Typescript类

1. 概念预览

• 产生背景: 传统的JavaScript程序是使用函数和基于原型的继承来创建可重用的组件的,但是对于熟悉使用面向对象的开发者来说并不是那么友好,因为他们是基于类的继承,并且对象是由类构建出来的.
• 发展: 于是从ECMAScript 2015开始,JavaScript支持使用基于类的面向对象方式.但是在typescript无需依赖于es6中的类,可以直接使用这些特性.

2. 代码示例

• 基本使用

class Greeter {
    greeting: string;
    constructor(message: string) {
        this.greeting = message;
    }
    greet() {
        return "Hello, " + this.greeting;
    }
}

let greeter = new Greeter("world");

• 继承 通过extends可以继承基类当中的属性和方法 我们通常将派生类称作子类,将基类称作超类 注意:如果超类中定义了构造函数,则子类必须在构造函数中调用super函数,这是一个强制的规定.

class Animal {
    move(distanceInMeters: number = 0) {
        console.log(`Animal moved ${distanceInMeters}m.`);
    }
}

class Dog extends Animal {
    bark() {
        console.log('Woof! Woof!');
    }
}

const dog = new Dog();
dog.bark();
dog.move(10);
dog.bark();

// 在这个例子中我们需要注意的是:如果超类中含有构造函数子类中必须调用super(),这是ts的一个规定
class Animal {
    name: string;
    constructor(theName: string) { this.name = theName; }
    move(distanceInMeters: number = 0) {
        console.log(`${this.name} moved ${distanceInMeters}m.`);
    }
}

class Snake extends Animal {
    constructor(name: string) { super(name); }
    move(distanceInMeters = 5) {
        console.log("Slithering...");
        super.move(distanceInMeters);
    }
}

class Horse extends Animal {
    constructor(name: string) { super(name); }
    move(distanceInMeters = 45) {
        console.log("Galloping...");
        super.move(distanceInMeters);
    }
}

let sam = new Snake("Sammy the Python");
let tom: Animal = new Horse("Tommy the Palomino");

sam.move();
tom.move(34);

• 公共,私有与受保护的修饰符 默认为:public public:成员可见 private:禁止从类外部访问成员,包括子类 protected:禁止从类外部访问成员,不包括子类,但是子类调用super()并不能将此类成员实例化

// 我们可以明确的将成员标记为public(公共)
class Animal {
    public name: string;
    public constructor(theName: string) { this.name = theName; }
    public move(distanceInMeters: number) {
        console.log(`${this.name} moved ${distanceInMeters}m.`);
    }
}
// private
class Animal {
    private name: string;
    constructor(theName: string) { this.name = theName; }
}
new Animal("Cat").name; // 错误: 'name' 是私有的.

• readonly修饰符 只读属性必须在声明时或构造函数里被初始化

// 错误示范: 这里表示在构造函数中赋值,并不是初始化
class Octopus {
    readonly name: string;
    readonly numberOfLegs: number = 8;
    constructor (theName: string) {
        this.name = theName;
    }
}
let dad = new Octopus("Man with the 8 strong legs");
dad.name = "Man with the 3-piece suit"; // 错误! name 是只读的.

// 正确示范初始化
class Octopus {
    readonly numberOfLegs: number = 8;
    constructor(readonly name: string) {
    }
}

• 存取器 TypeScript支持通过getters/setters来劫持对象成员的访问

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) {
    alert(employee.fullName);
}

• 静态属性 实例成员只有类为实例化的时候才能被初始化 静态成员可以直接访问类的构造器

class Grid {
    static origin = {x: 0, y: 0};
    calculateDistanceFromOrigin(point: {x: number; y: number;}) {
        let xDist = (point.x - Grid.origin.x);
        let yDist = (point.y - Grid.origin.y);
        return Math.sqrt(xDist * xDist + yDist * yDist) / this.scale;
    }
    constructor (public scale: number) { }
}

let grid1 = new Grid(1.0);  // 1x scale
let grid2 = new Grid(5.0);  // 5x scale

console.log(grid1.calculateDistanceFromOrigin({x: 10, y: 10}));
console.log(grid2.calculateDistanceFromOrigin({x: 10, y: 10}));

• 抽象类 抽象类:仅作为派生类的基类使用,一般不会将其直接实例化 如果我们引用抽象类作为变量的类型时,该变量必须完全遵守抽象类的约束 不同于接口的是,抽象类可以包含成员的实现细节 抽象方法:必须在子类中实现 abstract关键字是用于定义抽象类和在抽象类内部定义抽象方法的

abstract class Department {

    constructor(public name: string) {
    }

    printName(): void {
        console.log('Department name: ' + this.name);
    }

    abstract printMeeting(): void; // 必须在派生类中实现
}

class AccountingDepartment extends Department {

    constructor() {
        super('Accounting and Auditing'); // 在派生类的构造函数中必须调用 super()
    }

    printMeeting(): void {
        console.log('The Accounting Department meets each Monday at 10am.');
    }

    generateReports(): void {
        console.log('Generating accounting reports...');
    }
}

let department: Department; // 允许创建一个对抽象类型的引用
department = new Department(); // 错误: 不能创建一个抽象类的实例
department = new AccountingDepartment(); // 允许对一个抽象子类进行实例化和赋值
department.printName();
department.printMeeting();
department.generateReports(); // 错误: 方法在声明的抽象类中不存在

• 高级技巧 当你再TypeScript里声明了一个类的时候,实际上同时声明了很多东西 (1)首先,类就是实例的类型 (2)把类当做接口使用

/*(1)首先,类就是实例的类型*/
class Greeter {
    greeting: string;
    constructor(message: string) {
        this.greeting = message;
    }
    greet() {
        return "Hello, " + this.greeting;
    }
}

let greeter: Greeter;
greeter = new Greeter("world");
console.log(greeter.greet());
/*(2)把类当做接口使用*/
class Point {
    x: number;
    y: number;
}

interface Point3d extends Point {
    z: number;
}

let point3d: Point3d = {x: 1, y: 2, z: 3};