更新了…需要一个有关客户端如何从服务器端获取值而无需回发的Ajax示例

Question

basically I want to check whether a user is logged in with javascript when a button is clicked

I know ajax can do this, but I have no idea where to start... and I do not want to learn the whole ajax

is there any simple code sample I can "borrow"?

==============================UPDATE 2===================================

I posted another question for this one and get the answer below... in case someone has the same question as me...

Can I return true to asp.net OnClientClick After an Ajax call? Or maybe some other way?

==============================UPDATE=====================================

I read a few examples but I just cannot find the exact one I need. I can get the result from the server side and then... stuck... here is my code and please help with the questions in the comments. Thanks a lot!!!

in .ascx:

<asp:Button ID="buttonSubmit" Text="Submit" OnClientClick="return buttonSubmitOnclick()" OnClick="buttonSubmit_Click" runat="server"/>

in .js:

function buttonSubmitOnclick() {
  showLogin();
  //What to do here??? how to return true if the ajax call return 'true'?  
  return true;  //???
}


//========== Ajax ==============

function showLogin() {
  var xmlhttp;

  if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp = new XMLHttpRequest();
  }
  else {// code for IE6, IE5
    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
  }
  xmlhttp.onreadystatechange = function () {
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        if (xmlhttp.responseText == "false") {
            $find('modalPopupLogin').show();  //Show Login Dialog
        };
    }
  }
  xmlhttp.open("GET", "AjaxService.aspx?t=auth", true);   //??? should the last parameter be "false"???
  xmlhttp.send();
}

Answer 1

This is pretty straight forward. Since you are using asp.net probably the easiest way to do this is with a page method. Page Methods in asp.net allow you to write .NET code in your code behind and then call it using a javascript method. The page method must be declared static/shared since it will be accessed outside of the page life cycle.

Here are some examples:
- http://www.singingeels.com/Articles/Using_Page_Methods_in_ASPNET_AJAX.aspx
- http://encosia.com/using-jquery-to-directly-call-aspnet-ajax-page-methods/

Page Methods are pretty straight forward to call, with or without jQuery.

Answer 2

There is some good information on using Ajax without jQuery at w3 that has a working example of ajax with asp at the end. Not too much reading involved.

Updated for question changes: At the bottom explains the differences in using asynchronous true vs false .

Using async=true, the JS isn't waiting around for the response so you don't want to handle it 'here':

function buttonSubmitOnclick() {
  showLogin();
  //What to do here??? how to return true if the ajax call return 'true'?  
  return true;  //???
}

Once the xmlhttp.send() is run, js moves on, returns out of showLogin() and will return out of buttomSubmitOnclick, regardless of if the login response has come back yet.

Assuming async true, as you have it, you'll need to expand the conditional in the onreadystatechange function to handle when the responseText == "true".

xmlhttp.onreadystatechange = function () {
  if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
    if (xmlhttp.responseText == "false") {
      $find('modalPopupLogin').show();  //Show Login Dialog
    }else if (xmlhttp.responseText == "true") {
      //Continue the previous/submit action here because we are already authenticated
    }
  }
}