Question about JSON and callbacks

[pjakubo86]

Hi guys, I was hoping you could help me wrap my head around something. I just started with JSON today and am loving it, but I'm having a bit of a conceptual problem. Here's what's going on:

I have a class, let's call it theClass, like this:

function theClass() {
this.api_key = "2386235627365918365";
this.secret = 889h98ah98h9afaeg8;
this.JSONRequest = function() {var params = arguments[0];
for (var i=1; i < arguments.length; i++) {
    params += "&" + arguments[i];
}
params += "&v=1.0";
params += "&sig=" + hex_md5(params.replace(/&/g,"") + this.secret);
 
var requestURL = "http://a.jsonserver.com?" + params;
var headLoc = document.getElementsByTagName("head").item(0);
var scriptObj = document.createElement("script");
scriptObj.setAttribute("type", "text/javascript");
scriptObj.setAttribute("src", requestURL);
headLoc.appendChild(scriptObj);}
this.aFunctionInTheClass = function() {
var request = this.JSONRequest("api_key=" + this.api_key, "callback=authCreateTokenCallback", "method=some_method");
 

There's obviously a lot more to this class, but I don't want to bore you. So, someone should be able to say: var classInstance = new theClass(); and be able to call various functions in the class, one of which is this JSONRequest function. OK, well the problem lies in the callback. I need the callback to point to another function in theClass which sets some other member variables of theClass and such. Right now, the only way I can get this to work is to put the callback function outside the class, which is useless to me. If I try to put something like this.authCreateTokenCallback = function {} in theClass, I just get an error saying "Object expected" or something like that. How can I get the callback to point to that member function?

I hope I wasn't too confusing and thanks for your help!

[pbmods]

If I try to put something like this.authCreateTokenCallback = function {} in theClass, I just get an error saying "Object expected" or something like that.

You need parenthesis for that -

this.authCreateTokenCallback = function() {};
 

How can I get the callback to point to that member function?

If I understand your code correctly, you are loading a new script object whose source is dynamically generated by requestURL. You're passing callback as a parameter to the requestURL, but the callback isn't actually used by server; instead, you just want to invoke the callback when the script loads. Is that correct?

// You might want to consider making params a generic object.
this.JSONRequest = function(params, callback) {
.
.
.
scriptObj.setAttribute("src", requestURL);
headLoc.appendChild(scriptObj);
callback(this, whatever);
 

It's difficult to make this work effectively, since some browsers (FIrefox is a notable example) will load dynamic script elements asynchronously. In other words, when you attach the new script, Firefox won't wait until it has finished parsing the new file before executing the next line in your function.

Your best bet would be to create a new XMLHttpRequest object and eval the response text. Then you can set the object's onreadystatechange handler to an anonymous function that invokes the callback if readyState == 4.

A bare bones example:

this.JSONRequest = function(params, callback) {
.
.
.
var http = new XMLHttpRequest();
// set up your parameters here; it's been so long since I've had to manually create an XMLHttpRequest object...
http.onreadystatechange = function(response) {
    if(http.readyState == 4) {
        eval(response.responseText);
        callback(this, response);
    }
};
 

[pjakubo86]

Unfortunately, the entire reason I'm using JSON to do this is because I don't want to use XMLHttpRequests because I am doing cross-domain calls. It works OK in IE if the option to allow cross-domain calls is turned on, but it won't work in Firefox at all. I'll need to get my script signed and that seems like a big hassle.

I also thought about the asynchronous problem and it's actually a big problem. Not sure how I'll solve that.

Alternatively, do you know an easy way to get a script signed in Windows? I explored the options on mozilla.org and the tool they provide is for *nix only.

[pbmods]

Unfortunately, the entire reason I'm using JSON to do this is because I don't want to use XMLHttpRequests because I am doing cross-domain calls. It works OK in IE if the option to allow cross-domain calls is turned on, but it won't work in Firefox at all. I'll need to get my script signed and that seems like a big hassle.

This is done to prevent cross-site scripting attacks. For example, you could write a script that would send 100,000 XMLHttpRequests to your least favorite person's homepage, effectively initiating a DoS attack. Get enough people to run your script, and....!

Alternatively, do you know an easy way to get a script signed in Windows? I explored the options on mozilla.org and the tool they provide is for *nix only.

Not a clue. One way to work around this, though is to write a script on the server side that loads the information from the other sites, and then call this script via XMLHttpRequest.