ListView.RetrieveVirtualItem 事件

定义

ListView 处于虚拟模式且需要 ListViewItem 时发生。Occurs 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

在处理 Item 事件时,未将RetrieveVirtualItem 属性设置为某个项。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 VirtualMode 属性提供的更大示例的一部分。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 对象处于虚拟模式时,它会动态创建 ListViewItem 对象,而不是使用 Items 集合。When a ListView object is in virtual mode, it creates ListViewItem objects dynamically instead of using the Items collection. 当对象必须创建 ListViewItem 对象时,将引发此事件。This event is raised when the object must create a ListViewItem object. 此事件的处理程序应创建适当的 ListViewItem 或从缓存中检索该处理程序,然后通过 Item 属性将其传递回来。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