1

我想在 javascript 类中的函数上插入预执行和后执行挂钩。

可以说我有这样的课。

class Foo {
  method1(p1, p2) {
    this.p1 = p1;
    this.p2 = p2;
  }

  method2(p3) {
    this.p3 = p3;
  }
}

我想为这些预先存在的类方法定义一个前后挂钩。像这样的东西。

class Foo {
  before(funName, ...params){
    // Should print ('method1', [p1, p2]) when method 1 is called
    // and ('method2', [p3]) when method 2 is called
    console.log(funName, params)
  }
  after(funName, result){
    // Should print the function name followed by its result
    console.log(funName, result)
  }
  method1(p1, p2) {
    this.p1 = p1;
    this.p2 = p2;
  }
  method2(p3) {
    this.p3 = p3;
  }
}

export default Foo;

在现有代码中进行最少更改的情况下实现这些钩子的最佳方法是什么？

4

2 回答 2

3

这是该问题的粗略解决方案：

// we iterate over all method names
Object.getOwnPropertyNames(Foo.prototype).forEach((name) => {

  // First to do: we save the original method. Adding it to prototype
  // is a good idea, we keep 'method1' as '_method1' and so on
  Foo.prototype['_' + name] = Foo.prototype[name];

  // Next, we replace the original method with one that does the logging
  // before and after method execution. 
  Foo.prototype[name] = function() {

    // all arguments that the method receives are in the 'arguments' object
    console.log(`Method call: method1(${Object.values(arguments).join(', ')})`);

    // now we call the original method, _method1, on this with all arguments we received
    // this is probably the most confusing line of code here ;)
    // (I never user this['method'] before - but it works)
    const result = this['_' + name](...arguments);

    // here is the post-execution logging
    console.log(`Method result: ${result}`);

    // and we need to return the original result of the method
    return result;
  };
});

请注意，此代码不是类本身的一部分，请将其作为普通脚本执行。

这个简短的概念证明很有可能在现实世界的类中崩溃，并且需要一些额外的检查和特殊情况处理程序，尤其是为了获得正确的日志输出。但它适用于你的 Foo 类。

这是工作示例：https ://codesandbox.io/s/great-fog-c803c

于 2020-02-23T21:54:27.153 回答
0

代理解决方案

class Foo {
  classAlias = false;

  proxyHandle = {
    // little hack where we save refrenece to our class inside the object
    main : this,
    /**
     * The apply will be fired each time the function is called
     * @param  target Called function
     * @param  scope  Scope from where function was called
     * @param  args   Arguments passed to function
     * @return        Results of the function
     */
    apply : function (target, scope, args) {
      const func_name = target.name;

      console.log('before', func_name);

      // let's call some method of this class to actually check if this is the right instance
      // also remember that you have to exclude methods which you are gonna use
      // inside here to avoid “too much recursion” error
      this.main._free.instanceCheck();


      // here we bind method with our class by accessing reference to instance
      const results = target.bind(this.main)(...args);

      console.log('after', func_name, results);
      return results;
    }
  }

  constructor(classAlias) {
    // Get all methods of choosen class
    let methods = Object.getOwnPropertyNames( Foo.prototype );

    // Find and remove constructor as we don't need Proxy on it
    let consIndex = methods.indexOf('constructor');
    if ( consIndex > -1 ) methods.splice(consIndex, 1);

    // Replace all methods with Proxy methods
    methods.forEach( methodName => {
      this[methodName] = new Proxy( this[methodName], this.proxyHandle );
    });
    this.classAlias = classAlias;
  }

  // easies trick to do to avoid infinite loop inside apply is to set your functions
  // inside object, as getOwnPropertyNames from prototype will only get methods
  _free = {
    instanceCheck : () => {
      // this will check if this is our Foo class
      console.log(this.classAlias);
    }
  }

  log() {
    return 'Result';
  }
}

(new Foo('Proxy Class A')).log();

/* 
  Output:
    before log
    Proxy Class A
    after log Result
*/

只是想分享，因为我在评论中读到有人在设置代理时遇到问题。您可以在此处阅读有关代理的更多信息，并在此处阅读有关应用的更多信息。

请记住，在代理句柄中，this实际上是this.main. 为了更好地理解，您可以将其更改为classInstance或类似的内容。

于 2021-08-03T16:28:53.067 回答