装饰器

介绍

随着 TypeScript 和 ES6 中类的引入，现在存在某些需要附加功能来支持注释或修改类和类成员的场景。装饰器提供了一种为类声明和成员添加注释和元编程语法的方法。装饰器是 JavaScript 的第 2 阶段提案，可作为 TypeScript 的实验性功能使用。

注意装饰器是一项实验性功能，可能会在未来的版本中发生变化。

要启用对装饰器的实验性支持，您必须在命令行或 tsconfig.json 中启用 experimentalDecorators 编译器选项：

命令行：

tsc --target ES5 --experimentalDecorators

tsconfig.json：

{
  "compilerOptions": {
    "target": "ES5",
    "experimentalDecorators": true
  }
}

装饰器

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function sealed(target) {
  // do something with 'target' ...
}

装饰器工厂

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function color(value: string) {
  // this is the decorator factory, it sets up
  // the returned decorator function
  return function (target) {
    // this is the decorator
    // do something with 'target' and 'value'...
  };
}

装饰器组成

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function f() {}
function g() {}
@f @g x

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function f() {}
function g() {}
@f
@g
x

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每个装饰器的表达式都是从上到下计算的。
然后将结果作为函数从下到上调用。

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function first() {
  console.log("first(): factory evaluated");
  return function (target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    console.log("first(): called");
  };
}

function second() {
  console.log("second(): factory evaluated");
  return function (target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    console.log("second(): called");
  };
}

class ExampleClass {
  @first()
  @second()
  method() {}
}

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first(): factory evaluated
second(): factory evaluated
second(): called
first(): called

装饰器评价

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参数装饰器，后跟方法、访问器或属性装饰器应用于每个实例成员。
参数装饰器，后跟方法、访问器或属性装饰器应用于每个静态成员。
参数装饰器应用于构造函数。
类装饰器应用于类。

类装饰器

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function sealed(constructor: Function) {
  Object.seal(constructor);
  Object.seal(constructor.prototype);
}
@sealed
class BugReport {
  type = "report";
  title: string;

  constructor(t: string) {
    this.title = t;
  }
}

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function sealed(constructor: Function) {
  Object.seal(constructor);
  Object.seal(constructor.prototype);
}

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function reportableClassDecorator<T extends { new (...args: any[]): {} }>(constructor: T) {
  return class extends constructor {
    reportingURL = "http://www...";
  };
}

@reportableClassDecorator
class BugReport {
  type = "report";
  title: string;

  constructor(t: string) {
    this.title = t;
  }
}

const bug = new BugReport("Needs dark mode");
console.log(bug.title); // Prints "Needs dark mode"
console.log(bug.type); // Prints "report"

// Note that the decorator _does not_ change the TypeScript type
// and so the new property `reportingURL` is not known
// to the type system:
bug.reportingURL;

方法装饰器

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静态成员的类的构造函数，或者实例成员的类的原型。
成员的姓名。
成员的属性描述符。

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function enumerable(value: boolean) {
  return function (target: any,propertyKey: string,descriptor: PropertyDescriptor) {
    descriptor.enumerable = value;
  };
}
class Greeter {
  greeting: string;
  constructor(message: string) {
    this.greeting = message;
  }

  @enumerable(false)
  greet() {
    return "Hello, " + this.greeting;
  }
}

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function enumerable(value: boolean) {
  return function (target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    descriptor.enumerable = value;
  };
}

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访问器装饰器

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静态成员的类的构造函数，或者实例成员的类的原型。
成员的姓名。
成员的属性描述符。

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function configurable(value: boolean) {
  return function (
    target: any,
    propertyKey: string,
    descriptor: PropertyDescriptor
  ) {
    descriptor.configurable = value;
  };
}
class Point {
  private _x: number;
  private _y: number;
  constructor(x: number, y: number) {
    this._x = x;
    this._y = y;
  }

  @configurable(false)
  get x() {
    return this._x;
  }

  @configurable(false)
  get y() {
    return this._y;
  }
}

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function configurable(value: boolean) {
  return function (target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    descriptor.configurable = value;
  };
}

属性装饰器

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静态成员的类的构造函数，或者实例成员的类的原型。
成员的姓名。

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class Greeter {
  @format("Hello, %s")
  greeting: string;

  constructor(message: string) {
    this.greeting = message;
  }

  greet() {
    let formatString = getFormat(this, "greeting");
    return formatString.replace("%s", this.greeting);
  }
}

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import "reflect-metadata";

const formatMetadataKey = Symbol("format");

function format(formatString: string) {
  return Reflect.metadata(formatMetadataKey, formatString);
}

function getFormat(target: any, propertyKey: string) {
  return Reflect.getMetadata(formatMetadataKey, target, propertyKey);
}

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参数装饰器

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X+b6P3ujrZj16kV5W22oxjI9FuaTNzy/NDKLLGxnZ6EDcdm2GUblOaPlLjbqGVqWEGd7vekhxYKDjBPfqcGnDsgmtJvsWlJcMqdDxVooAr+Qzo76q5fxoj99PwYE9jFb

静态成员的类的构造函数，或者实例成员的类的原型。
成员的姓名。
函数参数列表中参数的序号索引。

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X+b6P3ujrZj16kV5W22oxgLtQmXmWNv51NCQdNlU8j9Hm20Kq/neZC5rFayNLj6o

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function validate(target: any, propertyName: string, descriptor: TypedPropertyDescriptor<any>) {}
function required(target: Object, propertyKey: string | symbol, parameterIndex: number) {}
class BugReport {
  type = "report";
  title: string;

  constructor(t: string) {
    this.title = t;
  }

  @validate
  print(@required verbose: boolean) {
    if (verbose) {
      return `type: ${this.type}\ntitle: ${this.title}`;
    } else {
     return this.title; 
    }
  }
}

us+MFmlit/XJwZmi89apGFhuYYeyUgprbbtM3yQrawl8swR+8A8x8rRi4FHP25N9Qew6edAD5NSHKstAN8cE93WXFa5aIwX1d1m2pWGxtMcFZbo9tIe1oHbQkPkBa6l17jKjsaiXIiflAZydbtHFPyP9u192BxzCGASL0jSU8wA=

import "reflect-metadata";
const requiredMetadataKey = Symbol("required");

function required(target: Object, propertyKey: string | symbol, parameterIndex: number) {
  let existingRequiredParameters: number[] = Reflect.getOwnMetadata(requiredMetadataKey, target, propertyKey) || [];
  existingRequiredParameters.push(parameterIndex);
  Reflect.defineMetadata( requiredMetadataKey, existingRequiredParameters, target, propertyKey);
}

function validate(target: any, propertyName: string, descriptor: TypedPropertyDescriptor<Function>) {
  let method = descriptor.value!;

  descriptor.value = function () {
    let requiredParameters: number[] = Reflect.getOwnMetadata(requiredMetadataKey, target, propertyName);
    if (requiredParameters) {
      for (let parameterIndex of requiredParameters) {
        if (parameterIndex >= arguments.length || arguments[parameterIndex] === undefined) {
          throw new Error("Missing required argument.");
        }
      }
    }
    return method.apply(this, arguments);
  };
}

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元数据

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DtuiSFsuC/ngo68vUqf0yCe0F37uTzxg9ideMMvjZon4b9RFdvcyYmvyKDuldrH7

npm i reflect-metadata --save

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LVmpK5gi1Kwl7ME78JadPFV8ksAwvFhWQdLhodE2s34=

tsc --target ES5 --experimentalDecorators --emitDecoratorMetadata

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{
  "compilerOptions": {
    "target": "ES5",
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

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import "reflect-metadata";

class Point {
  constructor(public x: number, public y: number) {}
}

class Line {
  private _start: Point;
  private _end: Point;

  @validate
  set start(value: Point) {
    this._start = value;
  }

  get start() {
    return this._start;
  }

  @validate
  set end(value: Point) {
    this._end = value;
  }

  get end() {
    return this._end;
  }
}

function validate<T>(target: any, propertyKey: string, descriptor: TypedPropertyDescriptor<T>) {
  let set = descriptor.set!;
  
  descriptor.set = function (value: T) {
    let type = Reflect.getMetadata("design:type", target, propertyKey);

    if (!(value instanceof type)) {
      throw new TypeError(`Invalid type, got ${typeof value} not ${type.name}.`);
    }

    set.call(this, value);
  };
}

const line = new Line()
line.start = new Point(0, 0)

// line.end = {}

// Fails at runtime with:
// > Invalid type, got object not Point

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class Line {
  private _start: Point;
  private _end: Point;

  @validate
  @Reflect.metadata("design:type", Point)
  set start(value: Point) {
    this._start = value;
  }
  get start() {
    return this._start;
  }

  @validate
  @Reflect.metadata("design:type", Point)
  set end(value: Point) {
    this._end = value;
  }
  get end() {
    return this._end;
  }
}

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