Running a periodic function from an object

[larztheloser]

Hey everyone,
Would anybody happen to know of a way to make this work:

function myObject()
{
this.someText="Hi!";
this.period=setInterval("alertsometext();",2000);
}
function alertsometext()
{
alert(this.someText);
}
n=new myObject();

Again I know there are other ways to do this (just calling a periodic "alert('hi' )", for instance), but this is for an animation class where I want to keep information about the layer packaged away in an object. Could I pass the object as an argument in the setInterval call (using the this keyword, somehow, I think)?

Thanks in advance,
Lars

[Dormilich]

you’re better off with a closure here.

function myObject()
{
    var someText = "Hi!";
    function alertsometext()
    {
        alert(someText);
    }
    this.period = setInterval(alertsometext, 2000);
}

var n = new myObject();

[larztheloser]

Thanks! The error turned out to be that you can't do it when it is attached with the this operator, but with a general variable you can (I personally have a little gripe with nested functions, that they make my classes huge).

[larztheloser]

WAIT - that variable is no longer tied to the object, now, is it? So you could run the event anywhere - in fact, in my testing someText now behaves like a global! I want someText to be inaccessible except through the object, because the wider application I'm doing this for will have multiple objects with multiple values of someText.

[Dormilich]

it’s not a global, it’s a closure, there are worlds between. btw. have you tried to change the value?
but it would not be difficult to write a function, that changes someText.

[larztheloser]

No, what I mean is - well, basically I don't want this to work:

function myObject()
{
var someText = "Hi!";
}
obj=new myObject();
alert(someText);

So I can do have 2 objects with different values of sometext without them interfering with each other:

function myObject()
{
var someText = "Hi!";
}
obj1=new myObject();
obj2=new myObject();
obj2.someText="Hello!"; //throws an error because it doesn't exist

Or is this sort of encapsulation beyond javascript?

[Dormilich]

So I can do have 2 objects with different values of sometext without them interfering with each other:

you already have that.

but you still have to understand the concept of closures.

so, let’s extent myObject, so that you can change the someText variable

// define someText while creating the object
function myObject(txt)
{
    // this is a private variable
    var someText = String(txt) || "Hi!";
    // this is a private function
    function alertsometext()
    {
        alert(someText);
    }
    // this is a public property
    this.period = setInterval(alertsometext, 2000);
    // a priviledged method to set someText
    this.someNewText = function (newValue)
    {
        someText = newValue;
    };
}

[larztheloser]

Wow. Javascript is even more powerful than I thought! Thanks, Dormilich, I shall give this a try!

[Dormilich]

recommended reading.

[Dormilich]

you might even want to add some security to your setInterval ID, so that it can’t be deleted by accident.

function myObject(txt, delay)
{
    var someText = String(txt) || "Hi!";
    function alertsometext()
    {
        alert(someText);
    }
    this.someNewText = function (newValue)
    {
        someText = newValue;
    };
    // no need to make period public
    var d = Number(delay) || 1000;
    var period = setInterval(alertsometext, d);
    this.stop = function ()
    {
        clearInterval(period);
    };
}