Reset the page scroll position after a PostBack

• Download source - 1.12 Kb

Introduction

This code cancel the move of the scrollbar after a Postback. It is a simple Control based on HtmlInputHidden which saves, before the postback, the position of the scrollbar and moves it afterwards back to the old position. The variable is saved in the viewstate. This code uses JavaScript to get the position of the scrollbar and move it.

Code

 using System;
 using System.Web.UI;

 namespace AnnulMvtPostBack
 {
  
  public class GetPositionPostBack: 
    System.Web.UI.HtmlControls.HtmlInputHidden, 
    IPostBackDataHandler
  {
    //The position is saving in the viewstate.
    public int VPos
    {
      get 
      {
        if(ViewState["VPos"]==null)
        {
          ViewState["VPos"] = 0;
        }
        return (int)ViewState["VPos"];
      }
      set { ViewState["VPos"] = value; }
    }

    public int HPos
    {
      get
      {
        if(ViewState["HPos"]==null)
        {
          ViewState["HPos"] = 0;
        }
        return (int)ViewState["HPos"];
      }
      set { ViewState["HPos"] = value; }
    }
    
    //Here the value of the HiddenControl is catching 
    //before the PostBack
    public bool LoadPostData(String postDataKey, 
      System.Collections.Specialized.NameValueCollection values) 
    {
      bool _returnV;
      bool _returnH;
        
      string Val = values[this.UniqueID].Trim();
      char Eperluette = Char.Parse("&");
      string[] _Val = Val.Split(Eperluette);
      if(_Val.Length>1)
      {
        if(!HPos.ToString().Equals(_Val[0])  && _Val[0].Trim()!=null )
        {
          HPos= Int32.Parse(_Val[0]);
          _returnH=true;  
        }
        else _returnH=false;

        if(!VPos.ToString().Equals(_Val[1])  && _Val[1].Trim()!=null )
        {
          VPos= Int32.Parse(_Val[1]);
          _returnV=true;  
        }
        else _returnV=false;  
      }
      else
      {
        HPos = 0;
        VPos=0;
        return false; //return true to execute RaisePostDataChangedEvent();
      }

      //_______return_________

      if(_returnV || _returnH) return false; 
       //return true to execute RaisePostDataChangedEvent();
      else return false;    
    }
    
    //This function is here just to understand the LoadPostData 
    //with the RaisePostDataChangedEvent() 
    public void RaisePostDataChangedEvent() 
    {
      // Part of the IPostBackDataHandler contract.  Invoked 
      // if we ever returned true from the
      // LoadPostData method (indicates that we want a change 
      // notification raised).  Since we
      // always return false, this method is just a no-op.
      
    }    

    //The control is rendered with a UniqueId and Javascript Code 
    //is use to get the position of the scrollbar
    protected override void OnPreRender(EventArgs e) 
    {        
      string _start = "<script language="'Javascript'">onscroll = function(){";
      _start += "document.getElementById('" + this.UniqueID + 
        "').value = document.body.scrollLeft+'&'+document.body.scrollTop;";
      _start += "}</script>";
      if(!Page.IsClientScriptBlockRegistered("start"))
      {
        Page.RegisterClientScriptBlock("start",_start);
      }
      //_______________Move the scrollbar____________________________________
      string _goScroll = "<script language="'javascript'">scrollTo("
        + this.HPos + ","+this.VPos+");</script>";
      if(!Page.IsStartupScriptRegistered("goScroll"))
      {
        Page.RegisterStartupScript("goScroll",_goScroll);
      }
    }    
  }
}

I just want to say that I have stolen this idea from another one on the Web (I don't remember who :( ) but I wasn't able to use it so I made my own code.

Mini Tip

• Don't forget to load it like a control.