ts中类的继承

定义类

class Person {
    name: string;   //属性
    constructor(_name: string) {
        this.name = _name;
    }   //构造函数
    sayHello(): string {
        return "Hi,everyone"
    }   //方法
}

let firstOne = new Person("Fred")   //实例化类

继承：继承使用关键字extends，调用父类使用super，子类继承父类的属性和方法，并且子类可以改写父类的属性和方法

class Animal {
    name: string;
    constructor(_name: string) {
        this.name = _name;
    }
    skinColour(color: string = "black"): void {
        console.log(`${this.name} skin colour is ${color}`)
    }
}

class Horse extends Animal {
    constructor(name: string) { super(name) }
    skinColour(color: string = "brown"): void {
        console.log(`I'am ${this.name}`);
        super.skinColour("brown");
    }
}

let horse = new Horse("horse");
horse.skinColour()
// I'am horse
// horse skin colour is brown

public、private、protected、readonly

• public（不声明默认都为public，也可以显示的设置为public）

  class Person {
      public name: string;   //属性
      public constructor(_name: string) {
          this.name = _name;
      }   //构造函数
      public sayHello(): string {
          return "Hi,everyone"
      }   //方法
  }
  
  let firstOne = new Person("Fred")   //实例化类

• private（private的成员不能被外部访问；比较带有private或protected成员的类型时，两个类型兼容的条件是private或protected的成员必须相同切来至同一个声明（同一个类））

  class Person {
      private name: string;   
      public constructor(_name: string) {
          this.name = _name;
      }   
  }
  
  class Employee {
      private name: string;   
      public constructor(_name: string) {
          this.name = _name;
      }   
  }
  
  let firstOne = new Person("Fred")
  console.log(firstOne.name)  //error: Property 'name' is private;
  let lastOne = new Employee("Fred")
  firstOne = lastOne  // error: Type 'Employee' is not assignable to type 'Person'.Types have separate declarations of a private property 'name'.

• protected（protected和private相似，但protected成员可以在派生类中访问（能被继承，但不能在实例中访问，若构造函数是protected，则不能被实例化，只能被继承））

  class Person {
      protected name: string;   
      protected constructor(_name: string) {
          this.name = _name;
      }   
  }
  
  class Employee extends Person {
      private department: string;   
      public constructor(name: string,department:string) {
          super(name);
          this.department = department;
      }   
  }
  
  let Bob = new Person;   //error: Constructor of class 'Person' is protected
  let fred = new Employee("fred","test");
  console.log(fred.name)  //error: Property 'name' is protected

• readonly（设置属性为只读，必须在声明时或构造函数里初始化）

  class Person {
      readonly name: string;   
      constructor(_name: string) {
          this.name = _name;
      }   
  }
  
  let fred = new Person("fred");
  fred.name = "Bob"   //error: Cannot assign to 'name' because it is a constant or a read-only property.

  参数属性（参数属性通过给构造函数参数添加一个访问限定符来声明（public,private,protected）,把声明和赋值合并至一处）

  class Person {
      constructor(private name: string) { }
      sayHello(): void {
          console.log(`my name is ${this.name}`)
      }
  }
  
  let fred = new Person("fred");
  fred.sayHello() //my name is fred

  存取器（get、set   只带有 get不带有set的存取器自动被推断为readonly）

  let passcode = "secret passcode";
  
  class Employee {
      private _fullName: string;
  
      get fullName(): string {
          return this._fullName;
      }
  
      set fullName(newName: string) {
          if (passcode && passcode == "secret passcode") {
              this._fullName = newName;
          }
          else {
              console.log("Error: Unauthorized update of employee!");
          }
      }
  }
  
  let employee = new Employee();
  employee.fullName = "Bob Smith";
  if (employee.fullName) {
      console.log(employee.fullName);
  }

  静态属性（static，不能被实例访问，在类里面访问时，需要加上类名）

  class Person {
      static height:number = 180;
      constructor(private name: string) { }
      sayHello(): void {
          console.log(`my name is ${this.name}, I height is ${Person.height}`)
      }
  }
  
  let fred = new Person("fred");
  fred.sayHello() //my name is fred, I height is 180

  抽象类（abstract，抽象类做为其它派生类的基类使用。 它们一般不会直接被实例化。抽象类中的抽象方法不包含具体实现并且必须在派生类中实现）

  abstract class Person {
      constructor(public name: string) { }
      abstract sayHello():void;
  }
  
  class Empoloy extends Person{
      constructor(){
          super("Fred")
      }
      sayHello(){
          console.log(`my name is ${this.name}`)
      }
  }
  
  let firstOne = new Empoloy();
  firstOne.sayHello();    //my name is Fred