ptg 1594 CHAPTER 36 Building Custom Controls EnsureChildControls(); txtPassword.Text = value; } } protected override void CreateChildControls() { txtUserName = new TextBox(); txtUserName.ID = “txtUserName”; this.Controls.Add(txtUserName); txtPassword = new TextBox(); txtPassword.ID = “txtPassword”; txtPassword.TextMode = TextBoxMode.Password; this.Controls.Add(txtPassword); } protected override void RenderContents(HtmlTextWriter writer) { writer.AddStyleAttribute(“float”, “left”); writer.RenderBeginTag(HtmlTextWriterTag.Div); writer.AddStyleAttribute(HtmlTextWriterStyle.Padding, “3px”); writer.RenderBeginTag(HtmlTextWriterTag.Div); writer.AddAttribute(HtmlTextWriterAttribute.For, txtUserName.ClientID); writer.RenderBeginTag(HtmlTextWriterTag.Label); writer.Write(“User Name:”); writer.RenderEndTag(); writer.RenderEndTag(); writer.AddStyleAttribute(HtmlTextWriterStyle.Padding, “3px”); writer.RenderBeginTag(HtmlTextWriterTag.Div); writer.AddAttribute(HtmlTextWriterAttribute.For, txtPassword.ClientID); writer.RenderBeginTag(HtmlTextWriterTag.Label); writer.Write(“Password:”); writer.RenderEndTag(); writer.RenderEndTag(); writer.RenderEndTag(); writer.AddStyleAttribute(“float”, “left”); writer.RenderBeginTag(HtmlTextWriterTag.Div); writer.AddStyleAttribute(HtmlTextWriterStyle.Padding, “3px”); writer.RenderBeginTag(HtmlTextWriterTag.Div); txtUserName.RenderControl(writer); writer.RenderEndTag(); From the Library of Wow! eBook ptg 1595 Overview of Custom Control Building 36 writer.AddStyleAttribute(HtmlTextWriterStyle.Padding, “3px”); writer.RenderBeginTag(HtmlTextWriterTag.Div); txtPassword.RenderControl(writer); writer.RenderEndTag(); writer.RenderEndTag(); writer.Write(“<br style=’clear:left’ />”); } protected override HtmlTextWriterTag TagKey { get { return HtmlTextWriterTag.Div; } } } } The control in Listing 36.11 works quite nicely in all recent browsers (Internet Explorer 6, Firefox, Opera 8) without requiring an HTML table for layout (see Figure 36.6). FIGURE 36.6 Performing CSS layout. From the Library of Wow! eBook ptg 1596 CHAPTER 36 Building Custom Controls View State and Control State The standard ASP.NET controls retain the values of their properties across postbacks. For example, if you change the text displayed by a Label control, the Label control continues to display the new text even if you repeatedly post the page containing the Label control back to the server. The ASP.NET Framework takes advantage of a hidden form field named __VIEWSTATE to preserve the state of control properties across postbacks. If you want your controls to preserve the values of their properties, you need to add the values of your control proper- ties to this hidden form field using the appropriate APIs—never access this hidden form field directly. The ASP.NET Framework supports two methods of preserving values across postbacks. You can take advantage of either View State or Control State. Supporting View State You can use the ViewState property of the Control or Page class to add values to View State. The ViewState property exposes a dictionary of key and value pairs. For example, the following statement adds the string Hello World! to View State: ViewState[“message”] = “Hello World!” Technically, you can add an instance of any serializable class to View State. In practice, however, you should add only simple values to View State, such as Strings, DateTimes, and Integers. Remember that anything that you add to View State must be added to the hidden __VIEWSTATE form field. If this field gets too big, it can have a significant impact on your page’s performance. The control in Listing 36.12 has two properties: Text and ViewStateText. The first prop- erty does not use View State, and the second property does use View State. The value of the ViewStateText property is preserved across postbacks automatically. LISTING 36.12 ViewStateControl.cs using System; using System.Web; using System.Web.UI; using System.Web.UI.WebControls; namespace myControls { public class ViewStateControl : WebControl { private string _text; public string Text From the Library of Wow! eBook ptg 1597 View State and Control State 36 { get { return _text; } set { _text = value; } } public string ViewStateText { get { if (ViewState[“ViewStateText”] == null) return String.Empty; else return (string)ViewState[“ViewStateText”]; } set { ViewState[“ViewStateText”] = value; } } protected override void RenderContents(HtmlTextWriter writer) { writer.Write(“Text: “ + Text); writer.WriteBreak(); writer.Write(“ViewStateText: “ + ViewStateText); writer.WriteBreak(); } } } The ViewStateText property uses the Control’s ViewState collection to preserve whatever value is assigned to the ViewStateText property across postbacks. When you add a value to the ViewState collection, the value is stuffed into the hidden __VIEWSTATE form field automatically. WARNING View State is loaded after the Page InitComplete event, and View State is saved after the Page PreRenderComplete event. This means that you should not attempt to retrieve a value from View State before or during the InitComplete event. You also should not attempt to add a value to View State after the PreRenderComplete event. The page in Listing 36.13 includes ViewStateControl. The text Hello World! is assigned to both control properties in the Page_Load() handler. However, if you post the page back to itself by clicking the button, only the value of the ViewStateText property is preserved across postbacks. From the Library of Wow! eBook ptg 1598 CHAPTER 36 Building Custom Controls LISTING 36.13 ShowViewState.aspx <%@ Page Language=”C#” %> <%@ Register TagPrefix=”custom” Namespace=”myControls” %> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”> <script runat=”server”> void Page_Load() { if (!Page.IsPostBack) { ViewStateControl1.Text = “Hello World!”; ViewStateControl1.ViewStateText = “Hello World!”; } } </script> <html xmlns=”http://www.w3.org/1999/xhtml” > <head runat=”server”> <title>Show View State</title> </head> <body> <form id=”form1” runat=”server”> <div> <custom:ViewStateControl id=”ViewStateControl1” Runat=”server” /> <asp:Button id=”btnSubmit” Text=”Submit” Runat=”server” /> </div> </form> </body> </html> From the Library of Wow! eBook ptg 1599 View State and Control State 36 Supporting Control State The ASP.NET Framework includes a feature named Control State, which is similar to View State. Just like View State, any values that you add to Control State are preserved in the hidden __VIEWSTATE form field. However, unlike View State, Control State cannot be disabled. Control State is intended to be used only for storing crucial information across postbacks. Control State was introduced to address a problem that developers encountered in the first version of ASP.NET Framework. You can disable View State for any control by assigning the value False to a control’s EnableViewState property. Often, this is a good idea for perfor- mance reasons; however, disabling View State also made several controls nonfunctional. For example, by default a GridView control retains the values of all the records that it displays in View State. If you display 500 database records with a GridView control, by default all 500 records are stuffed into the hidden __VIEWSTATE form field. To improve performance, you might want to disable View State for the GridView. However, a GridView uses the __VIEWSTATE form field to remember crucial information required for the proper functioning of the control, such as the current page number and the currently selected row. You don’t want the GridView to forget this critical information even when View State is disabled. The concept of Control State was introduced to enable you to save critical information in the hidden __VIEWSTATE form field even when View State is disabled. Microsoft makes it slightly more difficult to use Control State because they don’t want you to overuse this feature. You should use it only when storing super critical information. For example, the control in Listing 36.14 includes two properties named ViewStateText and ControlStateText. View State preserves the value of the first property, and Control State preserves the value of the second property. LISTING 36.14 ControlStateControl.cs using System; using System.Web; using System.Web.UI; using System.Web.UI.WebControls; namespace myControls { public class ControlStateControl : WebControl { private string _controlStateText; public string ViewStateText { From the Library of Wow! eBook ptg 1600 CHAPTER 36 Building Custom Controls get { if (ViewState[“ViewStateText”] == null) return String.Empty; else return (string)ViewState[“ViewStateText”]; } set { ViewState[“ViewStateText”] = value; } } public string ControlStateText { get { return _controlStateText; } set { _controlStateText = value; } } protected override void OnInit(EventArgs e) { Page.RegisterRequiresControlState(this); base.OnInit(e); } protected override object SaveControlState() { return _controlStateText; } protected override void LoadControlState(object savedState) { _controlStateText = (string)savedState; } protected override void RenderContents(HtmlTextWriter writer) { writer.Write(“ViewStateText: “ + ViewStateText); writer.WriteBreak(); writer.Write(“ControlStateText: “ + ControlStateText); writer.WriteBreak(); } } } From the Library of Wow! eBook ptg 1601 View State and Control State 36 The control in Listing 36.14 overrides the base Control class’s OnInit(), SaveControlState(), and LoadControlState() methods. In the OnInit() method, the RegisterRequiresControlState() method is called to indicate that the control needs to take advantage of Control State. The SaveControlState() and LoadControlState() methods are responsible for saving and loading the Control State. Control State is saved as an object. The object is serialized by ASP.NET Framework into the hidden __VIEWSTATE form field automatically. The page in Listing 36.15 illustrates the difference between View State and Control State. In the Page_Load() handler, the value Hello World! is assigned to both properties of the ControlStateControl. The control has View State disabled; however, if you click the button and post the page back to itself, the value of the ControlStateText property is not lost. LISTING 36.15 ShowControlState.aspx <%@ Page Language=”C#” %> <%@ Register TagPrefix=”custom” Namespace=”myControls” %> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”> <script runat=”server”> void Page_Load() { if (!Page.IsPostBack) { ControlStateControl1.ViewStateText = “Hello World!”; ControlStateControl1.ControlStateText = “Hello World!”; } } </script> <html xmlns=”http://www.w3.org/1999/xhtml” > <head id=”Head1” runat=”server”> <title>Show Control State</title> </head> <body> <form id=”form1” runat=”server”> <div> <custom:ControlStateControl id=”ControlStateControl1” EnableViewState=”false” Runat=”server” /> <asp:Button From the Library of Wow! eBook ptg 1602 CHAPTER 36 Building Custom Controls id=”btnSubmit” Text=”Submit” Runat=”server” /> </div> </form> </body> </html> Processing Postback Data and Events The ASP.NET Framework is built around web forms and controls pass information from the browser to the server by submitting a form to the server. This process of posting a form back to the server is called a postback. When an ASP.NET page processes a form that has been posted back to the server, two types of information can be passed to the controls in the page. First, if a control initiates a postback, a server-side event can be raised when the form is posted to the server. For example, if you click a Button control, a Click event is raised on the server when the form containing the Button is posted back to the server. This event is called a postback event. Second, the form data contained in the web form can be passed to a control. For example, when you submit a form that contains a TextBox control, the form data is passed to the TextBox control when the web form is submitted to the server. This form data is called the postback data. When building a custom control, you might need to process either postback data or a postback event. In this section, you learn how to implement the required control inter- faces for processing postbacks. Handling Postback Data If your control needs to process form data submitted to the server, you need to implement the IPostbackDataHandler interface. This interface includes the following two methods: . LoadPostData()—Receives the form fields posted from the browser. . RaisePostDataChangedEvent()—Enables you to raise an event indicating that the value of a form field has been changed. For example, the control in Listing 36.16 is a simple TextBox control that implements the IPostbackDataHandler interface to preserve the state of an input field across postbacks. From the Library of Wow! eBook ptg 1603 Processing Postback Data and Events 36 LISTING 36.16 CustomTextBox.cs using System; using System.Web.UI; using System.Web.UI.WebControls; namespace myControls { public class CustomTextBox : WebControl, IPostBackDataHandler { public event EventHandler TextChanged; public string Text { get { if (ViewState[“Text”] == null) return String.Empty; else return (string)ViewState[“Text”]; } set { ViewState[“Text”] = value; } } protected override void AddAttributesToRender(HtmlTextWriter writer) { writer.AddAttribute(HtmlTextWriterAttribute.Type, “text”); writer.AddAttribute(HtmlTextWriterAttribute.Value, Text); writer.AddAttribute(HtmlTextWriterAttribute.Name, this.UniqueID); base.AddAttributesToRender(writer); } protected override HtmlTextWriterTag TagKey { get { return HtmlTextWriterTag.Input; } } public bool LoadPostData(string postDataKey, System.Collections.Specialized.NameValueCollection postCollection) { if (postCollection[postDataKey] != Text) { From the Library of Wow! eBook . across postbacks. If you want your controls to preserve the values of their properties, you need to add the values of your control proper- ties to this hidden form field using the appropriate APIs—never. directly. The ASP. NET Framework supports two methods of preserving values across postbacks. You can take advantage of either View State or Control State. Supporting View State You can use the ViewState property. significant impact on your page’s performance. The control in Listing 36.12 has two properties: Text and ViewStateText. The first prop- erty does not use View State, and the second property does