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
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 ${distanceInMeters}m.`);
}
}
class Dog extends Animal {
bark() {
console.log('Woof! Woof!');
}
}
const dog = new Dog();
dog.bark();
dog.move(10);
dog.bark();
class Animal {
name: string;
constructor(theName: string) { this.name = theName; }
move(distanceInMeters: number = 0) {
console.log(`${this.name} moved ${distanceInMeters}m.`);
}
}
class Snake extends Animal {
constructor(name: string) { super(name); }
move(distanceInMeters = 5) {
console.log("Slithering...");
super.move(distanceInMeters);
}
}
class Horse extends Animal {
constructor(name: string) { super(name); }
move(distanceInMeters = 45) {
console.log("Galloping...");
super.move(distanceInMeters);
}
}
let sam = new Snake("Sammy the Python");
let tom: Animal = new Horse("Tommy the Palomino");
sam.move();
tom.move(34);
- 公共,私有与受保护的修饰符
默认为:
public
public
:成员可见
private
:禁止从类外部访问成员,包括子类
protected
:禁止从类外部访问成员,不包括子类,但是子类调用super()并不能将此类成员实例化
class Animal {
public name: string;
public constructor(theName: string) { this.name = theName; }
public move(distanceInMeters: number) {
console.log(`${this.name} moved ${distanceInMeters}m.`);
}
}
class Animal {
private name: string;
constructor(theName: string) { this.name = theName; }
}
new Animal("Cat").name;
- readonly修饰符
只读属性必须在声明时或构造函数里被初始化
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";
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);
let grid2 = new Grid(5.0);
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;
}
class AccountingDepartment extends Department {
constructor() {
super('Accounting and Auditing');
}
printMeeting(): void {
console.log('The Accounting Department meets each Monday at 10am.');
}
generateReports(): void {
console.log('Generating accounting reports...');
}
}
let department: Department;
department = new Department();
department = new AccountingDepartment();
department.printName();
department.printMeeting();
department.generateReports();
- 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
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());
class Point {
x: number;
y: number;
}
interface Point3d extends Point {
z: number;
}
let point3d: Point3d = {x: 1, y: 2, z: 3};