定义

装饰器是一种特殊类型的声明，它能够被附加到类声明，方法，访问符，属性或参数上，是一种在不改变原类和使用继承的情况下，动态的扩展对象功能。
装饰器使用@expression形式，其中expression必须评估为一个函数，该函数将在运行时调用，并带有有关装饰声明的信息。
前置操作

// tsconfig.json
{"compilerOptions": {"target": "ES5","experimentalDecorators": true}
}

使用

类装饰

类装饰器会把Class A的构造函数传入你的watcher函数当做第一个参数

const watcher: ClassDecorator = (target) => {target.prototype.age = 18target.prototype.getName = ():string=>{return 'hello'}
}
@watcher
class A {constructor() {}
}
const a = new A()
console.log((a as any).age);  // 18
console.log((a as any).getName());  //hello

装饰器工厂

如果想要传递参数，使装饰器变成类似工厂函数，只需要在装饰器函数内部再返回一个函数即可。

const watcher = (name: string): ClassDecorator => {return (target) => {target.prototype.age = 18target.prototype.getName = (): string => {return name}}}
@watcher('hello data')
class A {constructor() {}
}
const a = new A()
console.log((a as any).age); // 18
console.log((a as any).getName()); // hello data

装饰器组合

可以给一个类使用多个装饰器

const watcher = (name: string): ClassDecorator => {return (target) => {target.prototype.age = 18target.prototype.getName = (): string => {return name}}}const addAge = (age:number):ClassDecorator => {return (target) =>{target.prototype.addAge = (): number => {return age+1}}
}@watcher('hello data')  @addAge(20)
class A {constructor() {}
}
const a = new A()
console.log((a as any).age);
console.log((a as any).getName());
console.log((a as any).addAge());

方法/属性装饰器

装饰器参数为：

• target:对象的原型:对于静态成员来说是类的构造函数，对于实例成员是类的原型对象
• propertyKey：方法的名称
• descriptor：方法的属性描述符
  这三个属性实际就是Object.defineProperty的三个参数，如果是类的属性，则没有传递第三个参数

const met: MethodDecorator = (...args) => {console.log(args)
}class A {constructor() {}@metgetName(): string {return 'hello'}
}const xx = new A()

// 声明装饰器修饰方法/属性
function method(target: any, propertyKey: string, descriptor: PropertyDescriptor) {descriptor.writable = false;
};function property(target: any, propertyKey: string) {// 修改属性target[propertyKey] = '11223'
}class Person {@propertyname: string;constructor() {}@methodsay() {return 'instance method';}
}const xmz = new Person();// 修改实例方法say
xmz.say = () => {return '5566'   //由于 descriptor.writable = false; 所以此处修改不生效
}
console.log(xmz.name)
console.log(xmz.say())  // nstance method

参数装饰

装饰器参数为：

• target:当前对象的原型
• propertyKey：参数的名称
• index：参数数组中的位置

const init: ParameterDecorator = (...args)=>{console.log(args)
}
class A {constructor() {}getName(@init name: string = 'hello') {console.log(name) // [ {}, 'getName', 0 ]}
}

元数据

import data from './data.json'
import 'reflect-metadata'const Base = (base: string) => {return (target) => {target.prototype.base = base}
}const GetData = (url: string) => {console.log('getData run')const fn: MethodDecorator = (target: any, key, descriptor: PropertyDescriptor) => {const prop = Reflect.getMetadata('key', target)console.log(prop, 'prop')descriptor.value(prop ? data[prop] : data)}return fn;
}const Result = () => {console.log('result run')const fn: ParameterDecorator = (target: any, key, index) => {Reflect.defineMetadata('key', 'data', target)}return fn
}@Base('/api')
class Http {fileName: stringconstructor(name) {this.fileName = name}@GetData('json')getList(@Result() data: any) {console.log(data)}
}
const http = new Http('orange')