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ASP.NET Weblogs

Andrew Frederick

  • Handling Multiple Asynchronous Postbacks

    Sometimes multiple asynchronous postbacks get triggered. Now, I’m not talking about situations where we want to disable a submit button, so that the user doesn’t click it fifty times waiting for something to happen. Instead, I’m referring to situations where we do want each postback to happen in the order it was fired.

    However, when a page makes multiple asynchronous postbacks at the same time, the default action is that the PageRequestManager gives the most recent postback precedence. This cancels any prior asynchronous postback requests that have not yet been processed. (Get further explanation.)

    So, let’s create a way to “queue up” our asynchronous postback requests and fire them off in order, one by one. First, let’s create an aspx page with three buttons inside of an UpdatePanel:

    <asp:ScriptManager ID="ScriptManager1" runat="server" />
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <asp:Button ID="Button1" runat="server" Text="Button" />
            <asp:Button ID="Button2" runat="server" Text="Button" />
            <asp:Button ID="Button3" runat="server" Text="Button" />
        </ContentTemplate>
    </asp:UpdatePanel>

    There is no need to wire up any click events in the code-behind for our sample. While you could, all that we are concerned about is that they each cause a postback.

    Next, let’s add some deliberate latency into the code so that our postback requests can pile up. Every postback to the server will now take 3 ½ seconds, so that is the fastest each request can be processed.

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
      System.Threading.Thread.Sleep(3500)
    End Sub

    Now, let’s look at the JavaScript code that will manage the backed-up postback requests for us. Add this script block after the ScriptManager on the page but before the closing </html> tag.

    <script type="text/javascript">
        var prm = Sys.WebForms.PageRequestManager.getInstance();
        prm.add_initializeRequest(InitializeRequestHandler);
        prm.add_endRequest(EndRequestHandler);        
    
        var pbQueue = new Array();
        var argsQueue = new Array();       
    
        function InitializeRequestHandler(sender, args) {
            if (prm.get_isInAsyncPostBack()) {
                args.set_cancel(true);
                pbQueue.push(args.get_postBackElement().id);
                argsQueue.push(document.forms[0].__EVENTARGUMENT.value);
            }
        }       
    
        function EndRequestHandler(sender, args) {
            if (pbQueue.length > 0) {
                __doPostBack(pbQueue.shift(), argsQueue.shift());
            }
        }
    </script>

    The Code in Detail

    First, we use the PageRequestManager to set up handlers for the beginning and end of each asynchronous request:

    var prm = Sys.WebForms.PageRequestManager.getInstance();
    prm.add_initializeRequest(InitializeRequestHandler);
    prm.add_endRequest(EndRequestHandler);

    Queuing up the Postbacks…

    Then we create an array to store the originator of each asynchronous postback that cannot be processed immediately, as well as an array to store any event arguments associated with the postback:

    var pbQueue = new Array();
    var argsQueue = new Array();

    Then, at the beginning of each asynchronous postback, we check to see if the page is already in an asynchronous postback:

    function InitializeRequestHandler(sender, args) {
        if (prm.get_isInAsyncPostBack()) {...}

    If it is, we cancel the new postback request, and instead, add the event target and arguments to our arrays:

    args.set_cancel(true);
    pbQueue.push(args.get_postBackElement().id);
    argsQue.push(document.forms[0].__EVENTARGUMENT.value);

    …and Executing Them

    After each asynchronous postback completes, we check to see if there are any more queued up, and if so, we do a __doPostBack(). pbQueue.shift() pulls the first item out of the array and removes it.

    function EndRequestHandler(sender, args) {
        if (pbQueue.length > 0) {
            __doPostBack(pbQueue.shift(), argsQueue.shift());
        }
    }

    And that’s it. Run the page, and randomly click some buttons! If you watch the browser’s status bar, you’ll see the asynchronous postbacks piling up. Then, every 3 ½ seconds, you’ll see one of them being processed! (Remember, the 3 ½ seconds is just an arbitrary time that we added into this demonstration, and it has nothing to do with how the code really works.)

    Note: If for some reason, you wanted to execute the asynchronous postbacks in reverse chronological order (i.e., the most recent requests get processed first), just replace the array.shift() command in the EndRequestHandler() with array.pop().

  • Disabling a Trigger Control During Asynchronous PostBack

    Often, we want to disable the control that triggered an asynchronous postback until the postback has completed. This prohibites the user from triggering another postback until the current one is complete.

    The Code

    First add a ScriptManager to the page, immediately following the <form> tag.

    <asp:ScriptManager ID="ScriptManager1" runat="server" />

    Then add a Label wrapped in an UpdatePanel. This label will be populated with the date and time on each postback. We’ll also add a Button inside of the UpdatePanel to cause the postback.

    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <asp:Label ID="Label1" runat="server" Text="Label" /><br />
            <asp:Button ID="Button1" runat="server" Text="Update Time" OnClick="Button1_Click" />
        </ContentTemplate>
    </asp:UpdatePanel>

    We’ll also add an UpdateProgress control and associate it with our UpdatePanel just to let the user know that something’s happening.

    <asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
        <ProgressTemplate>
            Loading...
        </ProgressTemplate>
    </asp:UpdateProgress>

    Next, we’ll add a events in the code-behind to populate the Label and to introduce some latency, simulating a lengthy update to the page.

    VB.NET:

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Label1.Text = Now.ToString
    End Sub
    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        System.Threading.Thread.Sleep(4000)  'Pause for 4 seconds.
    End Sub

    And finally we’ll add the client-side code to disable our button during the postback. The Javascript disables the button during the beginRequest event of the PageRequestManager and enables it when control has been returned to the browser in the endRequest event. The control causing the postback is returned from the get_postBackElement() method of the BeginRequestEventArgs object which is passed to the function handling the beginRequest event.

    Add the follow script after the ScriptManager on the page:

    <script type="text/javascript">
        var pbControl = null;
        var prm = Sys.WebForms.PageRequestManager.getInstance();
        prm.add_beginRequest(BeginRequestHandler);
        prm.add_endRequest(EndRequestHandler);
        function BeginRequestHandler(sender, args) {
            pbControl = args.get_postBackElement();  //the control causing the postback
            pbControl.disabled = true;
        }
        function EndRequestHandler(sender, args) {
            pbControl.disabled = false;
            pbControl = null;
        }
    </script>

    And that’s it!

    The Complete Source Code:

    <%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="_Default" %>
    <%@ Register Assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
        Namespace="System.Web.UI" TagPrefix="asp" %>
    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
    <html xmlns="http://www.w3.org/1999/xhtml" >
        <head runat="server">
            <title>Untitled Page</title>
        </head>
        <body>
            <form id="form1" runat="server">
                <asp:ScriptManager ID="ScriptManager1" runat="server" />
                <script type="text/javascript">
                    var pbControl = null;
                    var prm = Sys.WebForms.PageRequestManager.getInstance();
                    prm.add_beginRequest(BeginRequestHandler);
                    prm.add_endRequest(EndRequestHandler);
                    function BeginRequestHandler(sender, args) {
                        pbControl = args.get_postBackElement();  //the control causing the postback
                        pbControl.disabled = true;
                    }
                    function EndRequestHandler(sender, args) {
                        pbControl.disabled = false;
                        pbControl = null;
                    }
                </script>
                <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                    <ContentTemplate>
                        <asp:Label ID="Label1" runat="server" Text="Label" /><br />
                        <asp:Button ID="Button1" runat="server" Text="Update Time" OnClick="Button1_Click" />
                    </ContentTemplate>
                </asp:UpdatePanel>
                <asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
                    <ProgressTemplate>
                        Loading...
                    </ProgressTemplate>
                </asp:UpdateProgress>
            </form>
        </body>
    </html>
  • Maintain Scroll Position after Asynchronous Postback

    Do you want to maintain the scroll position of a GridView, Div, Panel, or whatever that is inside of an UpdatePanel after an asynchronous postback?  Normally, if the updatepanel posts back, the item will scroll back to the top because it has been reloaded.  What you need to do is “remember” where the item was scrolled to and jump back to there after the postback.  Place the following script after the ScriptManager on your page.  And since the _endRequest event of the PageRequestManager happens before the page is rendered, you’ll never even see your item move!

    <script type="text/javascript">
        var xPos, yPos;
        var prm = Sys.WebForms.PageRequestManager.getInstance();
        prm.add_beginRequest(BeginRequestHandler);
        prm.add_endRequest(EndRequestHandler);
        function BeginRequestHandler(sender, args) {
            xPos = $get('scrollDiv').scrollLeft;
            yPos = $get('scrollDiv').scrollTop;
        }
        function EndRequestHandler(sender, args) {
            $get('scrollDiv').scrollLeft = xPos;
            $get('scrollDiv').scrollTop = yPos;
        }
    </script>
  • Selecting an AJAX AccordionPane by ID

    Typically, if we want to programmatically select a particular AccordionPane within an ASP.NET AJAX Accordion control, we set the SelectedIndex property.  This is great if we know the exact order of our AccordionPanes at all times, but this isn’t always so.  Let’s say, for instance, that you have four panes (making their indices 0, 1, 2, and 3, respectively).  If we make the third one (index 2) invisible, now the fourth one has an index of 2.  So let’s create a method to select the AccordionPane by its ID instead.

    VB.NET

    Public Sub SetPane(ByVal acc As AjaxControlToolkit.Accordion, ByVal PaneID As String)
        Dim Index As Integer = 0
        For Each pane As AjaxControlToolkit.AccordionPane In acc.Panes
            If (pane.Visible = True) Then
                If (pane.ID = PaneID) Then
                    acc.SelectedIndex = Index
                    Exit For
                End If
                Index += 1
            End If
        Next
    End Sub

    C#

    protected void SetPane(AjaxControlToolkit.Accordion acc, string PaneID) {
        int Index = 0;
        foreach (AjaxControlToolkit.AccordionPane pane in acc.Panes) {
            if (pane.Visible == true) {
                if (pane.ID == PaneID) {
                    acc.SelectedIndex = Index;
                    break;
                }
                Index++;
            }
        }
    }

    It is interesting to note that I attempted to create a client-side equivalent of this method.  However, the ID is not passed down from the server to the client-side behavior, making it impossible to access that propery from the client. 

  • Easy SQL “If Record Exists, Update It. If Not, Insert It.”

    A very common scenario is the one where we want to update the information in a record if it already exists in the table, and if it doesn’t exist, we want to create a new record with the information. 

    The most common solution to this problem is using IF EXISTS  (subquery).  This comes to mind first because it matches how we think about the problem (as you can see by the title of this article!).  We say “If the criterion exists in this initial subquery, then I’ll do this.  If not, I’ll do this other thing.”  This results in a three-step process:

    1. Do the subquery (SELECT whatever FROM wherever WHERE something).
    2. Evaluate the EXISTS statement (is it there or not?).
    3. Execute either the UPDATE or INSERT statement.

    Now, let’s try an “unnatural” shortcut.  I say unnatural because it doesn’t follow that “natural” logic occurring in our brain that I mentioned above.  Instead, let’s just do the update, and if it fails, then we’ll do the insert.  When the update fails, that just means that no rows were affected, not that an error was thrown.  Now we are down to a one-step (if the update succeeds) or two-step process (if we have to insert instead).  This is much more efficient!

    Example:

    This is not necessarily a practical example, but let’s say that we have a table called “Users” which has three fields:  “UserID”, “FirstName”, and “LastName”.  If a record already exists with the specified UserID, simply update it with the new @FirstName and @LastName values.  If it does not exist, create a new record with those values.

    CREATE PROCEDURE dbo.spAddUserName
         (
         @UserID AS int,
         @FirstName AS varchar(50),
         @LastName AS varchar(50)
         )
    AS
         BEGIN
              DECLARE @rc int    
    
              UPDATE [Users]
                 SET FirstName = @FirstName, LastName = @LastName
               WHERE UserID = @UserID   
    
              /* how many rows were affected? */
              SELECT @rc = @@ROWCOUNT    
    
              IF @rc = 0
                   BEGIN
                        INSERT INTO [Users]
                                    (FirstName, LastName)
                             VALUES (@FirstName, LastName)
                   END         
    
         END
  • View Source Trick for Pages with Partial Rendering

    Many people when developing want to look at the browser’s View Source to make sure that things are being outputted correctly or to see where in the DOM certain things are showing up. However, if you are using the Ajax concept of partial rendering (usually via UpdatePanels), what you get is the initial state of the page when it was first loaded before any partial page updates, not the state of the page as it is currently.

    To see the “current” source for the page, simply paste the following code into your browser’s location bar. It’s a little long, but it covers most browsers.

    javascript:if (typeof(window.document.body.outerHTML) != 'undefined'){'<xmp>'+window.document.body.outerHTML+'</xmp>'} else if (typeof(document.getElementsByTagName('html')[0].innerHTML) != 'undefined'){ '<xmp>'+document.getElementsByTagName('html')[0].innerHTML+'</xmp>'} else if (typeof(window.document.documentElement.outerHTML) != 'undefined'){ '<xmp>'+window.document.documentElement.outerHTML+'</xmp>'} else { alert('Your browser does not support this.') }
  • Controlling the ASP.NET Timer Control with JavaScript

    Have you ever wanted to control your <asp:Timer> control from client-side code?

    Let’s say you’ve named your timer ‘Timer1’. The first step is to create a reference to this component:

    var timer = Sys.Application.findComponent(‘<%= Timer1.ClientID %>’);

    Or, better yet, use the $find() shortcut:

    var timer = $find(‘<%= Timer1.ClientID %>’);

    You can then easily access the timer’s interval and enabled properties, as well as start and stop the timer.

    //returns the timer’s interval in milliseconds:
    var waitTime = timer.get_interval;       
    
    //sets the timer’s interval to 5000 milliseconds (or 5 seconds):
    timer.set_interval(5000);       
    
    //returns whether or not the timer is enabled:
    var isTimerEnabled = timer.get_enabled();       
    
    //disables the timer:
    timer.set_enabled(false);       
    
    //starts the timer:
    timer._startTimer();       
    
    //stops the timer:
    timer._stopTimer();

    For the more adventurous of you who would like to look at the client-side behavior code for the Timer control and who elected to download the source code for the AJAX Control Toolkit, you can probably find the .js file at:

    Drive:\Program Files\Microsoft ASP.NET\AJAX Control Toolkit\AJAXControlToolkit\Compat\Timer\Timer.js

    (If you don’t know what you are doing, do not make any changes to this file!)

  • A Client-side Ajax Login for ASP.NET

    A question was posed on the ASP.NET forums recently asking how to have a login control that doesn’t refresh the page.  The ideal solution would be to just drop an ASP.NET Login control inside an updatepanel.  However, the Login control (along with PasswordRecovery, ChangePassword, and CreateUserWizard controls whose contents have not been converted to editable templates) is not supported inside an UpdatePanel.  They are just not compatible with partial-page updates.

    The ASP.NET AJAX Library does include a proxy class that allows client-side authentication, the Sys.Services.AuthenticationService Class!  We can therefore create a very simple and straight-forward “roll-your-own” login solution that does not require a full postback.

    Getting Started:

    I am going to assume that you have completed all of the normal, necessary steps to allow authentication, but the authentication service is not enabled by default.  You must enable it in the web.config file.  Add the following to your web.config within the <configuration> section:

      <system.web.extensions>
        <scripting>
          <webServices>
            <authenticationService enabled="true" requireSSL="false"/>
          </webServices>
        </scripting>
      </system.web.extensions>

    Next, create two DIVs on your page:  one for the “anonymous view” and one for the “logged-in view”.  Set the the display for both DIVs to ‘none’.

      <div id="AnonymousView" style="display: none;">
          <input id="txtUsername" type="text" /><br />
          <input id="pwdPassword" type="password" /><br />
          <input id="chkRememberMe" type="checkbox" />Remember Me<br />
          <input id="btnLogIn" type="button" value="Log In" />
      </div>
      <div id="LoggedInView" style="display: none;">
          Logged in.<br />
          <input id="btnLogOut" type="button" value="Log Out" />
      </div>

    The get_isLoggedIn() property of the class then allows you to show and hide the DIVs appropriately.

      var ssa = Sys.Services.AuthenticationService;
      if (ssa.get_isLoggedIn()) {
          $get('LoggedInView').style.display = '';
      } else {
          $get('AnonymousView').style.display = '';
      }

    The Complete Code:

      <body>
        <form id="form1" runat="server">
            <asp:ScriptManager ID="ScriptManager1" runat="server" />
            <div id="AnonymousView" style="display: none;">
                <input id="txtUsername" type="text" /><br />
                <input id="pwdPassword" type="password" /><br />
                <input id="chkRememberMe" type="checkbox" />Remember Me<br />
                <input id="btnLogIn" type="button" value="Log In" />
            </div>
            <div id="LoggedInView" style="display: none;">
                Logged in.<br />
                <input id="btnLogOut" type="button" value="Log Out" />
            </div>
        </form>
    </body>
            <script type="text/javascript">
                // Hook up the click events of the log in and log out buttons.
                $addHandler($get('btnLogIn'), 'click', loginHandler);
                $addHandler($get('btnLogOut'), 'click', logoutHandler);
                var ssa = Sys.Services.AuthenticationService;
                if (ssa.get_isLoggedIn()) {
                    $get('LoggedInView').style.display = '';
                } else {
                    $get('AnonymousView').style.display = '';
                }
               
                function loginHandler() {
                    var username = $get('txtUsername').value;
                    var password = $get('pwdPassword').value;
                    var isPersistent = $get('chkRememberMe').checked;
                    var customInfo = null;
                    var redirectUrl = null;
                    // Log them in.
                    ssa.login(username,
                              password,
                              isPersistent,
                              customInfo,
                              redirectUrl,
                              onLoginComplete,
                              onError);
                }
               
                function logoutHandler() {
                    // Log them out.
                    var redirectUrl = null;
                    var userContext = null;
                    ssa.logout(redirectUrl,
                               onLogoutComplete,
                               onError,
                               userContext);
                }
               
                function onLoginComplete(result, context, methodName) {
                    // Logged in.  Hide the anonymous view.
                    $get('LoggedInView').style.display = '';
                    $get('AnonymousView').style.display = 'none';
                }
               
                function onLogoutComplete(result, context, methodName) {
                    // Logged out.  Hide the logged in view.
                    $get('LoggedInView').style.display = 'none';
                    $get('AnonymousView').style.display = '';
                }
               
                function onError(error, context, methodName) {
                    alert(error.get_message());
                }
             
            </script>

    Comments and Possible Enhancements:

    • There will always be a page refresh on logout.  This is necessary to ensure that any user-specific information is cleared from the page.
    • You can place the two DIVs inside a third DIV and style that, thus showing a consistant style (width, height, border, etc.) for both child DIVs.
    • You will have probably noticed that there is a redirectUrl parameter for both the login() and logout() methods.  This, along with querystring parameters, could easily be adapted to create a login page that users are redirected to for authentication and redirected from once logged in.  

    References:

    http://asp.net/ajax/documentation/live/ClientReference/Sys.Services/AuthenticationServiceClass/default.aspx

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