How to determine custom application page URL independently of the environment/farm?

Question

I have deployed a custom ribbon button and when a user clicks it, I open a custom application page - I have deployed the page to _layouts mapped folder. To open the page (with JavaScript), so far I'm providing an absolute URL which looks like this:

targetUrl = 'http://srvr/sites/Site1/_layouts/MyProject/AppPage.aspx';

where http://srvr/sites/Site1 is the "Site URL" property of my Visual Studio project (deployed as a farm solution).

My question is: how do I determine the URL of AppPage.aspx independently of the environment, i.e. in a relative manner? E.g. if I deploy to a farm where "Site1" doesn't exist, in what way should I construct the target URL? I assume what I'm trying to do is to deploy the application page in a way that won't rely on the existence of 'Site1' site collection. Would deploying to _admin mapped folder instead of _layouts make sense (I want the app page to be viewable by normal users, not just admins)?

(UPDATE) Here's the complete JS code that I use to open the application page (as a dialog):

function OpenMyAppPage() {   
    var options = SP.UI.$create_DialogOptions();
    options.url = 'http://srvr/sites/Site1/_layouts/MyProject/AppPage.aspx';
    options.url += "?Location=" + location.href;
    options.width = 400;
    options.height = 200;
    options.allowClose = false;
    options.allowMaximize = false;
    options.title = 'Example';
    SP.UI.ModalDialog.showModalDialog(options);
}

Note: simply changing the url to '/_layouts/MyProject/AppPage.aspx' gives me error 404.

Answer 1

Would this not work? Using the fact that Layouts is shared through the fram

targetUrl = '/_layouts/MyProject/AppPage.aspx' + "?Location=" + window.location.href;

or you could try:

targetUrl = SP.Utilities.Utility.getLayoutsPageUrl('MyProject/AppPage.aspx' + "?Location=" + window.location.href)

Answer 2

Try changing the options.Url to:

 options.url = _spPageContextInfo.webServerRelativeUrl+'/_layouts/MyProject/AppPage.aspx';

Edit: Didn't notice 2010 first, updated with webServerRelativeUrl