ListView.RetrieveVirtualItem ListView.RetrieveVirtualItem ListView.RetrieveVirtualItem ListView.RetrieveVirtualItem Event

定义

发生时ListView处于虚拟模式且需要ListViewItemOccurs when the ListView is in virtual mode and requires a ListViewItem.

public:
 event System::Windows::Forms::RetrieveVirtualItemEventHandler ^ RetrieveVirtualItem;
public event System.Windows.Forms.RetrieveVirtualItemEventHandler RetrieveVirtualItem;
member this.RetrieveVirtualItem : System.Windows.Forms.RetrieveVirtualItemEventHandler 
Public Custom Event RetrieveVirtualItem As RetrieveVirtualItemEventHandler 

异常

Item属性未设置为某个项时RetrieveVirtualItem处理事件。The Item property is not set to an item when the RetrieveVirtualItem event is handled.

示例

下面的代码示例演示了此事件的处理程序。The following code example demonstrates a handler for this event. 在此示例中,listView1 需要每个ListViewItem以显示其索引的平方。In this example, listView1 needs each ListViewItem to display the square of its index. 此代码示例是为提供一个更大示例的一部分VirtualMode属性。This code example is part of a larger example provided for the VirtualMode property.

//The basic VirtualMode function.  Dynamically returns a ListViewItem
//with the required properties; in this case, the square of the index.
void listView1_RetrieveVirtualItem(object sender, RetrieveVirtualItemEventArgs e)
{
    //Caching is not required but improves performance on large sets.
    //To leave out caching, don't connect the CacheVirtualItems event 
    //and make sure myCache is null.

    //check to see if the requested item is currently in the cache
    if (myCache != null && e.ItemIndex >= firstItem && e.ItemIndex < firstItem + myCache.Length)
    {
        //A cache hit, so get the ListViewItem from the cache instead of making a new one.
        e.Item = myCache[e.ItemIndex - firstItem];
    }
    else
    {
        //A cache miss, so create a new ListViewItem and pass it back.
        int x = e.ItemIndex * e.ItemIndex;
        e.Item = new ListViewItem(x.ToString());
    }
}
'The basic VirtualMode function.  Dynamically returns a ListViewItem
'with the required properties; in this case, the square of the index.
Private Sub listView1_RetrieveVirtualItem(ByVal sender As Object, ByVal e As RetrieveVirtualItemEventArgs) Handles listView1.RetrieveVirtualItem
    'Caching is not required but improves performance on large sets.
    'To leave out caching, don't connect the CacheVirtualItems event 
    'and make sure myCache is null.
    'check to see if the requested item is currently in the cache
    If Not (myCache Is Nothing) AndAlso e.ItemIndex >= firstItem AndAlso e.ItemIndex < firstItem + myCache.Length Then
        'A cache hit, so get the ListViewItem from the cache instead of making a new one.
        e.Item = myCache((e.ItemIndex - firstItem))
    Else
        'A cache miss, so create a new ListViewItem and pass it back.
        Dim x As Integer = e.ItemIndex * e.ItemIndex
        e.Item = New ListViewItem(x.ToString())
    End If


End Sub

注解

ListView对象是处于虚拟模式下,它将创建ListViewItem动态而不是使用对象Items集合。When a ListView object is in virtual mode, it creates ListViewItem objects dynamically instead of using the Items collection. 必须创建的对象时,将引发此事件ListViewItem对象。This event is raised when the object must create a ListViewItem object. 此事件的处理程序应创建相应ListViewItem或从缓存中检索它,并将其传回传递Item属性。A handler for this event should create the appropriate ListViewItem or retrieve it from the cache, and pass it back by way of the Item property.

有关处理事件的详细信息,请参阅处理和引发事件For more information about handling events, see Handling and Raising Events.

适用于