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How would I let 1 .aspx page use another .aspx page's code?

I have 2 aspx files in my project. The first.aspx page has some content on it and when I click on a button, it will launch a frame (second.aspx that only has code to show a calendar) on the same page.
Now once that calendar(second.aspx) loads on first.aspx, I want to click a link on the calendar that will .show() a hidden DIV on the first.aspx page.
How do I access code cross pages? In other words, how can I write some code in second.aspx that will affect first.aspx.

What you're asking for is not really possible. You're probably approaching it the wrong way. What you should do is turn your calendar page into a user control so that it can be used seamlessly in first.aspx.
Here is how to get started with user controls in asp.net:
After you turn it into a user control there are different approaches to getting access to the properties of the user control from your page. Here is one approach using the FindControl method.
Hope that helps.

The easiest solution would be to show and hide your div with jquery. Simple give your div a class like:
<div class="myCalendarDiv" style="display:none" />
And your Button should look like this:
<asp:Button id="myButton" OnClientClick="return ShowCalendar();" runat="server" />
<script type="text/javascript">
function ShowCalendar() {
$(".myCalendarDiv").show();
return false;
}
</script>
Another way would be instead of creating a seprate webpage for the calendar, as proposed you can use a jquery dialog, or make a usercontrol and embedd it on the same page.

(Posted on behalf of the OP).
So since I was dealing with an Iframe, I found out that you can target the parent window which would be first.aspx.
I used "window.parent.MYFUNCTION();" to call my JavaScript function on first.aspx and show the div.

Child webpages help

Using C#, ASP.Net
In my webpage i have the link for the another web page, if i click the link that should display a another page inside the main page like child page.
Another webpage should display like a popup window in my webpage. And also size of the another webpage should be small. How to do this.
Need Help.

For displaying one web page within another look at using frames (or an inline frame - iFrame)
The second part of your question I think is asking to be able to display the link in a separate window. To do this, use the target="blank" attribute in the anchor tag. to set the size of the child window you will need to use javascript:
<script type="text/javascript">
function showPopup(url) {
newwindow=window.open(url,'name','height=190,width=520,top=200,left=300,resizable');
if (window.focus) {newwindow.focus()}
}
</script>
So just call the showPopup method from your anchor click event
link text
Ideally you would move the javascript so that it is no longer inline, but this should work.

For the first part you need a master page
For the second part you might need a modal popup or it's jQuery equivalent
However, I don't understand what you mean with
And also size of the another webpage should be small. How to do this.

Another webpage should display like a
popup window in my webpage. And also
size of the another webpage should be
small. How to do this.
Did you mean a jQuery Modal Window?

page postback in asp.net?

in my application i have the playvideo page where video will play, below that i have the option for sharing the video like adding to favorite ,playlist and sending mail.
when i click on any of the link the page is postbacking and video will start from the first.
i place update panel for link button even though it is not working (video is playing from the first i.e., page is postbacking. can u help me. thank you

Actually, the part of page that is within the UpdatePanel does the postback. Make sure you have only those controls(for instance, your links) inside the UpdatePanel.
Alternatively, you can use multiple UpdatePanels; for instance one for your video and one for the links. In this case note that, when one UpdatePanel gets updated other UpdatePanels also gets updated, which you may not want; so all you have to do then is to mark the UpdateMode property to Conditional and call YourDesiredUpdatePanel.Update() method manually - whenever required.
Btw, updating selected portions of the page also reduces the load on the server
Or you may want to look into using client callbacks instead of a postback. But since client callback uses XMLHTTP, which means Microsoft implementation of AJAX, therefore callbacks are just awesome as long as your are working with IE.

You might want to try taking advantage of Page Methods to do the work you need done server side.
http://encosia.com/2008/05/29/using-jquery-to-directly-call-aspnet-ajax-page-methods/
Also, if you want to prevent a control from posting back, you can add return false to the end of your javascript onclick event on the control.
For example, if you had an asp button you were using you could do this:
<asp:Button ID="myButton" runat="server" OnClientClick="DoThingsInJavascript(); return false;" />
Or if you were just using a standard button you could say:
<input type="button" onclick="DoThingsInJavascript(); return false;" />

I've never really liked the update panel and I have sometimes found it's behaviour awful. Have you thought of trying something like a proper ajax call from Javascript

Methods for dynamically building JavaScript within an ASPX page?

I have a page that is referenced via a <script> tag from a page on another site. In the script src, I pass in the form I want my script to build (from a db table), and the div where the dynamically built form should go. The calling page looks something like this:
<div id="FormContainer"></div>
<script type="text/JavaScript" src="http://www.example.com/GenerateForm.aspx?FormId=1&div=FormContainer"></script>
GenerateForm.aspx contains the code that reads the QueryString parameters for the FormId, and the Div Id, and outputs JavaScript that will build the form.
My question is this. What are the different methods for "outputting" the JavaScript? Some of the JavaScript is static, and can be packaged into an external .js file and I have jQuery too. But should I add that on the GenerateForm.aspx markup page? Or should I use a ScriptManager?
And what about the dynamically built JavaScript? Currently I'm just using Response.Write() for a proof of concept, but instead, should I be doing something else? Use a Literal control on the page and set its value? Use a ScriptManager? Something else?
I know this is a verbose question, so thanks in advance!

If you want to use a seperate, referenced Javascript file, you probably want to do is use an ashx file. Basically this is just a generic handler that you'll use to write directly to the output stream without having to deal with the ASP.NET page lifecycle. If you add a basic Generic Handler (.ashx) to your site from the Add New Item dialog, the template should be enough direction, using context.Response.Write() to output your Javascript dynamically.
The ScriptManager is more useful if you want to output individual lines of Javascript to be ran at certain times, like after an event has fired. Then you can do ScriptManager.RegisterClientBlock(this, this.GetType(), "CodeBlock", "alert('Button clicked');", true); to show a client alert box after a button has been clicked, for example.
Static files should be handled just that way - statically. The server can handle the caching, and does not cause unnecessary processing if you reference the static script file directly from the script tag. However, if you need to load a static script dynamically, you could, for example, create a literal that had the <script> tag inside it. This way it uses the browser's cached version of the static file.

c# Ajax Lazy Loading

I need to populate 4 GridViews on an aspx page, but I only bind a datatable to one of them on page load. I need to pupulate the other 3 after page load.
does anyone know the best way to do this using ajax ?
Currently I'm using javascript to __doPostBack on a button that pupulates the 3 GridViews but unfortunately this forces a full page load even when using an update panel. I need the page to load, and then populate the GridViews as the datatables are returned.
any suggestions would be much apreciated.

The way you are doing it should work ok, although using jquery to populate a div via the $("#targetDiv").load("contentUrl"); function may be a cleaner way to do it. Anyway, in order to get your current implementation working, there could be a few things you want to look at:
I assume EnablePartialRendering is true on your ScriptManager (always worth checking!).
Make sure the eventTarget for the __dopostback call is set up as an async trigger for your update panels or that it is inside the UpdatePanel if you are only using one UpdatePanel. (See here for details)
Try returning false from the javascript code that executes in the onclick event handler if you have attached this to a button, to make sure the form is not being submitted normally by your browser when you click the button.

If I understand the question properly, you want the data to load after the page is in the browser. If this is the case, then you can fire an event with JavaScript when the page loads on the client.
One method I've used is to put a hidden (with CSS, not any property) button on the page and 'clicking' it with javascript. The event of the button click event will need to be wired in the page's code. Also the button would have to be in an update panel that either contains the grids you want to be bound or has the appropriate triggers to cause them to reload.
You might look at JQuery to get manage when this code gets fired. The $(document).ready(function(){ /* Your code here... */ }); method will fire after the entire DOM is available, which is faster than waiting on the entire page to load (images and so forth).