Flutter 三: Dart

1 数据类型

• 数字(number) int double

  • 字符串转换成 num int.parse(“1”) double.parse(“1”);
  • double 四舍五入保留两位小数 toStringAsFixed(2) 返回值为string
  • double 直接舍弃小数点后几位的数据 可使用字符串截取的方式

• 字符串(string)

  • 单引号 双引号 三引号
  • 三引号 可以输出文本格式 比如换行

• 布尔值(boolean)

• 列表(list)

  • 一旦定义了 内容的类型 就不可变
  • list内容是常量 添加const
    

• 集合(set)

• 映射(map)
• 符文(rune)
• 符号(Symbol)

- 数字(number)  int double
	-  字符串转换成 num   int.parse("1") double.parse("1");
	-  double 四舍五入保留两位小数 toStringAsFixed(2) 返回值为string
	-  double 直接舍弃小数点后几位的数据  可使用字符串截取的方式
//字符串转换成 num  
  var a = int.parse("1");
  var b = double.parse("1");
  print(a);
  print(b);
//结果
//1
//1.0

- 字符串(string)
	- 单引号 双引号   三引号
	- 三引号 可以输出文本格式 比如换行 
var a = "aaa"
      "bbb";
  print(a);
  a = 'aaa'
      'bbb';
  print(a);
  a = '''aaa 
  bbb''';
  print(a);  
结果
aaabbb
aaabbb
aaa 
  bbb


- 列表(list)
   - 一旦定义了 内容的类型 就不可变
   -  - list内容是常量  添加const
void main(){
 var list = [1,2,3];
 // list = ["1","2","3"]; // Error: A value of type 'String' can't be assigned to a variable of type 'int'.

  var cars = [ Car("TSLT"),Car("XRB")];
  print(cars);
  print(cars[0].name);
  print(cars[0].drive());

var names = const ["TSLT","XRBS"];  //内容不可改变
  print(names);
  names[1] = "TSLC"; // Cannot modify an unmodifiable list
  print(names);
}
class Car{

  var name;

  Car(this.name);

  bool drive(){
    print("${name}我着了...");
    return true;
  }
}
 结果
 [Instance of 'Car', Instance of 'Car']
TSLT
TSLT我着了...
true
[TSLT, XRBS]




void main(){
   var a = {1,"2",3.4,Car(5),true};
   print(a);

   a.add("aaa");
   print(a);

   var b = Set();
   b.add(1);
   b.add("2");
   b.add(3.4);
   b.add(Car(5));
   b.add(true);
   print(b);

   // var c = const {1,"2",3.4,Car(5),true}; //  Error:  The element '3.4' does not have a primitive equality.
   var c = const {1,"你好",true}; //  Error:  The element '3.4' does not have a primitive equality.
   print(c);
}
结果
{1, 2, 3.4, Instance of 'Car', true}
{1, 2, 3.4, Instance of 'Car', true, aaa}
{1, 2, 3.4, Instance of 'Car', true}
{1, 你好, true}

2 var 关键字

• var 关键字一旦赋值类型即确定

void main(){
  var a = 1;
  var b = "hello";
  var c = Car();
  c.drive();
  print(a.runtimeType);
  print(b.runtimeType);
  print(c.runtimeType);
} 

class Car{
  void drive(){
    print("我跑了");
  }
}

结果

我跑了
int
String
Car

3 dynamic 和 Object

• Object是Dart所有对象的父类
• Object和dynamic声明变量 可改变类型
• dynamic 可使用对象类型的属性和方法 Object声明的对象只能用Object的属性和方法

void main(){
  Object a;
  a = "hello";
  print("a 的类型是 ${a.runtimeType}");
  a = 1;
  print("a 的变化后类型是 ${a.runtimeType}");

  dynamic b;
  b = 1;
  print("b 的类型是 ${b.runtimeType}");
  b = 1.1;
  print("b 的变化后类型是 ${b.runtimeType}");

  a = "hello";
  b = "world";
  print(b.length);
  // print(a.length); //报错 The getter 'length' isn't defined for the class 'Object'.
}

结果

a 的类型是 String
a 的变化后类型是 int
b 的类型是 int
b 的变化后类型是 double
5

4 final 和 const

• const 编译时赋值
• final 运行时 第一次使用时 赋值

void main(){
  final a;
  a = 1;
  print(a);

  // const b; // 报错 Error: The const variable 'b' must be initialized
  const b = 2;
  print(b);
}

结果

1
2

5 空安全

void main(){
  Car? car;
  print(car?.drive());
  car = Car();
  print(car?.drive());
}
class Car{
  bool drive(){
    print("我着了...");
    return true;
  }
}

运行结果

lib/demo.dart:8:9: Warning: Operand of null-aware operation '?.' has type 'Car' which excludes null.
 - 'Car' is from 'package:demo/demo.dart' ('lib/demo.dart').
  print(car?.drive());
        ^
null
我着了...
true

函数 (方法)

• 不声明返回值的 默认是dynamic

run(){
  print("我跑了");
}

eat(){
  return "吃撑了";
}

void main(){
  print(run());
  print(eat());
}

结果

我跑了
null
吃撑了

• 函数可以作为变量

var run = (){
  print("我跑了");
};

var eat = (){
  return "吃撑了";
};

void main(){
  print(run());
  print(eat());
}

结果

我跑了
null
吃撑了

• 函数可以作为参数

run(String name){
  print("${name}跑了");
}

eat(String name){
  return "${name}吃撑了";
}




see(var seeWho){
  seeWho("老六");
}

void main(){
  var car  = Car("安妮");
  car.who(run);

  see((var name)=>print(name));
}
class Car{
  var name;

  Car(this.name);

  who(var run){
    run(name);
  }
}

运行结果：
安妮跑了
老六

• 可变函数

eat(String name,{String? food,int? num}){
  return "${name}吃${food}撑了${num}份";
}

speak(String name,[String? and,String? chat]){
  return "${name}吃${and}${chat}";
}


void main(){
  print(eat("张三"));
  print(eat("张三",food: "麻辣烫",num: 10));
  print(speak("张三"));
  print(speak("张三","李四"));
  print(speak("张三","李四","聊天"));
}

运行结果：
张三吃null撑了null份
张三吃麻辣烫撑了10份
张三吃nullnull
张三吃李四null
张三吃李四聊天

• 扩展函数
  • 常用类型转换 / 时间处理 / 或者Widget的扩展（例如padding）
  • 多个扩展函数若已定义 不可重复定义 即函数名不可重复
  • dynamic 不可使用扩展函数

/**
 * 扩展函数
 * 格式：
 * extension 函数名 on 类
 *
 */
extension toNum on String{
  int toInt(){
    return int.parse(this);
  }

  double toDouble(){
    return double.parse(this);
  }
}


void main(){
  var a = "12".toInt();
  print(a);
  print(a.runtimeType);

  dynamic b = "12";
}

结果：
12
int

• 类
  • 默认构造函数
  • 命名构造函数 Student2.fromJson()

import 'dart:convert';


void main(){
   var student = Student("张三", "三年二班", 18, "2005-12-12 12:12:12");
   print("student>>${student}");
   var student1 = Student1("张三", "三年二班", 18, "2005-12-12 12:12:12");
   print("student1>>${student1}");
   var student11 = Student1.name("李四");  //命名构造函数
   print("student11>>${student11}");

   String jsonString = '{"name": "John","classGrade": "三年二班","birthday": "2005-12-12 12:12:12", "age": 30}';
   var student2 = Student2.fromJson(jsonString);
   print(student2.toString());
}

class Student{

  //不添加构造函数  这些属性 会报错  或者可以加上？
  String name,classGrade;
  int age;
  String birthday;

  Student(this.name, this.classGrade, this.age, this.birthday);

  Map<String,dynamic> toJson(){
    return {'name':name,'classGrade':classGrade,'age':age,'birthday':birthday};
  }

  @override
  String toString() {
    return 'Student{name: $name, classGrade: $classGrade, age: $age, birthday: $birthday}';
  }
}

class Student1{

  //不添加构造函数  这些属性 会报错  或者可以加上？
  String? name,classGrade;
  int? age;
  String? birthday;

  Student1(this.name, this.classGrade, this.age, this.birthday);

  Student1.name(this.name);



  @override
  String toString() {
    return 'Student1{name: $name, classGrade: $classGrade, age: $age, birthday: $birthday}';
  }
}

class Student2{
  String name = "",classGrade = "";
  int age = 0;
  String birthday = "";

//命名构造函数
  Student2.fromJson(String jsonStr){
    var json = jsonDecode(jsonStr);
    print(jsonStr);
    this.name = json['name'];
    this.classGrade = json['classGrade'];
    this.age = json['age'];
    this.birthday = json['birthday'];
  }

  @override
  String toString() {
    return 'Student2{name: $name, classGrade: $classGrade, age: $age, birthday: $birthday}';
  }
}

运行结果：
student>>Student{name: 张三, classGrade: 三年二班, age: 18, birthday: 2005-12-12 12:12:12}
student1>>Student1{name: 张三, classGrade: 三年二班, age: 18, birthday: 2005-12-12 12:12:12}
student11>>Student1{name: 李四, classGrade: null, age: null, birthday: null}
{"name": "John","classGrade": "三年二班","birthday": "2005-12-12 12:12:12", "age": 30}
Student2{name: John, classGrade: 三年二班, age: 30, birthday: 2005-12-12 12:12:12}

• 常量构造函数
  • 常量构造函数 const修饰构造函数
  • 常量构造函数用const 修饰时 属性值一样 则对象相等

import 'dart:convert';


class Student{
  String name;
  int age;

  Student(this.name, this.age);
}

class Student1{

  final String name;
  final int age;

  const Student1(this.name, this.age);
}

void main(){
  var student = Student("张三",18);
  var student0 = Student("张三",18);
  print(student.hashCode);
  print(student0.hashCode);
  print(student == student0);

  print("\n");

   var student1 = const Student1("张三",18);
   var student11 = const Student1("张三",18);
   var student12 = const Student1("张三",19);
   print(student1.hashCode);
   print(student11.hashCode);
   print(student12.hashCode);
   print(student1 == student11);  //true
   print(student12 == student11); //false

}

结果：
333142976
1033297191
false


87118132
87118132
590927718
true
false

• 工厂构造函数

import 'dart:convert';


class Student{
  String? name;
  int? age;

  Student.init({String name ="",int age = 0}){
    this.name = name;
    this.age = age;
  }

  static Student instance = Student.init();

  factory Student(){
     return instance;
  }

}


void main(){
  var student = Student.instance;
  var student1 = Student.instance;
  print(student.hashCode);
  print(student1.hashCode);
}

结果：
11925193
11925193

• 类
  • 默认属性方法修饰符是public
  • _下划线表示私有
  • 单继承 多实现
    

class A{
  void printA(){
    print("A");
  }
}

class B{
  void printA(){
    print("B");
  }
}

abstract class M{
  void printM(){
    print("M");
  }
}

abstract class N{
  void printN(){
    print("N");
  }
}

interface class X{
  void printX(){
    print("X");
  }
}

interface class Y{
  void printY(){
    print("Y");
  }
}
//单继承 多实现
class C extends A implements M,N,X,Y{
  @override
  void printM() {
    // TODO: implement printM
  }

  @override
  void printN() {
    // TODO: implement printN
  }

  @override
  void printX() {
    // TODO: implement printX
  }

  @override
  void printY() {
    // TODO: implement printY
  }

}

• 类的混入

mixin class P{
  void printP(){
    print("P");
  }
}

mixin class Q{
  void printQ(){
    print("Q");
  }
}

class D with P,Q{
  void printD(){
    print("D");
  }
}



void main(){
   var d = D();
   d.printD();
   d.printP();
   d.printQ();
}

结果：
D
P
Q