MSDN Magazine - April 2008 - (Page 42)

Figure 8 Choosing Template Based on Content namespace Samples.Controls { public class ListViewItemCreatingEventArgs : EventArgs { private int _dataItemIndex; private int _displayIndex; public ListViewItemCreatingEventArgs(int dataItemIndex, int displayIndex) { _dataItemIndex = dataItemIndex; _displayIndex = displayIndex; } public int DisplayIndex { get { return _displayIndex; } set { _displayIndex = value; } } public int DataItemIndex { get { return _dataItemIndex; } set { _dataItemIndex = value; } } protected override void InstantiateItemTemplate(Control container, int displayIndex) { if (_shouldInstantiate) { base.InstantiateItemTemplate(container, displayIndex); _shouldInstantiate = false; } } protected override ListViewDataItem CreateDataItem(int dataItemIndex, int displayIndex) { // Fire a NEW event: ItemCreating if (ItemCreating != null) ItemCreating(this, new ListViewItemCreatingEventArgs(dataItemIndex, displayIndex)); // Cache for later _displayIndex = displayIndex; // Call the base method return base.CreateDataItem(_dataItemIndex, displayIndex); } } public class ListView : System.Web.UI.WebControls.ListView { public event EventHandler ItemCreating; private int _displayIndex; private bool _shouldInstantiate = false; } protected override void OnItemCreated(ListViewItemEventArgs e) { base.OnItemCreated(e); // You can proceed with template instantiation now _shouldInstantiate = true; InstantiateItemTemplate(e.Item, _displayIndex); } } user code—the absolute index of the item in the data source and the page-specific index. For example, for a page size of 10, when the ListView is working on rendering the first item of the second page, dataItemIndex contains 11 items and displayIndex contains 1 item. In order to use the new ItemCreating event, simply declare the method and handler on your custom ListView control, as you can see in the following code: In your code, you can handle the event like so: void ListView1_ItemCreating( object sender, ListViewItemCreatingEventArgs e) { string url = “standard.ascx”; if (e.DisplayIndex % DataPager1.PageSize == 0) url = “firstItem.ascx”; } ListView1.ItemTemplate = Page.LoadTemplate(url); the item template within the data-binding process. Have a look at the code in Figure 8. The CreateDataItem method fires the ItemCreating event and caches the display index for later use. In addition, the InstantiateItemTemplate method is overridden to delay the actual template instantiation. A private boolean flag is used for that purpose. As mentioned, the ListView starts the data-binding process after instantiating the item template. In the implementation shown in the code in Figure 8, however, no item template is really instantiated until the ItemCreated event is fired. When the ItemCreated event is raised, the data item object is bound to the ListView item container through the DataItem property. By handling the ItemCreated event in your code, you can decide which item template to use based on the bound data item, as you can see here: protected override void OnItemCreated(ListViewItemEventArgs e) { base.OnItemCreated(e); _shouldInstantiate = true; InstantiateItemTemplate(e.Item, _displayIndex); } Here, two different user controls are employed to render the data items. The specific user control is determined by the display index. All items share the same template except the first. Figure 7 shows the page in action. When you think about the complexity of common real-world pages, this solution appears too simple. More often than not, you need to use different templates based on the content to display. You need to further enhance the custom ListView control to change 42 msdnmagazine Cutting Edge In this case, the base method fires the ItemCreated event to the page. After that, the custom ListView control resets the Boolean flag and invokes the method to instantiate the item template. In the end, the item template is instantiated a bit later than in the built-in ListView control, but you can set the ItemTemplate property for each item programmatically in the ItemCreated event handler after looking at the content of the bound data item (see Figure 9). Figure 10 shows a sample page where a blue template is used for men and a pink template is used for women.

Table of Contents Feed for the Digital Edition of MSDN Magazine - April 2008

MSDN Magazine - April 2008 Contents Toolbox CLR Inside Out Basic Instincts Cutting Edge Foundations Test Run Service Station Windows with C++ Going Places { End Bracket }

MSDN Magazine - April 2008

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