ListView.VirtualMode 属性

定义

获取或设置一个值,该值指示你是否为 ListView 控件提供了自己的数据管理操作。Gets or sets a value indicating whether you have provided your own data-management operations for the ListView control.

public:
 property bool VirtualMode { bool get(); void set(bool value); };
public bool VirtualMode { get; set; }
member this.VirtualMode : bool with get, set
Public Property VirtualMode As Boolean

属性值

Boolean

如果 ListView 使用您提供的数据管理操作,则为 true;否则为 falsetrue if ListView uses data-management operations that you provide; otherwise, false. 默认为 falseThe default is false.

例外

VirtualMode 设置为 true 且存在以下条件之一:VirtualMode is set to true and one of the following conditions exist:

  • VirtualListSize is greater than 0 and RetrieveVirtualItem is not handled.
  • -或--or-

  • Items, CheckedItems, or SelectedItems contains items.
  • -或--or- - 对 Items 进行了编辑。- Edits are made to Items.

    示例

    此示例演示一个简单的 ListView,其内容为前10000个正方形。This example illustrates a simple ListView whose contents are the first ten thousand squares. 它处理搜索并使用缓存来提高性能。It handles searching and uses a cache for increased performance.

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Text;
    using System.Windows.Forms;
    
    public class Form1 : Form
    {
        private ListViewItem[] myCache; //array to cache items for the virtual list
        private int firstItem; //stores the index of the first item in the cache
    
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }
    
        public Form1()
        {
            //Create a simple ListView.
            ListView listView1 = new ListView();
            listView1.View = View.SmallIcon;
            listView1.VirtualMode = true;
            listView1.VirtualListSize = 10000;
    
            //Hook up handlers for VirtualMode events.
            listView1.RetrieveVirtualItem += new RetrieveVirtualItemEventHandler(listView1_RetrieveVirtualItem);
            listView1.CacheVirtualItems += new CacheVirtualItemsEventHandler(listView1_CacheVirtualItems);
            listView1.SearchForVirtualItem += new SearchForVirtualItemEventHandler(listView1_SearchForVirtualItem);
    
            //Add ListView to the form.
            this.Controls.Add(listView1);
    
            //Search for a particular virtual item.
            //Notice that we never manually populate the collection!
            //If you leave out the SearchForVirtualItem handler, this will return null.
            ListViewItem lvi = listView1.FindItemWithText("111111");
    
            //Select the item found and scroll it into view.
            if (lvi != null)
            {
                listView1.SelectedIndices.Add(lvi.Index);
                listView1.EnsureVisible(lvi.Index);
            }
        }
    
        //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());
            }
        }
    
        //Manages the cache.  ListView calls this when it might need a 
        //cache refresh.
        void listView1_CacheVirtualItems(object sender, CacheVirtualItemsEventArgs e)
        {
            //We've gotten a request to refresh the cache.
            //First check if it's really neccesary.
            if (myCache != null && e.StartIndex >= firstItem && e.EndIndex <= firstItem + myCache.Length)
            {
                //If the newly requested cache is a subset of the old cache, 
                //no need to rebuild everything, so do nothing.
                return;
            }
    
            //Now we need to rebuild the cache.
            firstItem = e.StartIndex;
            int length = e.EndIndex - e.StartIndex + 1; //indexes are inclusive
            myCache = new ListViewItem[length];
            
            //Fill the cache with the appropriate ListViewItems.
            int x = 0;
            for (int i = 0; i < length; i++)
            {
                x = (i + firstItem) * (i + firstItem);
                myCache[i] = new ListViewItem(x.ToString());
            }
        }
    
        //This event handler enables search functionality, and is called
        //for every search request when in Virtual mode.
        void listView1_SearchForVirtualItem(object sender, SearchForVirtualItemEventArgs e)
        {
            //We've gotten a search request.
            //In this example, finding the item is easy since it's
            //just the square of its index.  We'll take the square root
            //and round.
            double x = 0;
            if (Double.TryParse(e.Text, out x)) //check if this is a valid search
            {
                x = Math.Sqrt(x);
                x = Math.Round(x);
                e.Index = (int)x;
            }
            //If e.Index is not set, the search returns null.
            //Note that this only handles simple searches over the entire
            //list, ignoring any other settings.  Handling Direction, StartIndex,
            //and the other properties of SearchForVirtualItemEventArgs is up
            //to this handler.
        }
    }
    
    Public Class Form1
        Inherits Form
        Private myCache() As ListViewItem 'array to cache items for the virtual list
        Private firstItem As Integer 'stores the index of the first item in the cache
        Private WithEvents listView1 As ListView
    
        Public Shared Sub Main()
            Application.Run(New Form1)
        End Sub
    
        Public Sub New()
            'Create a simple ListView.
            listView1 = New ListView()
            listView1.View = View.SmallIcon
            listView1.VirtualMode = True
            listView1.VirtualListSize = 10000
    
            'Add ListView to the form.
            Me.Controls.Add(listView1)
    
            'Search for a particular virtual item.
            'Notice that we never manually populate the collection!
            'If you leave out the SearchForVirtualItem handler, this will return null.
            Dim lvi As ListViewItem = listView1.FindItemWithText("111111")
    
            'Select the item found and scroll it into view.
            If Not (lvi Is Nothing) Then
                listView1.SelectedIndices.Add(lvi.Index)
                listView1.EnsureVisible(lvi.Index)
            End If
    
        End Sub
    
        '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
    
        'Manages the cache.  ListView calls this when it might need a 
        'cache refresh.
        Private Sub listView1_CacheVirtualItems(ByVal sender As Object, ByVal e As CacheVirtualItemsEventArgs) Handles listView1.CacheVirtualItems
            'We've gotten a request to refresh the cache.
            'First check if it's really neccesary.
            If Not (myCache Is Nothing) AndAlso e.StartIndex >= firstItem AndAlso e.EndIndex <= firstItem + myCache.Length Then
                'If the newly requested cache is a subset of the old cache, 
                'no need to rebuild everything, so do nothing.
                Return
            End If
    
            'Now we need to rebuild the cache.
            firstItem = e.StartIndex
            Dim length As Integer = e.EndIndex - e.StartIndex + 1 'indexes are inclusive
            myCache = New ListViewItem(length) {}
    
            'Fill the cache with the appropriate ListViewItems.
            Dim x As Integer = 0
            Dim i As Integer
            For i = 0 To length
                x = (i + firstItem) * (i + firstItem)
                myCache(i) = New ListViewItem(x.ToString())
            Next i
    
        End Sub
    
        'This event handler enables search functionality, and is called
        'for every search request when in Virtual mode.
        Private Sub listView1_SearchForVirtualItem(ByVal sender As Object, ByVal e As SearchForVirtualItemEventArgs) Handles listView1.SearchForVirtualItem
            'We've gotten a search request.
            'In this example, finding the item is easy since it's
            'just the square of its index.  We'll take the square root
            'and round.
            Dim x As Double = 0
            If [Double].TryParse(e.Text, x) Then 'check if this is a valid search
                x = Math.Sqrt(x)
                x = Math.Round(x)
                e.Index = Fix(x)
            End If
            'Note that this only handles simple searches over the entire
            'list, ignoring any other settings.  Handling Direction, StartIndex,
            'and the other properties of SearchForVirtualItemEventArgs is up
            'to this handler.
        End Sub
    
    End Class
    

    注解

    VirtualMode 属性设置为 true 会将 ListView 置于虚拟模式下。Setting the VirtualMode property to true puts the ListView into virtual mode. 在虚拟模式下,普通 Items 集合是未使用的。In Virtual mode, the normal Items collection is unused. 相反,ListViewItem 对象是动态创建的,因为 ListView 需要它们。Instead, ListViewItem objects are created dynamically as the ListView requires them.

    虚拟模式在许多情况下都很有用。Virtual mode can be useful under many circumstances. 如果 ListView 对象必须由内存中已经存在的超大型集合进行填充,则为每个条目创建 ListViewItem 对象可能会浪费。If a ListView object must be populated from a very large collection already in memory, creating a ListViewItem object for each entry can be wasteful. 在虚拟模式下,仅创建所需的项。In virtual mode, only the items required are created. 在其他情况下,可能需要频繁地重新计算 ListViewItem 对象的值,对整个集合执行此操作将产生不可接受的性能。In other cases, the values of the ListViewItem objects may need to be recalculated frequently, and doing this for the whole collection would produce unacceptable performance. 在虚拟模式下,只计算所需的项。In virtual mode, only the required items are calculated.

    若要使用虚拟模式,必须处理 RetrieveVirtualItem 事件,每次 ListView 需要项时都会引发该事件。In order to use virtual mode, you must handle the RetrieveVirtualItem event, which is raised every time the ListView requires an item. 此事件处理程序应创建属于指定索引处的 ListViewItem 对象。This event handler should create the ListViewItem object that belongs at the specified index. 此外,VirtualListSize 属性必须设置为虚拟列表的大小。In addition, the VirtualListSize property must be set to the size of the virtual list.

    处理 SearchForVirtualItem 事件可以在虚拟模式下进行搜索。Handling the SearchForVirtualItem event enables searching in virtual mode. 如果未处理此事件,则 FindItemWithTextFindNearestItem 方法将返回 nullIf this event is not handled, the FindItemWithText and FindNearestItem methods will return null.

    您可以处理 CacheVirtualItems 事件以便维护 ListViewItem 对象的缓存。You can handle the CacheVirtualItems event in order to maintain a cache of ListViewItem objects. 如果创建 ListViewItem 对象的计算或查找成本高昂,则维护缓存可以提高性能。If the calculation or lookup to create a ListViewItem object is expensive, maintaining a cache can improve performance.

    如果 View 属性设置为 Tile,则 VirtualMode 设置为 true时,该值将自动更改为 LargeIconIf the View property is set to Tile, the value will automatically be changed to LargeIcon when VirtualMode is set to true.

    在虚拟模式下,将禁用 Items 收集。In virtual mode, the Items collection is disabled. 尝试访问它会导致 InvalidOperationExceptionAttempting to access it results in an InvalidOperationException. CheckedItems 集合和 SelectedItems 集合也是如此。The same is true of the CheckedItems collection and the SelectedItems collection. 如果要检索选定或选中的项,请改用 SelectedIndicesCheckedIndices 集合。If you want to retrieve the selected or checked items, use the SelectedIndices and CheckedIndices collections instead.

    适用于