ASP.NET 4 Unleased - p 163 docx

CHAPTER 36 Building Custom Controls View State and Control State The standard ASP.NET controls retain the values of their properties across postbacks.. The ASP.NET Framework takes advant

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();

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

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 propprop-erty 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

{

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

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>

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

{

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();

}

}

}

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

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

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)

{