MSDN Magazine - April 2008 - (Page 41)

Figure 6 Firing an ItemCreating Event 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; } } } public class ListView : System.Web.UI.WebControls.ListView { public event EventHandler ItemCreating; protected override ListViewDataItem CreateDataItem(int dataItemIndex, int displayIndex) { // Fire a NEW event: ItemCreating if (ItemCreating != null) ItemCreating(this, new ListViewItemCreatingEventArgs (dataItemIndex, displayIndex)); // Call the base method return base.CreateDataItem(_dataItemIndex, displayIndex); } } } You can’t use ASP-style code blocks because they are treated like literals if used to set a property of a server control. You can’t use a data binding expression either because the expression is evaluated too late for the needs of the DataPager. The Load event is too late and would cause the DataPager to fire an exception. The simplest workaround is to set the PagedControlID property programmatically in the page’s Init event, like so: protected void Page_Init(object sender, EventArgs e) { DataPager1.PagedControlID = ListView1.UniqueID; } Although programmatic injection of controls is a solution that worked really well in practice, it never captured my heart. So I decided to try a different route when a customer asked me to modify a ListView-based menu in a Web page. In a menu similar to the one in Figure 1, I needed to render the items of one submenu horizontally rather than vertically. The ListView control generates its markup by looping through the data source and applying the following algorithm. First, it checks whether an item separator is required. If so, it instantiates the template and creates the data item object. The data item object is the container of the item template and carries information about the index of the item in the view and the bound data source. When the item template is instantiated, the ItemCreated event fires. The next step is data binding. After this has completed, the ItemDataBound event is fired. As you can see, there’s no public event that you can handle that allows programmatically changing the template for each item. You can change the template in the Init or Load page event, but that would be for all bound items. If you handle ItemCreated and set the ItemTemplate property there, the change will affect the next item, but not the item being currently processed. You would need an ItemCreating event, but no such an event is fired by the ListView control. The solution, then, is to create your own ListView control, as in Figure 6. By overriding the CreateDataItem method, you have a chance to run your code just before the item template is instantiated. The CreateDataItem method is declared protected and virtual in the ListView class. As you can see in Figure 6, the method override is quite simple. You first fire a custom ItemCreating event and then proceed by calling the base method. The ItemCreating event passes a couple of integers back to the Multiple Item Templates Just like other template-based and data-bound controls, the ListView repeats the same item template for each bound data item. So what if you want to change it for a certain subset of items? Honestly, I have to admit that I never had the need to use more than one item template in years of ASP.NET programming. Several times I customized the appearance of a small group of items in DataGrid and GridView controls based on runtime conditions; however, that always entailed applying a different set of style attributes. Only in very few cases did I programmatically add new controls (mostly Label controls or table cells) to the existing template. In data-bound controls that fire data binding events, this is not a really difficult task—at least not if you have intimate knowledge of the internal structure of the controls you are manipulating. Figure 7 Multiple Item Templates Cutting Edge april2008 41

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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