如何将其他参数传递给node.js中的封装回调函数

Question

Have used this site for years but never posted a question before :-) Here goes..

I have a javascript object, which I would like to assign itself a value based on a google API when it's initialised. That's all fine, but the google response doesn't contain the reference to which object called it, so I need to find a way to pass the ID down the chain.

I hope the below example makes sense, basically the API response I want will not contain a reference to the object that started it - so I need a way of associating it back to the object that called it.

NOTE : THIS IS PSUDO CODE

    function myClass(param) {
      this.property = param;
      this.distanceFromSomething = 0;
      this.init = function() {
        // this will set this.distanceFromSomething
        var url = 'http:// GOOGLE MAPS API URL BUILT HERE';
        http.get(url, function(res) {
          var body = '';
          res.on('data', function(chunk) {body += chunk;});
          res.on('end', function() {
            var response = JSON.parse(body)
            var distance = response.distance;
            this.distanceFromSomething = distance;
            // 'this' is no longer defined since it's asynchronous... :-(
            // alternative...
            setDistance(ID, distance);
            // and I cannot get the ID of the current object from here either, since it's encapsulated :-(
            // How can this callback function understand which object it relates to?
          }); 
        };
      };
      this.init();
    }

    var entity = new myClass(foo);
    var undefined = entity.distanceFromSomething;  :-(

Answer 1

In jcubic's answer, you won't get the desired result since distanceFromSomething won't be set until http.get is completed, so you need some sort of a callback.

function myClass(param) {
    var self = this;
    this.property = param;
    this.distanceFromSomething = 0;
    this.init = function(cb) {
        // this will set this.distanceFromSomething
        var url = 'http:// GOOGLE MAPS API URL BUILT HERE';
        http.get(url, function(res) {
            var body = '';
            res.on('data', function(chunk) {body += chunk;});
            res.on('end', function() {
                var response = JSON.parse(body)
                var distance = response.distance;
                self.distanceFromSomething = distance;
                setDistance(ID, distance);
                if(cb) {
                    cb.bind(self)(); // "this" inside the callback will be bound to myClass instance.
                }
            }); 
        };
    };
}

var cls = new myClass({});
cls.init(function() {
    console.log(this.distanceFromSomething); //init is done.
});

Answer 2

You can change the scope of the function (this) using apply, call or bind (bind will return another function with scope changed).

so in your case just do

function myClass(param) {
  var self = this;
  this.property = param;
  this.distanceFromSomething = 0;
  this.init = function() {
    // this will set this.distanceFromSomething
    var url = 'http:// GOOGLE MAPS API URL BUILT HERE';
    http.get(url, function(res) {
      var body = '';
      res.on('data', function(chunk) {body += chunk;});
      res.on('end', function() {
        var response = JSON.parse(body)
        var distance = response.distance;
        this.distanceFromSomething = distance;
        // 'this' will be from myClass scope
        // alternative...
        setDistance(ID, distance);
        // and I cannot get the ID of the current object from here either, since it's encapsulated :-(
        // How can this callback function understand which object it relates to?
      }.bind(self)); // <--- HERE bind to self
    };
  };
  this.init();
}

or you can just use self inside a callback.

EDIT And in initialized code you will need to wait until the value is fetched:

var entity = new myClass(foo);
(function delay() {
    if (!entity.distanceFromSomething) {
        return setTimeout(delay, 100);
    }
    alert(entity.distanceFromSomething);
});

Answer 3

Thanks guys, I used a combination of your solutions which worked!

    function myClass(param) {
    this.property = param;
    this.distanceFromSomething = 0;
    this.init = function() {
    // this will set this.distanceFromSomething
    var url = 'http:// GOOGLE MAPS API URL BUILT HERE';
    var self = this; // 'this' is still defined at this point
    http.get(url, function(res) {
        var body = '';
        res.on('data', function(chunk) {body += chunk;});
        res.on('end', function() {
            var response = JSON.parse(body)
            var distance = response.distance;
            self.distanceFromSomething = distance;
            setDistance(self.id, distance);  // <-- this function receives self.id :-)
        }); 
    }.bind(self));
};

}