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JohnnyCode

desperate on javascript issue

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Hi,

I have implemented a 'class' in javascript like this

function CMesh()
{
this.m_faVerticies=new Array();
this.m_fWorld=new Array();
this.m_fFile;
CMesh.prototype.FeedAFile = function(uri) {
// code

var xmlhttp=new XMLHttpRequest();

xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
var bytes = xmlhttp.responseBody;

this.m_fFile= bytes.toArray();
this.m_faVerticies = new Array();
var n=0;
for (var i = 0; i < this.m_fFile.length; i=i+4)
{
var fl=decodeFloat(i,this.m_fFile, 1, 8, 23, -126, 127, true);
this.m_faVerticies[n]=fl;
n++;
}
alert( this.m_faVerticies.length+'A');// ALERTS CORRECT VALUE-9A
}
}
xmlhttp.open("GET",uri,false);

xmlhttp.send();

}
}

If I call this function - FeedAFile, on an instance, I get from my debug alert "9A", which is correct.
but If I read the member m_faVerticies on a next alert(), all hapyness is gone

mesh=new CMesh();
mesh.FeedAFile('onefloat.bin');// THE FUNCTION ALERTS '9A'
alert(mesh.m_faVerticies.length); // THIS FOLLOWING ALERT OUTPUTS 0

I have no idea as where the member .m_faVerticies changed. Have I implemented the class with a public variable incorrectly?

Thanks in advance

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I'm not a JS guru.
I can only guess that you assign m_faVerticies to a wrong object than mesh.
Can you alert value "this" inside of FeedAFile and value "mesh" outside of FeedAFile?

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Can you alert value "this" inside of FeedAFile and value "mesh" outside of FeedAFile?

Thanks , but this will alert only the object type, not something like poiner value or so

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I tested your code (cut down version) and seems m_faVerticies is assigned to correct object.
So there must be some other reasons.

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thanks very much. .... interesting
was it alerting 0 or more? Also I did'nt provide defintion for decodeFloat(i,this.m_fFile, 1, 8, 23, -126, 127, true); so then you perhaps defined it to return just constant float. (I am not on a PC with source right now, cannot post the definition, but I think I will work around and test the code by altering it, I will try add floats to array by hand and so on)
I was on Win 7 64bit, ie 9. What browser and OS have you got? Thank you for your effort, I hope you find this code usefull.

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I have to comment out some code to make it run.
The two alerts are "3A" and "3", so they are correct.
I tested on WinXP, in both IE 8 and FireFox 12.
[source lang="xml"]<html>
<head></head>
<body>
<script>
function CMesh()
{
this.m_faVerticies=new Array();
this.m_fWorld=new Array();
this.m_fFile;
CMesh.prototype.FeedAFile = function(uri) {
// code
// var xmlhttp=new XMLHttpRequest();

// xmlhttp.onreadystatechange=function()
{
if (1)
{
// var bytes = xmlhttp.responseBody;
// this.m_fFile= bytes.toArray();
this.m_faVerticies = new Array();
var n=0;
for (var i = 0; i < 9; i=i+4)
{
var fl=1;
this.m_faVerticies[n]=fl;
n++;
}
alert( this.m_faVerticies.length+'A');// ALERTS CORRECT VALUE-9A
}
}
// xmlhttp.open("GET",uri,false);
// xmlhttp.send();
}
}
mesh=new CMesh();
mesh.FeedAFile('onefloat.bin');
alert(mesh.m_faVerticies.length);
</script>
</body>
</html>
[/source] Edited by wqking

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Maybe try to place the public function FeedAFile() outside the class definition. Not sure if this will make any difference, I've always placed the public functions outside the class definition. Functions inside the class defintion(that are not prototype) are still public from what I remember but they can also access and change private variables.


function CMesh()
{
this.m_faVerticies=new Array();
this.m_fWorld=new Array();
this.m_fFile;

var privateVar = 1;
// This maybe incorrect but I think ONLY the following
// type of functions can be inside the class declaration
this.MyFunction = function(foo) {
// access and change private and public variables
}
}

CMesh.prototype.FeedAFile = function(uri) {
// code
var xmlhttp=new XMLHttpRequest();

xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
var bytes = xmlhttp.responseBody;
this.m_fFile= bytes.toArray();
this.m_faVerticies = new Array();
var n=0;
for (var i = 0; i < this.m_fFile.length; i=i+4)
{
var fl=decodeFloat(i,this.m_fFile, 1, 8, 23, -126, 127, true);
this.m_faVerticies[n]=fl;
n++;
}
alert( this.m_faVerticies.length+'A');// ALERTS CORRECT VALUE-9A
}
}
xmlhttp.open("GET",uri,false);
xmlhttp.send();
}
Edited by gretty

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This code looks very fishy ...


function CMesh() {
this.m_faVerticies=new Array();
// etc

CMesh.prototype.FeedAFile = function(uri) {
var xmlhttp=new XMLHttpRequest();
xmlhttp.onreadystatechange=function() { // lambda function
// cut
this.m_fFile= bytes.toArray();
// cut
}
}
}


1. take CMesh.prototype.FeedAFile out of the class "constructor". You are needlessly reassigning the prototype function with every new object - it is unnecessary. Every new object will inherit the same function from the prototype chain anyway - that is what prototype is for.
2. "this.m_fFile" in the lambda function will point to the lambda function itself - which does not have any of the CMesh properties (edit: ie, all of the data will be thrown away the moment "FeedAFile" function ends).

Something like:

function CMesh() {
this.m_faVerticies=new Array();
// etc
}
CMesh.prototype.FeedAFile = function(uri) {
var xmlhttp=new XMLHttpRequest();
var that = this; // because closure is fun - aka, we want to give the lambda function access to "this" of the CMesh object.
xmlhttp.onreadystatechange=function() { // lambda function
// cut
that.m_fFile= bytes.toArray(); // notice "that" instead of "this".
// cut
}
}
Edited by tanzanite7

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2. "this.m_fFile" in the lambda function will point to the lambda function itself - which does not have any of the CMesh properties (edit: ie, all of the data will be thrown away the moment "FeedAFile" function ends).

Something like:

function CMesh() {
this.m_faVerticies=new Array();
// etc
}
CMesh.prototype.FeedAFile = function(uri) {
var xmlhttp=new XMLHttpRequest();
var that = this; // because closure is fun - aka, we want to give the lambda function access to "this" of the CMesh object.
xmlhttp.onreadystatechange=function() { // lambda function
// cut
that.m_fFile= bytes.toArray(); // notice "that" instead of "this".
// cut
}
}


Very good spot.
Though I rarely used JS, I used Flash AS3 a lot which is quite JS like. In AS3 the "this" in a closure is also quite tricky.

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