Maintain focus between postbacks in ASP.NET 2.0, also in ASP.NET AJAX

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• Download source code (AJAX) - 3.09 Kb

The Problem

Let's assume you have two TextBoxes. The first one has a TextChanged event attached and AutoPostBack set to true. When you edit TextBox1 and move to TextBox2, the control TextBox2 loses its focus.

Case 1: The normal postback in ASP.NET 2.0 without smart navigation.

The trick is to save the name of the control which received focus at last, and then restore it after postback. There is a hidden field __LASTFOCUS, and we are going to use it.

First of all, let us go through all "relevant" controls and set the attribute onfocus to save their name when they receive focus. "Relevant" are controls which potentially can receive focus. Because we do not want to set this attribute to each control explicitly, we just define types of "relevant" controls and enumerate them recursively.

protected void Page_Load(object sender, EventArgs e)
{
     //sets onfocus event to all apropriate controls on the page.
     if (!IsPostBack)
          HookOnFocus(this.Page as Control);
}
/// <summary>
/// This function goes recursively all child controls and sets 
/// onfocus attribute if the control has one of defined types.
/// </summary>
/// <param name="CurrentControl">the control to hook.</param>
private void HookOnFocus(Control CurrentControl)
{
    //checks if control is one of TextBox, DropDownList, ListBox or Button
    if ((CurrentControl is TextBox) ||
        (CurrentControl is DropDownList) ||
        (CurrentControl is ListBox) ||
        (CurrentControl is Button))
    //adds a script which saves active control on receiving focus 
    //in the hidden field __LASTFOCUS.
        (CurrentControl as WebControl).Attributes.Add(
           "onfocus",
           "try{document.getElementById('__LASTFOCUS').value=this.id} 
               catch(e) {}");
        //checks if the control has children
        if (CurrentControl.HasControls())
            //if yes do them all recursively
            foreach (Control CurrentChildControl in CurrentControl.Controls)
                HookOnFocus(CurrentChildControl);
}

Add aditional control types in the first if condition under the function private void HookOnFocus(Control CurrentControl).

Now, we need to set focus after the postback. Insert the following JavaScript as a constant in your page:

/// <summary>
/// This script sets a focus to the control with a name to which
/// REQUEST_LASTFOCUS was replaced. Setting focus heppens after the page
/// (or update panel) was rendered. To delay setting focus the function
/// window.setTimeout() will be used.
/// </summary>
private const string SCRIPT_DOFOCUS =
    @"window.setTimeout('DoFocus()', 1);
    function DoFocus()
    {
        try {
            document.getElementById('REQUEST_LASTFOCUS').focus();
        } catch (ex) {}
    }";

and this statement to Page_Load:

protected void Page_Load(object sender, EventArgs e)
{
    //sets onfocus event to all apropriate controls on the page.
    if (!IsPostBack)
        HookOnFocus(this.Page as Control);

    //replaces REQUEST_LASTFOCUS in SCRIPT_DOFOCUS with 
    //the posted value from Request    
    //["__LASTFOCUS"]
    //and registers the script to start after the page was rendered
    Page.ClientScript.RegisterStartupScript(
        typeof(MyPage),
        "ScriptDoFocus",
        SCRIPT_DOFOCUS.Replace("REQUEST_LASTFOCUS", 
                               Request["__LASTFOCUS"]),
        true);
}

Case 2: A "tiny Postback" inside the UpdatePanel in ASP.NET 2.0 AJAX

The only difference is that setting the focus must happen after each update inside UpdatePanel. In this case, the script registered using Page.ClientScript.RegisterStartupScript() will execute only after each global postback. We need to use ScriptManager.RegisterStartupScript() instead. The Page_Load will look like this:

protected void Page_Load(object sender, EventArgs e)
{
    //sets onfocus event to all apropriate controls on the page.
    if (!IsPostBack)
            HookOnFocus(this.Page as Control);

    //replaces REQUEST_LASTFOCUS in SCRIPT_DOFOCUS with the posted value from 
    //Request ["__LASTFOCUS"]
    //and registers the script to start after Update panel was rendered
    ScriptManager.RegisterStartupScript(
        this,
        typeof(MyPage),
        "ScriptDoFocus",
        SCRIPT_DOFOCUS.Replace("REQUEST_LASTFOCUS", Request["__LASTFOCUS"]),
        true);
}

Links

• This article on my page.
• ASP.NET AJAX.