

print(names); // [Bob, Alice, John]

names.add('Tom');

print(names); // [Bob, Alice, John, Tom]

for (var name in names) {
  print(name);
}

Set

dart

Set<int> numbers = {1, 2, 3, 4, 5};

var nums = new Set<int>();

print(numbers); // {1, 2, 3, 4, 5}

numbers.add(6);
numbers.add(6);

numbers.contains(6); // true

print(numbers); // {1, 2, 3, 4, 5, 6}

var objs = new Set<dynamic>();
objs.add("Hello");
objs.add("HuTu");

final strs = objs.cast<String>();
strs.forEach((element) {
  print(element+'s');
});

Map

dart

Map<String, int> scores = {
  'Alice': 100,
  'Bob': 90,
  'John': 80
};

print(scores); // {Alice: 100, Bob: 90, John: 80}

scores['Tom'] = 70;

print(scores); // {Alice: 100, Bob: 90, John: 80, Tom: 70}

scores.remove('Tom');

print(scores); // {Alice: 100, Bob: 90, John: 80}

for (var key in scores.keys) {
  print('$key: ${scores[key]}');
}

Dynamic

dart

dynamic name = 'Bob';

print(name); // Bob

name = 20;

print(name); // 20

var age = 20 * name as int;

print(age); // 400

Object

dart

Object name = '123';

print(name);

Runes

dart

String name = 'Bob';

print(name.runes.toList()); // (66, 111, 98)

print(name.codeUnits); // [66, 111, 98]

Runes input = new Runes('\u{1f605}');
print(input); // (128517)
print(new String.fromCharCodes(input)); // 😅

Symbols

dart

Symbol name = #name;

print(name); // Symbol("name")

// Convert Symbol to String
print(name.toString()); // "Symbol("name")"

// Convert Symbol to String
print(name
    .toString()
    .replaceAll('Symbol("', '')
    .replaceAll('")', '')); // "name"

// Symbol usages cases
var person = {
  #name: 'John',
  #age: 30,
};

print(person[#name]); // John
print(person[#age]); // 30

用于反射

dart

import 'dart:mirrors';

class MyClass {
  void sayHello(String name) {
    print('Hello, $name!');
  }
}

void main() {
  MyClass myInstance = MyClass();
  InstanceMirror im = reflect(myInstance);

  // 使用符号调用sayHello方法
  im.invoke(#sayHello, ['World']);
}

Type

dart

Type type = String;

print(type); // String

print(type.runtimeType); // Type

print(type is Type); // true

print(type is String); // false

print(type is Object); // true

print(type is dynamic); // true

print(type is int); // false

获取类型

dart

void main() {
  var list = [1, 2, 3];
  printType(list);
  
  var string = "Hello, Dart!";
  printType(string);
}

void printType(dynamic obj) {
  Type type = obj.runtimeType;
  print("The type of the object is $type");
}

Enum

dart

enum Color { red, green, blue }

void main() {
  print(Color.red); // Color.red
  print(Color.red.index); // 0
  print(Color.values); // [Color.red, Color.green, Color.blue]
}

4. Operators

Arithmetic Operators

dart

int a = 11;

print(a ~/ 2); // 5

print(a % 2); // 1

print(a++); // 11
print(a); // 12

print(++a); // 13
print(a); // 13

5. 自定义类操作符

dart

class Point {
  int x, y;

  Point(this.x, this.y);

  Point operator +(Point p) {
    return new Point(x + p.x, y + p.y);
  }

  Point operator *(int n) {
    return Point(x * n, y * n);
  }

  Point operator /(int n) {
    return Point(x ~/ n, y ~/ n);
  }

  Point operator -() {
    return Point(-x, -y);
  }

  Point operator -(Point p) {
    return Point(x - p.x, y - p.y);
  }

  @override
  String toString() {
    return 'Point($x, $y)';
  }
}

void main() {
  Point p1 = Point(10, 20);
  Point p2 = Point(30, 40);

  print(p1 + p2); // Point(40, 60)
  print(p1 * 2); // Point(20, 40)
  print(p1 / 2); // Point(5, 10)
  print(-p1); // Point(-10, -20)

  Point p3 = p1 - p2;
  print(p3); // Point(-20, -20)
}

6. Functions

Function Definition

dart

void main() {
  print(sayHello('Bob')); // Hello, Bob!
  print(sayHello2('Alice')); // Hello, Alice!
  print(sayHello3('John')); // Hello, John!
}

String sayHello(String name) {
  return 'Hello, $name!';
}

final sayHello2 = (String name) => 'Hello, $name!';

sayHello3(String name) => 'Hello, $name!';

Optional Parameters

dart


void main() {
  print(sayHello('Bob')); // Hello, Bob!
  print(sayHello('Alice', 'Good Morning')); // Good Morning, Alice!
  print(sayHello('John', 'Good Afternoon', 'How are you?')); // Good Afternoon, John! How are you?
}

String sayHello(String name, [String? greeting, String? message]) {
  if (greeting != null && message != null) {
    return '$greeting $name! $message';
  } else if (greeting != null) {
    return '$greeting $name!';
  } else {
    return 'Hello, $name!';
  }
}

Named Parameters

dart

void main() {
  print(sayHello('Bob')); // Hello, Bob!
  print(sayHello('Alice', greeting: 'Good Morning')); // Good Morning, Alice!
  print(sayHello('John', greeting: 'Good Afternoon', message: 'How are you?')); // Good Afternoon, John! How are you?
}

String sayHello(String name, {String? greeting, String? message}) {
  if (greeting != null && message != null) {
    return '$greeting $name! $message';
  } else if (greeting != null) {
    return '$greeting $name!';
  } else {
    return 'Hello, $name!';
  }
}

Default Parameters

dart

sayHello(
    {String name = 'Anonymous',
    String greeting = 'Hello',
    String message = 'Are you fine?'}) {
  return '$greeting $name! $message';
}

void main() {
  print(sayHello());
  print(sayHello(name: 'John', greeting: 'Hi', message: 'How are you?'));
}

Function as a Parameter

dart

void main() {
  print(sayHello('Bob', (name) => 'Hello, $name!'));
  print(sayHello('Alice', (name) => 'Good Morning, $name!'));
  print(sayHello('John', (name) => 'Good Afternoon, $name!'));
}

String sayHello(String name, String Function(String) greeting) {
  return greeting(name);
}

Function as a Return Type

dart

void main() {
  var greeting = sayHello('Bob');
  print(greeting('Hello'));
}

Function sayHello(String name) {
  return (String message) => '$message' + ' ' + name;
}

Anonymous Function

dart

void main() {
  var list = ['Bob', 'Alice', 'John'];

  list.forEach((name) {
    print('Hello, $name!');
  });
}

Closures

dart

Function makeAdder(int addBy) {
  return (int i) => addBy + i;
}

void main() {
  var add2 = makeAdder(2);
  var add4 = makeAdder(4);

  print(add2(3)); // 5
  print(add4(3)); // 7
}

7. Cascade Operator

dart

class Person {
  String? name;
  int age = 0;
  String? address;

  void show() {
    print('name: $name, age: $age, address: $address');
  }
}

void main() {
  var person = Person()
    ..name = 'Bob'
    ..age = 20
    ..address = 'New York';

  person.show();
}

8. Control Flow

switch case

dart

void main() {
  var grade = 'A';

  switch (grade) {
    case 'A':
      print('Excellent');
      break;
    case 'B':
      print('Good');
      break;
    case 'C':
      print('Fair');
      break;
    case 'D':
      print('Poor');
      break;
    default:
      print('Invalid grade');
  }
}

9. Loops

for loop

dart

void main() {
  var numbers = [1, 2, 3, 4, 5];

  for (var number in numbers) {
    print(number);
  }
}

while loop

dart

void main() {
  var count = 0;

  while (count < 5) {
    print(count);
    count++;
  }
}

do while loop

dart

void main() {
  var count = 0;

  do {
    print(count);
    count++;
  } while (count < 5);
}

for i loop

dart

void main() {
  for (var i = 0; i < 5; i++) {
    print(i);
  }
}

10. Exception Handling

throw exception

dart

void main() {
  var age = 20;

  if (age < 18) {
    throw Exception('Age must be greater than 18');
  } else {
    print('You are eligible to vote');
  }
}

try catch

dart

void main() {
  var age = 2;

  try {
    if (age < 18) {
      throw "You are not eligible to vote";
    } else {
      print('You are eligible to vote');
    }
  } on String catch (e) {
    print("---- String Exception ---- $e");
  } catch (e) {
    print(e);
  } finally {
    print('Thank you');
  }
}

11. Generics

dart

class Data<T> {
  T value;

  Data(this.value);

  T getValue() {
    return value;
  }
}

void main() {
  var data = Data<int>(10);
  print(data.getValue()); // 10

  var data2 = Data<String>('Hello, Dart!');
  print(data2.getValue()); // Hello, Dart!
}

12. Future

future 是一个异步操作的结果，它会在未来的某个时间点返回一个值。

dart

Future<String> fetchData() {
  return Future.delayed(Duration(seconds: 2), () {
    return "Hello, Dart!";
  });
}

void main() {
  print('Fetching data...');
  fetchData().then((value) {
    print(value);
  });
  print('Data fetched');
}

13. Async Await

dart

Future<String> fetchData() {
  return Future.delayed(Duration(seconds: 2), () {
    return "Hello, Dart!";
  });
}

void main() async {
  print('Fetching data...');
  var data = await fetchData();
  print(data);
  print('Data fetched');
}

14. Abstract Class

dart

abstract class Animal {
  void makeSound();
}

class Dog extends Animal {
  @override
  void makeSound() {
    print('Bark');
  }
}

class Cat extends
Animal {
  @override
  void makeSound() {
    print('Meow');
  }
}

void main() {
  var dog = Dog();
  dog.makeSound(); // Bark

  var cat = Cat();
  cat.makeSound(); // Meow
}

15. Interface

接口是一种规范，它定义了类应该实现的方法。

dart

class Animal {
  void makeSound() {
    print('Animal makes a sound');
  }
}

class Dog implements Animal {
  @override
  void makeSound() {
    print('Bark');
  }
}

class Cat implements Animal {
  @override
  void makeSound() {
    print('Meow');
  }
}

void main() {
  var dog = Dog();
  dog.makeSound(); // Bark

  var cat = Cat();
  cat.makeSound(); // Meow
}

16. Extend Class

dart

class Animal {
  void makeSound() {
    print('Animal makes a sound');
  }
}

class Dog extends Animal {
  void bark() {
    print('Bark');
  }
}

class Cat extends Animal {
  void meow() {
    print('Meow');
  }
}

void main() {
  var dog = Dog();
  dog.makeSound(); // Animal makes a sound
  dog.bark(); // Bark

  var cat = Cat();
  cat.makeSound(); // Animal makes a sound
  cat.meow(); // Meow
}

17. Mixin

dart

mixin Swim {
  void swim() {
    print('Swimming');
  }
}

mixin Fly {
  void fly() {
    print('Flying');
  }
}

class Animal {
  void makeSound() {
    print('Animal makes a sound');
  }
}

class Fish extends Animal with Swim {
  void swim() {
    print('Fish is swimming');
  }
}

class Bird extends Animal with Fly {
  void fly() {
    print('Bird is flying');
  }
}

class Duck extends Animal with Swim, Fly {
  sayHello() {
    print('Hello');
  }
}

void main() {
  var fish = Fish();
  fish.makeSound(); // Animal makes a sound
  fish.swim(); // Fish is swimming

  var bird = Bird();
  bird.makeSound(); // Animal makes a sound
  bird.fly(); //  Bird is flying

  var duck = Duck();
  duck.makeSound(); // Animal makes a sound
  duck.swim(); // Swimming
  duck.fly(); // Flying
  duck.sayHello(); // Hello
}

18. Library

Import Library

dart

import 'dart:math';

void main() {
  print(min(10, 20)); // 10
  print(max(10, 20)); // 20
}

import Netwrok Library

dart

import 'https://pub.dev/packages/http' as http;

void main() {
  var url = Uri.parse('https://jsonplaceholder.typicode.com/posts');
  http.get(url).then((response) {
    print(response.statusCode);
    print(response.body);
  });
}

split library

part 用于将一个大的库文件分割成多个小的库文件。

dart

library my_library;
part 'part1.dart';
part 'part2.dart';

1. Hello World ​

2. Variables and Constants ​

变量和常量定义 ​

变量空值 ​

3. 内置类型 ​

Number ​

Boolean ​

String ​

List ​

Set ​

Map ​

Dynamic ​

Object ​

Runes ​

Symbols ​

Type ​

Enum ​

4. Operators ​

Arithmetic Operators ​

5. 自定义类操作符 ​

6. Functions ​

Function Definition ​

Optional Parameters ​

Named Parameters ​

Default Parameters ​

Function as a Parameter ​

Function as a Return Type ​

Anonymous Function ​

Closures ​

7. Cascade Operator ​

8. Control Flow ​

switch case ​

9. Loops ​

for loop ​

while loop ​

do while loop ​

for i loop ​

10. Exception Handling ​

throw exception ​

try catch ​

11. Generics ​

12. Future ​

13. Async Await ​

14. Abstract Class ​

15. Interface ​

16. Extend Class ​

17. Mixin ​

18. Library ​

Import Library ​

import Netwrok Library ​

split library ​

1. Hello World

2. Variables and Constants

变量和常量定义

变量空值

3. 内置类型

Number

Boolean

String

List

Set

Map

Dynamic

Object

Runes

Symbols

Type

Enum

4. Operators

Arithmetic Operators

5. 自定义类操作符

6. Functions

Function Definition

Optional Parameters

Named Parameters

Default Parameters

Function as a Parameter

Function as a Return Type

Anonymous Function

Closures

7. Cascade Operator

8. Control Flow

switch case

9. Loops

for loop

while loop

do while loop

for i loop

10. Exception Handling

throw exception

try catch

11. Generics

12. Future

13. Async Await

14. Abstract Class

15. Interface

16. Extend Class

17. Mixin

18. Library

Import Library

import Netwrok Library

split library