TypeScript 自定义装饰器

（预测未来最好的方法就是把它创造出来——尼葛洛庞帝）

装饰器

装饰器一种更现代的代码模式，通过使用@的形式注入在属性，寄存器，方法，方法参数和类中，比如在Angular，Nestjs和midway等流行框架中也都用到了装饰器。
官方说明
对装饰器更详细的相关说明

tc39提案

tc39提案一共分为五个阶段

Stage 0 - 设想（Strawman）：只是一个想法，可能有 Babel插件。
Stage 1 - 建议（Proposal）：这是值得跟进的。
Stage 2 - 草案（Draft）：初始规范。
Stage 3 - 候选（Candidate）：完成规范并在浏览器上初步实现。
Stage 4 - 完成（Finished）：将添加到下一个年度版本发布中。

装饰器目前已经在阶段3，相信不久的将来，js中也会支持装饰器。在typescript5.0中已经完全支持了装饰器。
该提案进度更详细的相关说明

装饰器实践

tsconfig.json

{
    
    
  "compilerOptions": {
    
    
    "target": "ES6", // 为发出的JavaScript设置JavaScript语言版本，并包含兼容的库声明
    "experimentalDecorators": true, // 启用对遗留实验装饰器的实验支持
    "module": "ES6", // 指定生成的模块代码
    "esModuleInterop": true, // 发出额外的JavaScript以简化对导入CommonJS模块的支持。这为类型兼容性启用了“allowSyntheticDefaultImports”
    "moduleResolution": "node", // 指定TypeScript如何从给定的模块说明符中查找文件
    "outDir": "dist", // 为所有发出的文件指定输出文件夹
    "rootDir": "src", // 指定源文件中的根文件夹
  },
  "include": [ // 需要编译的目录文件
    "src/**/*",
  ],
  "exclude": [ // 需要排除的目录文件
    "node_modules"
  ]
}

官方更详细的配置说明

类装饰器

typescript

// 通过装饰器对类进行扩展，减少对代码侵入性和业务间的耦合性
// constructor参数为类的构造函数
const classExtends = () => {
    
    
  const ClassDecorator = (constructor: Function) => {
    
    
    console.log('ClassDecorator---');
    // 扩展类属性
    constructor.prototype.name = 'ClassDecoratorName';
    // 扩展类方法
    constructor.prototype.run = () => {
    
    
      console.log('ClassDecorator run test');
    }
  };
  interface Test {
    
    
    name: string;
    run (): void;
  }

  @ClassDecorator
  class Test {
    
    
  }
  new Test().run();
  const obj = {
    
     name: 'adsa' };
  Reflect.get(obj, 'name');
};
classExtends();

// 通过装饰器入参的形式对类进行扩展，使用参数可以对业务进行更强的定制化处理
const classExtendByParams = () => {
    
    
  const ClassDecorator = (param: string) => {
    
    
    return function (constructor: Function) {
    
    
      // 针对入参做不同的处理
      constructor.prototype.run = () => {
    
    
        if (param === 'agent') {
    
    
          console.log('classExtendByParams run agent');
        } else if (param === 'user') {
    
    
          console.log('classExtendByParams run user');
        }
      };
    }
  };
  interface Test {
    
    
    name: string;
    run (): void;
  }
  @ClassDecorator('agent')
  class Test {
    
    
  }
  new Test().run();
};
classExtendByParams();

// 通过装饰器工厂方法进行扩展，工厂方法装饰器可以和类型更好的兼容
const classExtendOfFactory = () => {
    
    
  const ClassDecorator = (param: string) => {
    
    
    return function <T extends new (...args: any[]) => any> (constructor: T) {
    
    
      return class extends constructor {
    
    
        run () {
    
    
          if (param === 'agent') {
    
    
            console.log('classExtendOfFactory run agent');
          } else if (param === 'user') {
    
    
            console.log('classExtendOfFactory run user');
          }
        };
      }
    }
  };
  const Test = ClassDecorator('user')(
    class {
    
     }
  );
  new Test().run();
};
classExtendOfFactory();

javascript

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    
    
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
// 通过装饰器对类进行扩展，减少对代码侵入性和业务间的耦合性
// constructor参数为类的构造函数
const classExtends = () => {
    
    
    const ClassDecorator = (constructor) => {
    
    
        console.log('ClassDecorator---');
        // 扩展类属性
        constructor.prototype.name = 'ClassDecoratorName';
        // 扩展类方法
        constructor.prototype.run = () => {
    
    
            console.log('ClassDecorator run test');
        };
    };
    let Test = class Test {
    
    
    };
    Test = __decorate([
        ClassDecorator
    ], Test);
    new Test().run();
    const obj = {
    
     name: 'adsa' };
    Reflect.get(obj, 'name');
};
classExtends();
// 通过装饰器入参的形式对类进行扩展，使用参数可以对业务进行更强的定制化处理
const classExtendByParams = () => {
    
    
    const ClassDecorator = (param) => {
    
    
        return function (constructor) {
    
    
            // 针对入参做不同的处理
            constructor.prototype.run = () => {
    
    
                if (param === 'agent') {
    
    
                    console.log('classExtendByParams run agent');
                }
                else if (param === 'user') {
    
    
                    console.log('classExtendByParams run user');
                }
            };
        };
    };
    let Test = class Test {
    
    
    };
    Test = __decorate([
        ClassDecorator('agent')
    ], Test);
    new Test().run();
};
classExtendByParams();
// 通过装饰器工厂方法进行扩展，工厂方法装饰器可以和类型更好的兼容
const classExtendOfFactory = () => {
    
    
    const ClassDecorator = (param) => {
    
    
        return function (constructor) {
    
    
            return class extends constructor {
    
    
                run() {
    
    
                    if (param === 'agent') {
    
    
                        console.log('classExtendOfFactory run agent');
                    }
                    else if (param === 'user') {
    
    
                        console.log('classExtendOfFactory run user');
                    }
                }
                ;
            };
        };
    };
    const Test = ClassDecorator('user')(class {
    
    
    });
    new Test().run();
};
classExtendOfFactory();

方法装饰器

typescript

/**
 * 入参解释
 * target: 对于静态成员来说是类的构造函数，对于实例成员来说是类的原型对象
 * propertyKey: 属性的名称
 * descriptor: 属性的描述器
 */
/**
 * PropertyDescriptor参数解释
 * PropertyDescriptor的参数各项为js的属性描述符，在创建变量或方法等对象时，js会默认赋予这些描述符
 * 详细的阅读https://www.tektutorialshub.com/javascript/javascript-property-descriptors-enumerable-writable-configurable/
 * descriptor 参数
 * value 方法自身
 * writable 该方法是否可以被变更
 * enumerable 是否可以被枚举
 * configurable 决定是否可配置，如果为false，则value，writable，enumerable不能被修改
 */

// 通过装饰器对方法进行扩展，减少对代码侵入性和业务间的耦合性
const methodExtends = () => {
    
    
  const MethodDecorator = (target: any, propertyKey: string, descriptor: PropertyDescriptor) => {
    
    
    // 获取方法本身
    const method = descriptor.value;
    // 对该方法的生命周期进行操作
    descriptor.value = (...args) => {
    
    
      console.log('MethodDecorator before run');
      const data = method.call(this, args);
      console.log('MethodDecorator after run');
      return data;
    };
  };
  class Test {
    
    

    @MethodDecorator
    method () {
    
    
      return ';;;;';
    }

  }
  console.log(new Test().method());
};
methodExtends();

// 通过装饰入参的形式对方法进行扩展，使用参数可以对业务进行更强的定制化处理
const methodExtendsByParams = () => {
    
    
  const MethodDecorator = (param: string) => {
    
    
    console.log('methodExtendsByParams', param);
    return (target: any, propertyKey: string, descriptor: PropertyDescriptor) => {
    
    
      // 获取方法本身
      const method = descriptor.value;
      // 对该方法的生命周期进行操作
      descriptor.value = (...args) => {
    
    
        console.log('before run');
        const data = method.call(this, args);
        console.log('after run');
        return data;
      };
    }
  };
  class Test {
    
    

    @MethodDecorator('user')
    method () {
    
    
      return ';;;;';
    }

  }
  console.log(new Test().method());
};
methodExtendsByParams();

javascript

/**
 * 入参解释
 * target: 对于静态成员来说是类的构造函数，对于实例成员来说是类的原型对象
 * propertyKey: 属性的名称
 * descriptor: 属性的描述器
 */
/**
 * PropertyDescriptor参数解释
 * PropertyDescriptor的参数各项为js的属性描述符，在创建变量或方法等对象时，js会默认赋予这些描述符
 * 详细的阅读https://www.tektutorialshub.com/javascript/javascript-property-descriptors-enumerable-writable-configurable/
 * descriptor 参数
 * value 方法自身
 * writable 该方法是否可以被变更
 * enumerable 是否可以被枚举
 * configurable 决定是否可配置，如果为false，则value，writable，enumerable不能被修改
 */
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    
    
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
// 通过装饰器对方法进行扩展，减少对代码侵入性和业务间的耦合性
const methodExtends = () => {
    
    
    const MethodDecorator = (target, propertyKey, descriptor) => {
    
    
        // 获取方法本身
        const method = descriptor.value;
        // 对该方法的生命周期进行操作
        descriptor.value = (...args) => {
    
    
            console.log('MethodDecorator before run');
            const data = method.call(this, args);
            console.log('MethodDecorator after run');
            return data;
        };
    };
    class Test {
    
    
        method() {
    
    
            return ';;;;';
        }
    }
    __decorate([
        MethodDecorator
    ], Test.prototype, "method", null);
    console.log(new Test().method());
};
methodExtends();
// 通过装饰入参的形式对方法进行扩展，使用参数可以对业务进行更强的定制化处理
const methodExtendsByParams = () => {
    
    
    const MethodDecorator = (param) => {
    
    
        console.log('methodExtendsByParams', param);
        return (target, propertyKey, descriptor) => {
    
    
            // 获取方法本身
            const method = descriptor.value;
            // 对该方法的生命周期进行操作
            descriptor.value = (...args) => {
    
    
                console.log('before run');
                const data = method.call(this, args);
                console.log('after run');
                return data;
            };
        };
    };
    class Test {
    
    
        method() {
    
    
            return ';;;;';
        }
    }
    __decorate([
        MethodDecorator('user')
    ], Test.prototype, "method", null);
    console.log(new Test().method());
};
methodExtendsByParams();

方法参数装饰器

typescript

// 通过装饰器对方法中的属性进行扩展
/**
 * target 实例自身
 * methodName 方法名
 * paramIndex 参数的下标位置
 */
const methodParamExtends = () => {
    
    
  const methodPramDecorator = (param: string) => {
    
    
    return (target: any, methodName: string, paramIndex: any) => {
    
    
      console.log('target', target, methodName, paramIndex);
      target.decoratorData = '222';
    }
  };
  class Test {
    
    
    decoratorData!: string;
    init (@methodPramDecorator('agent') type: string) {
    
    
      return type;
    }
  }
  const test = new Test();
  const data = test.init('20230611');
  console.log(data, test.decoratorData);
  return data;
};
methodParamExtends();

javascript

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    
    
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __param = (this && this.__param) || function (paramIndex, decorator) {
    
    
    return function (target, key) {
    
     decorator(target, key, paramIndex); }
};
// 通过装饰器对方法中的属性进行扩展
/**
 * target 实例自身
 * methodName 方法名
 * paramIndex 参数的下标位置
 */
const methodParamExtends = () => {
    
    
    const methodPramDecorator = (param) => {
    
    
        return (target, methodName, paramIndex) => {
    
    
            console.log('target', target, methodName, paramIndex);
            target.decoratorData = '222';
        };
    };
    class Test {
    
    
        init(type) {
    
    
            return type;
        }
    }
    __decorate([
        __param(0, methodPramDecorator('agent'))
    ], Test.prototype, "init", null);
    const test = new Test();
    const data = test.init('20230611');
    console.log(data, test.decoratorData);
    return data;
};
methodParamExtends();

寄存器装饰器

typescript

// 通过装饰器对寄存器进行扩展
/**
 * target 实例自身
 * propertyKey 属性key
 * descriptor 属性描述符
 */
const setgetExtends = () => {
    
    
  const SetGetDecorator = (param: string) => {
    
    
    return (target: any, propertyKey: string, descriptor: PropertyDescriptor) => {
    
    
      const set = descriptor.set;
      const get = descriptor.get;
      descriptor.set = function (value) {
    
    
        console.log('GetDecorator before run', value);
        if (typeof value === 'string') {
    
    
          value += ` set decrator ${
      
      param}`;
        }
        set.call(this, value);
        console.log('GetDecorator after run', value);
      };
      descriptor.get = function () {
    
    
        console.log('GetDecorator before run', target);
        let data = get.call(this);
        console.log('GetDecorator after run');
        if (typeof data === 'string') {
    
    
          data += ` get decrator ${
      
      param}`;
        }
        return data;
      }
    }
  };

  class Test {
    
    

    #name: string;

    @SetGetDecorator('custom setget')
    set name (name: string) {
    
    
      this.#name = name;
    }
    get name () {
    
    
      return this.#name;
    }

  }
  const user = new Test();
  user.name = 'user';
  console.log('setgetExtends', user.name);
};
setgetExtends();

javascript

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    
    
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __classPrivateFieldSet = (this && this.__classPrivateFieldSet) || function (receiver, privateMap, value) {
    
    
    if (!privateMap.has(receiver)) {
    
    
        throw new TypeError("attempted to set private field on non-instance");
    }
    privateMap.set(receiver, value);
    return value;
};
var __classPrivateFieldGet = (this && this.__classPrivateFieldGet) || function (receiver, privateMap) {
    
    
    if (!privateMap.has(receiver)) {
    
    
        throw new TypeError("attempted to get private field on non-instance");
    }
    return privateMap.get(receiver);
};
// 通过装饰器对寄存器进行扩展
/**
 * target 实例自身
 * propertyKey 属性key
 * descriptor 属性描述符
 */
const setgetExtends = () => {
    
    
    var _name;
    const SetGetDecorator = (param) => {
    
    
        return (target, propertyKey, descriptor) => {
    
    
            const set = descriptor.set;
            const get = descriptor.get;
            descriptor.set = function (value) {
    
    
                console.log('GetDecorator before run', value);
                if (typeof value === 'string') {
    
    
                    value += ` set decrator ${
      
      param}`;
                }
                set.call(this, value);
                console.log('GetDecorator after run', value);
            };
            descriptor.get = function () {
    
    
                console.log('GetDecorator before run', target);
                let data = get.call(this);
                console.log('GetDecorator after run');
                if (typeof data === 'string') {
    
    
                    data += ` get decrator ${
      
      param}`;
                }
                return data;
            };
        };
    };
    class Test {
    
    
        constructor() {
    
    
            _name.set(this, void 0);
        }
        set name(name) {
    
    
            __classPrivateFieldSet(this, _name, name);
        }
        get name() {
    
    
            return __classPrivateFieldGet(this, _name);
        }
    }
    _name = new WeakMap();
    __decorate([
        SetGetDecorator('custom setget')
    ], Test.prototype, "name", null);
    const user = new Test();
    user.name = 'user';
    console.log('setgetExtends', user.name);
};
setgetExtends();

属性装饰器

typescript

// 通过属性装饰器对属性进行扩展
const paramExtends = () => {
    
    
  const ParamDecorator = (param: string) => {
    
    
    return function (target: any, key: any) {
    
    
      target[key] = param;
      console.log(`init param ${
      
      key} to ${
      
      param}`);
    }
  }
  class Test {
    
    
    @ParamDecorator('www.baidu.com')
    private url!: string;
    getName () {
    
    
      return this.url;
    }
  }
  const test = new Test();
  const data = test.getName();
  console.log('data', data);
};
paramExtends();

javascript

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    
    
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
// 通过属性装饰器对属性进行扩展
const paramExtends = () => {
    
    
    const ParamDecorator = (param) => {
    
    
        return function (target, key) {
    
    
            target[key] = param;
            console.log(`init param ${
      
      key} to ${
      
      param}`);
        };
    };
    class Test {
    
    
        getName() {
    
    
            return this.url;
        }
    }
    __decorate([
        ParamDecorator('www.baidu.com')
    ], Test.prototype, "url", void 0);
    const test = new Test();
    const data = test.getName();
    console.log('data', data);
};
paramExtends();

装饰器执行顺序

示例代码

typescript

// 不同装饰器的执行顺序

const CustomClassDecorator = (constructor: Function) => {
    
    
  console.log('类装饰器');
};
const CustomMethodDecorator = (target: any, propertyKey: string, descriptor: PropertyDescriptor) => {
    
    
  console.log('方法装饰器');
};
const CustomMethodParamDecorator = (target: any, methodName: string, paramIndex: any) => {
    
    
  console.log('方法参数装饰器');
}
const CustomParamDecorator = (target: any, key: any) => {
    
    
  console.log(`参数装饰器`);
}
const CustomSetGetDecorator = (target: any, propertyKey: string, descriptor: PropertyDescriptor) => {
    
    
  console.log('寄存器装饰器');
}

@CustomClassDecorator
class Test {
    
    

  @CustomParamDecorator
  sex!: string;

  #name !: string;

  @CustomSetGetDecorator
  set name (name: string) {
    
    
    this.#name = name;
  }
  get name () {
    
    
    return this.#name
  }

  @CustomMethodDecorator
  handleName (@CustomMethodParamDecorator prefix: string) {
    
    
    return prefix + this.name;
  }

}
const instance = new Test();
const data = instance.handleName('prefix');
console.log(data);

javascript

// 不同装饰器的执行顺序
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    
    
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __param = (this && this.__param) || function (paramIndex, decorator) {
    
    
    return function (target, key) {
    
     decorator(target, key, paramIndex); }
};
var __classPrivateFieldSet = (this && this.__classPrivateFieldSet) || function (receiver, privateMap, value) {
    
    
    if (!privateMap.has(receiver)) {
    
    
        throw new TypeError("attempted to set private field on non-instance");
    }
    privateMap.set(receiver, value);
    return value;
};
var __classPrivateFieldGet = (this && this.__classPrivateFieldGet) || function (receiver, privateMap) {
    
    
    if (!privateMap.has(receiver)) {
    
    
        throw new TypeError("attempted to get private field on non-instance");
    }
    return privateMap.get(receiver);
};
var _name;
const CustomClassDecorator = (constructor) => {
    
    
    console.log('类装饰器');
};
const CustomMethodDecorator = (target, propertyKey, descriptor) => {
    
    
    console.log('方法装饰器');
};
const CustomMethodParamDecorator = (target, methodName, paramIndex) => {
    
    
    console.log('方法参数装饰器');
};
const CustomParamDecorator = (target, key) => {
    
    
    console.log(`参数装饰器`);
};
const CustomSetGetDecorator = (target, propertyKey, descriptor) => {
    
    
    console.log('寄存器装饰器');
};
let Test = class Test {
    
    
    constructor() {
    
    
        _name.set(this, void 0);
    }
    set name(name) {
    
    
        __classPrivateFieldSet(this, _name, name);
    }
    get name() {
    
    
        return __classPrivateFieldGet(this, _name);
    }
    handleName(prefix) {
    
    
        return prefix + this.name;
    }
};
_name = new WeakMap();
__decorate([
    CustomParamDecorator
], Test.prototype, "sex", void 0);
__decorate([
    CustomSetGetDecorator
], Test.prototype, "name", null);
__decorate([
    CustomMethodDecorator,
    __param(0, CustomMethodParamDecorator)
], Test.prototype, "handleName", null);
Test = __decorate([
    CustomClassDecorator
], Test);
const instance = new Test();
const data = instance.handleName('prefix');
console.log(data);

执行结果

参数装饰器
寄存器装饰器
方法参数装饰器
方法装饰器
类装饰器
prefixundefined

装饰器原理

以下代码是重写过的，方便理解和回顾。由于Reflect.decorate装饰器反射机制还不支持，且相关资料较少，所以在本文中不进行深入研究。

/**
 * Test = __decorate([ClassDecrator], Test)
 * decorators 装饰器列表
 * target 类实例
 * key 属性名称
 * desc 变量属性描述符
 */
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    
    
  /**
   * 获取请求参数，咱本示例中，请求参数为2
   * Test = __decorate([ClassDecrator], Test)
   */
  var c = arguments.length
  // 初始化r变量
  var r = null;
  // 如果请求参数小于三个，在本示例中满足
  if (c < 3) {
    
    
    // 将类实例赋予r，也就是将Test赋给r
    r = target;
  } else {
    
    
    // 如果属性描述符为空
    if (desc === null) {
    
    
      // 返回指定属性名的描述符
      desc = Object.getOwnPropertyDescriptor(target, key);
      r = desc;
    } else {
    
    
      // 如果存在描述符，则直接赋予r
      r = desc;
    }
  }
  // 由此可见，在类装饰器下，r为类实例本身，在方法等装饰器下，r为属性描述符

  var d;
  // 是否支持es6的Reflect，暂时跳过，文章后面单独会将
  if (typeof Reflect === "object" && typeof Reflect.decorate === "function") {
    
    
    r = Reflect.decorate(decorators, target, key, desc)
  }
  // 如果不支持es6的Reflect，则向下执行
  else {
    
    
    // 在这里倒叙循环执行每一个装饰器，由此看出ts装饰器的执行顺序
    for (var i = decorators.length - 1; i >= 0; i--) {
    
    
      d = decorators[i];
      if (d) {
    
    
        var temp = null;
        if (c < 3) {
    
    
          temp = d(r);
        } else {
    
    
          if (c > 3) {
    
    
            temp = d(target, key, r);
          } else {
    
    
            temp = d(target, key);
          }
        }
        if (temp) {
    
    
          r = temp;
        }
      }
    }
  }
  // 如果参数大于3个，则将属性名和属性描述符赋予该实例
  if (c > 3 && r) {
    
    
    Object.defineProperty(target, key, r);
  }
  // 返回该实例实例或属性描述符
  return r;
};