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Retrieving the previous page in history instead of the PostBack page

Roy Oliver

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23 Dec 2005

An article on retrieving the actual previous page instead of the post back page using JavaScript and ASP.NET.

Introduction

We all are quite familiar with the Back and Forward buttons of a web browser. They work exactly as they should, but when dealing with server-side pages that take advantage of post-backs, such as ASP.NET pages, the Back and Forward buttons work a little too well.

For instance, if you were to interact with a drop down list on a page or any type of control that automatically displays results to the same page, then the previous state before the change took place will be added into the browser's memory.

So in this case, what happens when you click the Back button?

That's right, you get the previous state of that page and not the actual page that you browsed from or were sent from.

The Solution

Oh, it's simple. Just write your own backward and forward JavaScript functions and track the number of times the page commits an action or post-back. There are three easy steps to accomplish this. Before we get started, note that templating is popular with server-side pages (especially with ASP.NET). Templating can cause the actual ID of a web-control to change after the page is rendered, so we will take advantage of the ClientID property found in all ASP.NET web-controls since it knows what the rendered ID will be.

Step 1

Insert the following JavaScript functions in a referenced (.js) file or within script tags:

<html>
    <head>
        <title>Backward Forward Example</title>

        <script>

            function Backward(oSpyID)
            {
               // The hidden post-back spy or counter field

               var spy = null;
               // Total number of post-backs

               var refreshes = new Number(0);
               // Allows the actual previous page to be selected

               var offset = new Number(1);
   
               spy = document.getElementById(oSpyID);
       
               refreshes = new Number(spy.value) + offset;
                           
               history.go(-refreshes);
               // Redirects to the actual previous page

            }



            function Forward()
            {
               history.forward(1);
               // Redirects if the next page exists,

               // including the post-back versions.

            }

        </script>
    </head>
    <body>
        <form runat="server">
            ...
        </form>
    </body>
</html>

Step 2

Create two HyperLink controls and one input server tag (the post-back spy). Note that if you want a more graphical look, you can wrap these HyperLink controls around backward and forward images.

<asp:HyperLink CssClass="navPages" ID="hpBackward" Runat="server">
      <img src="../images/nav-arrow-backward.gif" /></asp:HyperLink>
<asp:HyperLink CssClass="navPages" ID="hpForward" Runat="server">
      <img src="../images/nav-arrow-forward.gif" /></asp:HyperLink>
<input type="hidden" id="inputPostBackSpy" runat="server" />

Step 3

In the code-behind file or in the server script tags, declare the HyperLinks and set their attributes respectfully.

      Protected WithEvents inputPostBackSpy As HtmlInputHidden
      Protected WithEvents hpBackward As HyperLink
      Protected WithEvents hpForward As HyperLink

      Private Property PostBackSpy() As Integer
         Get
            Return Convert.ToInt32(inputPostBackSpy.Value)
         End Get
         Set(ByVal Value As Integer)
            inputPostBackSpy.Value = Value.ToString()
         End Set
      End Property

      Private Sub Page_Load(ByVal sender As System.Object, _
         ByVal e As System.EventArgs) Handles MyBase.Load
        
         If Not Me.IsPostBack Then

            hpBackward.Attributes.Add("onclick", _
               "Backward('" & inputPostBackSpy.ClientID & "')")
            hpForward.Attributes.Add("onclick", "Forward()")

            PostBackSpy = 0

         Else

            PostBackSpy = PostBackSpy + 1

         End If
      End Sub

That's It!

That's all there is to it.

Combining custom JavaScripts with ASP.NET or any type of server-side environment will always allow better performance.

License

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