ListView.RetrieveVirtualItem 事件

定义

命名空间:

System.Windows.Forms

程序集:

System.Windows.Forms.dll

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

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 

事件类型

RetrieveVirtualItemEventHandler

例外

InvalidOperationException

在处理 Item 事件时，未将RetrieveVirtualItem 属性设置为某个项。

示例

下面的代码示例演示此事件的处理程序。 在此示例中，listView1 需要每个 ListViewItem 显示其索引的平方。 此代码示例是为 属性提供的更大示例的 VirtualMode 一部分。

//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当 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。 当 对象必须创建 对象 ListViewItem 时，将引发此事件。 此事件的处理程序应创建相应的 ListViewItem 或从缓存中检索它，并通过 属性将其传递回 Item 。

有关处理事件的详细信息，请参阅 处理和引发事件。