Professional ASP.NET 3.5 in C# and Visual Basic Part 129 pdf

Listing 26-31: Handling postback events in a server control VB Imports System Imports System.Collections.Generic Imports System.ComponentModel Imports System.Text Imports System.Web Impo

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Now that you can create a postback, you may want to add events to your control that execute

dur-ing the page postback To raise server-side events from a client-side object, you implement the

Sys-tem.Web.IPostBackEventHandler interface Listing 26-31 shows how to do this for a button control You also create a server-side Click event you can handle when the page posts back

Listing 26-31: Handling postback events in a server control

VB

Imports System

Imports System.Collections.Generic

Imports System.ComponentModel

Imports System.Text

Imports System.Web

Imports System.Web.UI

Imports System.Web.UI.WebControls

<DefaultProperty("Text")> _

<ToolboxData("<{0}:ServerControl1 runat=server></{0}:ServerControl1>")> _

Public Class ServerControl1

Inherits System.Web.UI.WebControls.WebControl

Implements IPostBackEventHandler

’ Code removed for clarity

Public Event Click()

Public Sub OnClick(ByVal args As EventArgs)

RaiseEvent Click()

End Sub

Public Sub RaisePostBackEvent(ByVal eventArgument As String) _

Implements System.Web.UI.IPostBackEventHandler.RaisePostBackEvent

OnClick(EventArgs.Empty)

End Sub

End Class

C#

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Linq;

using System.Text;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

namespace ServerControl1

{

[DefaultProperty("Text")]

[ToolboxData("<{0}:ServerControl1 runat=server></{0}:ServerControl1>")]

public class ServerControl1 : WebControl, IPostBackEventHandler

{

// Code removed for clarity

#region IPostBackEventHandler Members

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public event EventHandler Click;

public virtual void OnClick(EventArgs e) {

if (Click != null) {

Click(this,e);

} } public void RaisePostBackEvent(string eventArgument) {

OnClick(EventArgs.Empty);

}

#endregion }

}

Now, when the user clicks the button and the page posts back, the server-side Click event fires, allowing

you to add server-side handling code to the event

Handling PostBack Data

Now that you have learned how to store data in ViewState and add postback capabilities to a control,

look at how you can enable the control to interact with data the user enters into one of its form fields

When a page is posted back to the server by ASP.NET, all the form data is also posted to the server If the

control can interact with data that is passed with a page, you can store the information in ViewState and

complete the illusion of a stateful application

To interact with postback data, your control must be able to access the data To do this, it implements

the System.Web.IPostBackDataHandler interface This interface allows your control to examine the form

data that is passed back to the server during the postback

The IPostBackDataHandler interface requires that you implement two methods:LoadPostData and

RaisePostBackDataChangedEvent TheLoadPostDatamethod is called for all server controls on the page

that have postback data If a control does not have any postback data, the method is not called; however,

you can explicitly ask for the method to be called by using theRegisterRequiresPostBackmethod

Listing 26-32 shows how you implement the IPostBackDataHandler interface method in a text box

Listing 26-32: Accessing Postback data in a server control

VB

Imports System

Imports Sustem.Collections.Generic

Imports System.Text

Imports System.Web

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Inherits SystemWebControl

Implements IPostBackEventHandler, IPostBackDataHandler

’ Code removed for clarity

Public Function LoadPostData(ByVal postDataKey As String, _

ByVal postCollection As _

System.Collections.Specialized.NameValueCollection) _

As Boolean Implements System.Web.UI.IPostBackDataHandler.LoadPostData

Me.Text = postCollection(postDataKey)

Return False

End Function

Public Sub RaisePostDataChangedEvent() _

Implements System.Web.UI.IPostBackDataHandler.RaisePostDataChangedEvent

End Sub

End Class

C#

using System;

using System.Linq;

using System.Text;

using System.Web;

{

public class ServerControl1 : WebControl,

IPostBackEventHandler, IPostBackDataHandler

{

// Code removed for clarity

public bool LoadPostData(string postDataKey,

System.Collections.Specialized.NameValueCollection postCollection)

{

this.Text = postCollection[postDataKey];

return false;

}

public void RaisePostDataChangedEvent()

{

}

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As you can see, theLoadPostDatamethod passes any form data submitted to the method as a name

value collection that the control can access ThepostDataKeyparameter allows the control to access

the postback data item specific to it You use these parameters to save text to theTextproperty of the

TextBox control If you remember the earlier ViewState example, theTextproperty saves the new value

to ViewState; when the page renders, the TextBox value automatically repopulates

In addition to the input parameters, theLoadPostDatamethod also returns a Boolean value This

value indicates whether theRaisePostBackDataChangedEventmethod is also called after the

LoadPostDatamethod completes execution In the sample, it returnsfalsebecause no events exist,

but if you create aTextChangedevent to indicate the Textbox text has changed, you raise that event in

theRaisePostDataChangedEventmethod

Composite Controls

So far, in looking at Server controls, you have concentrated on emitting a single HTML control; but

this can be fairly limiting Creating extremely powerful controls often requires that you nest several

HTML elements together ASP.NET allows you to easily create controls that serve as a container for other

controls These types of controls are called composite controls.

To demonstrate how easy creating a composite control can be, try to change an existing control into a

composite control Listing 26-33 shows how you can do this

Listing 26-33: Creating a composite control

VB

Imports System

Imports System.Text

Imports System.Web

Inherits System.Web.UI.WebControls.CompositeControl

Protected textbox As TextBox

Protected Overrides Sub CreateChildControls()

Me.Controls.Add(textbox) End Sub

End Class

C#

using System;

using System.Linq;

using System.Text;

using System.Web;

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{

public class ServerControl1 : CompositeControl

{

protected TextBox textbox = new TextBox();

protected override void CreateChildControls() {

this.Controls.Add(textbox);

} }

}

A number of things in this listing are important First, notice that the control class is now inheriting

fromCompositeControl, rather thanWebControl Deriving fromCompositeControlgives you a few

extra features specific to this type of control

Second, notice that noRendermethod appears in this code Instead, you simply create an instance of

another type of server control and add that to theControlscollection in theCreateChildControls

method When you run this sample, you see that it renders a text box just like the last control did In fact, the HTML that it renders is almost identical

Exposing Child Control Properties

When you drop a composite control (such as the text box from the last sample) onto the design

sur-face, notice that even though you are using a powerful ASP.NET TextBox control within the control,

none of that control’s properties are exposed to you in the Properties Explorer In order to expose

child control properties through the parent container, you must create corresponding properties in the

parent control For example, if you want to expose the ASP.NET text boxTextproperty through

the parent control, you create aTextproperty Listing 26-34 shows how to do this

Listing 26-34: Exposing control properties in a composite control

VB

Imports System

Imports System.Text

Imports System.Web

Inherits System.Web.UI.WebControls.CompositeControl

Protected textbox As TextBox

Public Property Text() As String

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Get EnsureChildControls() Return textbox.Text End Get

Set(ByVal value As String) EnsureChildControls() textbox.Text = value End Set

End Property

Me.Controls.Add(textbox) Me.ChildControlsCreated=True End Sub

End Class

C#

using System;

using System.Linq;

using System.Text;

using System.Web;

{

public class ServerControl1 : CompositeControl

{

protected TextBox textbox = new TextBox();

public string Text {

get { EnsureChildControls();

return textbox.Text;

} set { EnsureChildControls();

textbox.Text = value;

} } protected override void CreateChildControls() {

this.Controls.Add(textbox);

this.ChildControlsCreated=true;

} }

}

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Notice that you use this property simply to populate the underlying control’s properties Also notice

that before you access the underlying control’s properties, you always call theEnsureChildControls

method This method ensures that children of the container control have actually been initialized before you attempt to access them

Templated Controls

In addition to composite controls, you can also create templated controls Templated controls allow the user

to specify a portion of the HTML that is used to render the control, and to nest other controls inside of a container control You might be familiar with the Repeater or DataList control These are both templated controls that let you specify how you want the bound data to be displayed when the page renders

To demonstrate a templated control, the following code gives you a simple example of displaying a mes-sage from a user on a Web page Because the control is a templated control, the developer has complete control over how the message is displayed

To get started, create the Message server control that will be used as the template inside of a container

control Listing 26-35 shows the class which simply extends the existing Panel control by adding two

additional properties, Name and Text, and a new constructor

Listing 26-35: Creating the templated control’s inner control class

VB

Public Class Message

Inherits System.Web.UI.WebControls.Panel

Implements System.Web.UI.INamingContainer

Private _name As String

Private _text As String

Public Sub New(ByVal name As String, ByVal text As String)

_text = text _name = name End Sub

Public ReadOnly Property Name() As String

Get Return _name End Get

End Property

Public ReadOnly Property Text() As String

Get Return _text End Get

End Property

End Class

C#

using System;

using System.Text;

using System.Web;

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{

public class Message : Panel, INamingContainer

{

private string _name;

private string _text;

public Message(string name, string text) {

_text = text;

_name = name;

} public string Name {

get { return _name;}

} public string Text {

get { return _text;}

} }

}

As you will see in a moment, you can access the public properties exposed by theMessageclass in

order to insert dynamic content into the template You will also see how you can display the values

of theNameandTextproperties as part of the rendered template control

Next, as shown in Listing 26-36, create a new server control which will be the container for the

Messagecontrol This server control is responsible for rendering any template controls nested in it

Listing 26-36: Creating the template control container class

VB

Imports System

Imports System.Text

Imports System.Web

<ToolboxData("<{0}:TemplatedControl runat=server></{0}:TemplatedControl>")> _

Public Class TemplatedControl

Inherits System.Web.UI.WebControls.WebControl

Private _name As String

Private _text As String

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Private _message As Message

Private _messageTemplate As ITemplate

<Browsable(False)> Public ReadOnly Property Message() As Message

Get

Return _message

End Get

End Property

<PersistenceMode(PersistenceMode.InnerProperty), _

TemplateContainer(GetType(Message))> _

Public Property MessageTemplate() As ITemplate

Get

Return _messageTemplate

End Get

Set(ByVal value As ITemplate)

_messageTemplate = value

End Set

End Property

<Bindable(True), DefaultValue("")> Public Property Name() As String

Get

Return _name

End Get

Set(ByVal value As String)

_name = value

End Set

End Property

<Bindable(True), DefaultValue("")> Public Property Text() As String

Get

Return _text

End Get

Set(ByVal value As String)

_text = value

End Set

End Property

Public Overrides Sub DataBind()

CreateChildControls()

ChildControlsCreated = True

MyBase.DataBind()

End Sub

Me.Controls.Clear()

_message = New Message(Name, Text)

If Me.MessageTemplate Is Nothing Then

Me.MessageTemplate = New DefaultMessageTemplate()

End If

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Me.MessageTemplate.InstantiateIn(_message) Controls.Add(_message)

End Sub

Protected Overrides Sub RenderContents( _

ByVal writer As System.Web.UI.HtmlTextWriter) EnsureChildControls()

ChildControlsCreated = True MyBase.RenderContents(writer) End Sub

End Class

C#

using System;

using System.Linq;

using System.Text;

using System.Web;

{

[ToolboxData("<{0}:TemplatedControl runat=server></{0}: TemplatedControl >")]

public class TemplatedControl : WebControl

{

private string _name;

private string _text;

private Message _message;

private ITemplate _messageTemplate;

[Browsable(false)]

public Message Message {

get { return _message;

} } [PersistenceMode(PersistenceMode.InnerProperty)]

[TemplateContainer(typeof(Message))]

public virtual ITemplate MessageTemplate {

get { return _messageTemplate;}

set { _messageTemplate = value;}

}

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