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dony authored 2022-04-10 06:07 . Typescript装饰器

Typescript装饰器

1 装饰器是什么

  • 它是一个表达式
  • 该表达式被执行后,返回一个函数
  • 函数的入参分别为 target、name 和 descriptor
  • 执行该函数后,可能返回 descriptor 对象,用于配置 target 对象

2 装饰器的分类

  • 类装饰器(Class decorators)
  • 属性装饰器(Property decorators)
  • 方法装饰器(Method decorators)
  • 参数装饰器(Parameter decorators)

需要注意的是,若要启用实验性的装饰器特性,你必须在命令行或 tsconfig.json 里启用 experimentalDecorators 编译器选项:

命令行

tsc --target ES5 --experimentalDecorators

tsconfig.json

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

3 类装饰器

类装饰器声明:

declare type ClassDecorator = <TFunction extends Function>(
  target: TFunction
) => TFunction | void;

类装饰器顾名思义,就是用来装饰类的。它接收一个参数:

  • target: TFunction - 被装饰的类
function Greeter(target: Function): void {
  target.prototype.greet = function (): void {
    console.log("Hello Semlinker!");
  };
}

@Greeter
class Greeting {
  constructor() {
    // 内部实现
  }
}

let myGreeting = new Greeting();
myGreeting.greet(); // console output: 'Hello Semlinker!';

上面的例子中,我们定义了 Greeter 类装饰器,同时我们使用了 @Greeter 语法糖,来使用装饰器。

不过上面的例子总是输出 Hello Semlinker!, 想要自定义输出问候语,实现如下:

function Greeter(greeting: string) {
  return function (target: Function) {
    target.prototype.greet = function (): void {
      console.log(greeting);
    };
  };
}

@Greeter("Hello TS!")
class Greeting {
  constructor() {
    // 内部实现
  }
}

let myGreeting = new Greeting();
myGreeting.greet(); // console output: 'Hello TS!';

4 属性装饰器

属性装饰器声明:

declare type PropertyDecorator = (target:Object, propertyKey: string | symbol ) => void;

属性装饰器顾名思义,用来装饰类的属性。它接收两个参数:

  • target: Object - 被装饰的类
  • propertyKey: string | symbol - 被装饰类的属性名
function logProperty(target: any, key: string) {
  delete target[key];

  const backingField = "_" + key;

  Object.defineProperty(target, backingField, {
    writable: true,
    enumerable: true,
    configurable: true
  });

  // property getter
  const getter = function (this: any) {
    const currVal = this[backingField];
    console.log(`Get: ${key} => ${currVal}`);
    return currVal;
  };

  // property setter
  const setter = function (this: any, newVal: any) {
    console.log(`Set: ${key} => ${newVal}`);
    this[backingField] = newVal;
  };

  // Create new property with getter and setter
  Object.defineProperty(target, key, {
    get: getter,
    set: setter,
    enumerable: true,
    configurable: true
  });
}

class Person { 
  @logProperty
  public name: string;

  constructor(name : string) { 
    this.name = name;
  }
}

const p1 = new Person("semlinker");
p1.name = "kakuqo";

以上代码我们定义了一个 logProperty 函数,来跟踪用户对属性的操作,当代码成功运行后,在控制台会输出以下结果:

Set: name => semlinker
Set: name => kakuqo

5 方法装饰器

方法装饰器声明:

declare type MethodDecorator = <T>(target:Object, propertyKey: string | symbol,    
  descriptor: TypePropertyDescript<T>) => TypedPropertyDescriptor<T> | void;

方法装饰器顾名思义,用来装饰类的方法。它接收三个参数:

  • target: Object - 被装饰的类
  • propertyKey: string | symbol - 方法名
  • descriptor: TypePropertyDescript - 属性描述符
function LogOutput(tarage: Function, key: string, descriptor: any) {
  let originalMethod = descriptor.value;
  let newMethod = function(...args: any[]): any {
    let result: any = originalMethod.apply(this, args);
    if(!this.loggedOutput) {
      this.loggedOutput = new Array<any>();
    }
    this.loggedOutput.push({
      method: key,
      parameters: args,
      output: result,
      timestamp: new Date()
    });
    return result;
  };
  descriptor.value = newMethod;
}

class Calculator {
  @LogOutput
  double (num: number): number {
    return num * 2;
  }
}

let calc = new Calculator();
calc.double(11);
// console ouput: [{method: "double", output: 22, ...}]
console.log(calc.loggedOutput); 

6 参数装饰器

参数装饰器声明:

declare type ParameterDecorator = (target: Object, propertyKey: string | symbol, 
  parameterIndex: number ) => void

参数装饰器顾名思义,是用来装饰函数参数,它接收三个参数:

  • target: Object - 被装饰的类
  • propertyKey: string | symbol - 方法名
  • parameterIndex: number - 方法中参数的索引值
function Log(target: Function, key: string, parameterIndex: number) {
  let functionLogged = key || target.prototype.constructor.name;
  console.log(`The parameter in position ${parameterIndex} at ${functionLogged} has
 been decorated`);
}

class Greeter {
  greeting: string;
  constructor(@Log phrase: string) {
 this.greeting = phrase; 
  }
}

// console output: The parameter in position 0 
// at Greeter has been decorated

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