Building Templated Databound Controls ~ OneAoneJ

Creating Templated Controls

A template enables you to customize the layout of a control. Furthermore, a template can contain expressions that are not evaluated until runtime. The ASP.NET Framework supports two types of templates. First, you can create a one-way databinding template. You use a one-way databinding template to display data items. In a one-way databinding template, you use the Eval() expression to display the value of a data item.

Second, you have the option of creating a two-way databinding template. A two-way databinding template can be used not only to display data items, but also to update data items. You can use the Bind() expression in a two-way databinding template to both display a data item and extract the value of a data item. Typically, you use templates with a databound control. For example, the ListView, GridView, Repeater, DataList, FormView, and DetailsView controls all support an ItemTemplate that enables you to format the data items that these controls display.

Implementing the ITemplate Interface

You create a one-way databinding template by adding a property to a control that returns an object that implements the ITemplate interface. The ITemplate interface includes one method:

InstantiateIn—Instantiates the contents of a template in a particular control.

You are not required to implement the InstantiateIn() method yourself. The ASP.NET Framework creates the method for you automatically. You call the InstantiateIn method in your control to add the contents of a template to your control. For example, the control in Listing represents an article. The Article control includes a template named ItemTemplate. The ItemTemplate is used to lay out the elements of the article: the title, author, and contents.

LISTING Article.cs

using System;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

namespace myControls

{

public class Article : CompositeControl

{

private string _title;

private string _author;

private string _contents;

private ITemplate _itemTemplate;

public string Title

{

get { return _title; }

set { _title = value; }

}

public string Author

{

get { return _author; }

set { _author = value; }

}

public string Contents

{

get { return _contents; }

set { _contents = value; }

}

[TemplateContainer(typeof(Article))]

[PersistenceMode(PersistenceMode.InnerProperty)]

public ITemplate ItemTemplate

{

get { return _itemTemplate; }

set { _itemTemplate = value; }

}

protected override void CreateChildControls()

{

_itemTemplate.InstantiateIn(this);

}

Notice that the Article control contains a property named ItemTemplate that returns an object that implements the ITemplate interface. Notice that this property is decorated with two attributes: a TemplateContainer and a PersistenceMode attribute. The TemplateContainer attribute is used to specify the type of control that will contain the template. In the case of the Article control, the template will be contained in the Article control itself. Therefore, the Article control’s type is passed to the TemplateContainer attribute.

The PersistenceMode attribute indicates how a property is persisted in an ASP.NET page. The possible values are Attribute, EncodedInnerDefaultProperty, InnerDefaultProperty, and InnerProperty. We want to declare the ItemTemplate like this:

<custom:Article

runat=”server”>

... template contents ...

</ItemTemplate>

</custom:Article>

Because we want to declare the ItemTemplate inside the Article control, the PersistenceMode attribute needs to be set to the value InnerProperty. The Article control overrides the base WebControl class’s CreateChildControls() method. The ItemTemplate is added as a child control to the Article control. Any

controls contained in the template become child controls of the current control. The page in Listing illustrates how you can use the Article control and its ItemTemplate.

LISTING ShowArticle.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”>

void Page_Load()

{

Article1.Title = “Creating Templated Databound Controls”;

Article1.Author = “Stephen Walther”;

Article1.Contents = “Blah, blah, blah, blah...”;

Article1.DataBind();

}

</script>

<title>Show Article</title>

</head>

<body>

<div>

<custom:Article id=”Article1” Runat=”server”>

<%# Container.Contents %>

</ItemTemplate>

</custom:Article>

</div>

</form>

</body>

</html>

In the Page_Load() method, the Title, Author, and Contents properties of the article are set. Notice that these properties are used within databinding expressions within the Article control’s ItemTemplate. For example, the value of the Title property is displayed with the following databinding expression:

<%# Container.Title %>

The Container keyword refers to the current binding container. In this case, the binding container is the Article control itself. Therefore, you can refer to any property of the Article control by using the Container keyword. Notice that the Article control’s DataBind() method is called at the end of the Page_Load() method. Don’t forget to call this method when you include databinding expressions in a template. If you don’t call this method, then the databinding expressions are never evaluated and displayed.

Creating a Default Template

Normally, you don’t implement the InstantiateIn() method; you let the ASP.NET Framework do it for you. However, if you want to supply a control with a default template, then you need to implement this method. The modified Article control in Listing includes a default template for the ItemTemplate. The default template is used when an ItemTemplate is not supplied.

LISTING ArticleWithDefault.cs

using System;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

namespace myControls

{

public class ArticleWithDefault : CompositeControl

{

private string _title;

private string _author;

private string _contents;

private ITemplate _itemTemplate;

public string Title

{

Creating Templated Controls 1575

get { return _title; }

set { _title = value; }

}

public string Author

{

get { return _author; }

set { _author = value; }

}

public string Contents

{

get { return _contents; }

set { _contents = value; }

}

[TemplateContainer(typeof(ArticleWithDefault))]

[PersistenceMode(PersistenceMode.InnerProperty)]

public ITemplate ItemTemplate

{

get { return _itemTemplate; }

set { _itemTemplate = value; }

}

protected override void CreateChildControls()

{

if (_itemTemplate == null)

_itemTemplate = new ArticleDefaultTemplate();

_itemTemplate.InstantiateIn(this);

}

public class ArticleDefaultTemplate : ITemplate

{

public void InstantiateIn(Control container)

{

Label lblTitle = new Label();

lblTitle.DataBinding += new EventHandler(lblTitle_DataBinding);

Label lblAuthor = new Label();

lblAuthor.DataBinding += new EventHandler(lblAuthor_DataBinding);

Label lblContents = new Label();

lblContents.DataBinding += new EventHandler(lblContents_DataBinding);

container.Controls.Add(lblTitle);

container.Controls.Add(new LiteralControl(“<br />”));

container.Controls.Add(lblAuthor);

container.Controls.Add(new LiteralControl(“<br />”));

container.Controls.Add(lblContents);

}

void lblTitle_DataBinding(object sender, EventArgs e)

{

Label lblTitle = (Label)sender;

ArticleWithDefault container = (ArticleWithDefault)lblTitle.

➥NamingContainer;

lblTitle.Text = container.Title;

}

void lblAuthor_DataBinding(object sender, EventArgs e)

{

Label lblAuthor = (Label)sender;

ArticleWithDefault container = (ArticleWithDefault)lblAuthor.

➥NamingContainer;

lblAuthor.Text = container.Author;

}

void lblContents_DataBinding(object sender, EventArgs e)

{

Label lblContents = (Label)sender;

ArticleWithDefault container = (ArticleWithDefault)lblContents.

➥NamingContainer;

lblContents.Text = container.Contents;

}

Supporting Simplified Databinding

The databinding expressions used in the previous two sections might seem a little odd. For example, we used the following databinding expression to refer to the Title property:

<%# Container.Title %>

When you use a databinding expression with one of the standard ASP.NET controls, such as the GridView control, you typically use a databinding expression that looks like this:

<%# Eval(“Title”) %>

Why the difference? The standard ASP.NET controls support a simplified databinding syntax. If you want to support this simplified syntax in your custom controls, then you must implement the IDataItemContainer interface. The IDataItemContainer includes the following three properties, which you are required to implement:

DataItem—Returns the value of the data item.

DataItemIndex—Returns the index of the data item from its data source.

DisplayIndex—Returns the index of the data item as it is displayed in a control.

Typically, you implement the IDataItemContainer when creating a databound control. For example, you wrap up each record retrieved from a database table in an object that implements the IDataItemContainer interface. That way, you can use a simplified databinding expression to refer to the value of a particular database record column.

Supporting Two-Way Databinding

Two-way databinding is a feature that was introduced with the ASP.NET 2.0 Framework. Two-way databinding enables you to extract values from a template. You can use a twoway databinding expression not only to display the value of a data item, but also to update the value of a data item. You create a template that supports two-way databinding expressions by creating a property that returns an object that implements the IBindableTemplate interface. This interface inherits from the ITemplate interface. It has the following two methods:

InstantiateIn—Instantiates the contents of a template in a particular control.

ExtractValues—Returns a collection of databinding expression values from a template.

For example, the ProductForm control in Listing represents a form for editing an existing product. The control includes a property named EditItemTemplate that represents a two-way databinding template.

LISTING ProductForm.cs

using System;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.ComponentModel;

using System.Collections.Specialized;

namespace myControls

{

public class ProductForm : CompositeControl

{

public event EventHandler ProductUpdated;

private IBindableTemplate _editItemTemplate;

private ProductFormItem _item;

private IOrderedDictionary _results;

public IOrderedDictionary Results

{

get { return _results; }

}

public string Name

{

get

{

EnsureChildControls();

return _item.Name;

}

set

{

EnsureChildControls();

_item.Name = value;

}

public decimal Price

{

get

{

EnsureChildControls();

return _item.Price;

}

set

{

EnsureChildControls();

_item.Price = value;

}

[TemplateContainer(typeof(ProductFormItem), BindingDirection.TwoWay)]

[PersistenceMode(PersistenceMode.InnerProperty)]

public IBindableTemplate EditItemTemplate

{

get { return _editItemTemplate; }

set { _editItemTemplate = value; }

}

protected override void CreateChildControls()

{

_item = new ProductFormItem();

_editItemTemplate.InstantiateIn(_item);

Controls.Add(_item);

}

protected override bool OnBubbleEvent(object source, EventArgs args)

{

_results = _editItemTemplate.ExtractValues(_item);

if (ProductUpdated != null)

ProductUpdated(this, EventArgs.Empty);

return true;

}

public class ProductFormItem : WebControl, IDataItemContainer

{

private string _name;

private decimal _price;

public string Name

{

get { return _name; }

set { _name = value; }

}

public decimal Price

{

get { return _price; }

set { _price = value; }

}

public object DataItem

{

get { return this; }

}

public int DataItemIndex

{

get { return 0; }

}

public int DisplayIndex

{

get { return 0; }

}

You should notice two special things about the EditItemTemplate property. First, notice that the property returns an object that implements the IBindableTemplate interface. Second, notice that the TemplateContainer attribute that decorates the property includes a BindingDirection parameter. You can assign one of two possible values to BindingDirection: OneWay and TwoWay. The ProductForm includes an OnBubbleEvent() method. This method is called when a child control of the ProductForm control raises an event. For example, if someone clicks a Button control contained in the EditItemTemplate, the OnBubbleEvent() method is called. In Listing, the OnBubbleEvent() method calls the EditItemTemplate’s ExtractValues() method. This method is supplied by the ASP.NET Framework because the EditItemTemplate is marked as a two-way databinding template. The ExtractValues() method returns an OrderedDictionary collection that contains name/value pairs that correspond to each of the databinding expressions contained in the EditItemTemplate. The ProductForm control exposes this collection of values with its Results property. After the values are extracted, the control raises a ProductUpdated event.

The page in Listing illustrates how you can use the ProductForm control to update the properties of a product.

LISTING ShowProductForm.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”>

void Page_Load()

{

if (!Page.IsPostBack)

{

ProductForm1.Name = “Laptop”;

ProductForm1.Price = 433.12m;

ProductForm1.DataBind();

}

protected void ProductForm1_ProductUpdated(object sender, EventArgs e)

{

lblName.Text = ProductForm1.Results[“Name”].ToString();

lblPrice.Text = ProductForm1.Results[“Price”].ToString();

}

</script>

<title>Show ProductForm</title>

</head>

<body>

<div>

<custom:ProductForm id=”ProductForm1”

Runat=”server” OnProductUpdated=”ProductForm1_ProductUpdated”>

<asp:Label id=”lblName” Text=”Product Name:” AssociatedControlID=”txtName” Runat=”server” />

<asp:TextBox id=”txtName” Text=’<%# Bind(“Name”) %>’ Runat=”server” />

<asp:Label id=”lblPrice” Text=”Product Price:” AssociatedControlID=”txtPrice” Runat=”server” />

<asp:TextBox id=”txtPrice” Text=’<%# Bind(“Price”) %>’ Runat=”server” />

<asp:Button id=”btnUpdate” Text=”Update” Runat=”server” />

</EditItemTemplate>

</custom:ProductForm>

<hr />

New Product Name:

<asp:Label id=”lblName” Runat=”server” />

New Product Price:

<asp:Label id=”lblPrice” Runat=”server” />

</div>

</form>

</body>

</html>

In the Page_Load() method in Listing, the ProductForm Name and Price properties are set. Next, the DataBind() is called in order to cause the ProductForm control to evaluate its databinding expressions. Notice that the ProductForm control’s EditItemTemplate includes Bind() expressions instead of Eval() expressions. You use Bind() expressions in a two-way databinding template.

The EditItemTemplate includes a Button control. When you click the Button control, the ProductForm control’s OnBubbleEvent() method executes, the values are retrieved from the EditItemTemplate, and the ProductUpdated event is raised. The page in Listing handles the ProductUpdated event and displays the new values with two Label controls

Creating Templated Databound Controls

A databound control can be bound to a DataSource control such as the SqlDataSource or ObjectDataSource controls. The ASP.NET Framework provides you with a number of base classes that you can use when creating a custom databound control. So, let’s look at some tables and figures. Typically, you inherit from one of the leaf nodes. You create a control that derives from the base CompositeDataBoundControl, HierarchicalDataBoundControl, or ListControl class. This is the easiest base class to use when you want to display one or more database records and use templates.

Creating a DivView Control

Let’s start simple. In this section, we create a custom databound control named the DivView control. The DivView control displays a set of data items (database records) in HTML <div> tags. The DivView control inherits from the base CompositeDataBoundControl class and overrides a single method of the base class. The DivView control overrides the base class’s CreateChildControls() method. The DivView control is contained in Listing.

LISTING DivView.cs

using System;

using System.Collections;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

namespace AspNetUnleashed

{

public class DivView : CompositeDataBoundControl

{

private ITemplate _itemTemplate;

[TemplateContainer(typeof(DivViewItem))]

[PersistenceMode(PersistenceMode.InnerProperty)]

public ITemplate ItemTemplate

{

get { return _itemTemplate; }

set { _itemTemplate = value; }

}

protected override int CreateChildControls(IEnumerable dataSource, bool

➥dataBinding)

{

int counter = 0;

foreach (object dataItem in dataSource)

{

DivViewItem contentItem = new DivViewItem(dataItem, counter);

_itemTemplate.InstantiateIn(contentItem);

Controls.Add(contentItem);

counter++;

}

DataBind(false);

return counter;

}

protected override HtmlTextWriterTag TagKey

{

get

{

return HtmlTextWriterTag.Div;

}

public class DivViewItem : WebControl, IDataItemContainer

{

private object _dataItem;

private int _index;

public object DataItem

{

get { return _dataItem; }

}

public int DataItemIndex

{

get { return _index; }

}

public int DisplayIndex

{

get { return _index; }

}

protected override HtmlTextWriterTag TagKey

{

get

{

return HtmlTextWriterTag.Div;

}

public DivViewItem(object dataItem, int index)

{

_dataItem = dataItem;

_index = index;

}

The DivView control supports an ItemTemplate that is used to format each of its data items. You are required to supply an ItemTemplate when you use the DivView control. All the work happens in the CreateChildControls() method. Notice that this is not the same CreateChildControls() method that is included in the base System.Web.UI.Control class. The DivView control overrides the CompositeDataBounControl’s CreateChildControls() method. The CreateChildControls() method accepts the following two parameters:

dataSource—Represents all the data items from the data source.

dataBinding—Represents whether the CreateChildControls() method is called when the data items are being retrieved from the data source.

The CreateChildControls() method is called every time that the DivView control renders its data items. When the control is first bound to a DataSource control, the dataSource parameter represents the data items retrieved from the DataSource control. After a postback, the dataSource parameter contains a collection of null values, but the correct number of null values.

After a postback, the contents of the data items can be retrieved from View State. As long as the correct number of child controls is created, the Framework can rebuild the contents of the databound control. You can use the dataBinding parameter to determine whether the data items from the data source actually represent anything. Typically, the dataBinding parameter has the value True when the page first loads and the value False after each postback. Notice that the DataBind() method is called after the child controls are created. You must call the DataBind() method when a template includes databinding expressions. Otherwise, the databinding expressions are never evaluated. The page in Listing illustrates how you can bind the DivView control to a SqlDataSource control.

LISTING ShowDivView.aspx

<%@ Page Language=”C#” %>

<%@ Register TagPrefix=”custom” Namespace=”AspNetUnleashed” %>

<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN”

“http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”>

.movies

{

width:500px;

}

.movies div

{

border:solid 1px black;

padding:10px;

margin:10px;

}

</style>

<title>Show DivView</title>

</head>

<body>

<div>

<custom:DivView id=”lstMovies” DataSourceID=”srcMovies” CssClass=”movies” Runat=”Server”>

Director: <%# Eval(“Director”) %>

</ItemTemplate>

</custom:DivView>

<asp:SqlDataSource id=”srcMovies” ConnectionString=”<%$ ConnectionStrings:Movies %>”

SelectCommand=”SELECT Title, Director FROM Movies” Runat=”server” />

<br />

<asp:LinkButton id=”lnkReload” Text=”Reload” Runat=”server” />

</div>

</form>

</body>

</html>

In Listing the SqlDataSource control represents the Movies database table. The DivView control includes an ItemTemplate that formats each of the columns from this database table.

Tuesday, June 11, 2013

Building Templated Databound Controls