ListView.VirtualMode 属性

参考

定义

命名空间:: System.Windows.Forms

程序集:: System.Windows.Forms.dll

重要

一些信息与预发行产品相关，相应产品在发行之前可能会进行重大修改。对于此处提供的信息，Microsoft 不作任何明示或暗示的担保。

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。

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 false。默认值为 false。

例外

InvalidOperationException

VirtualMode 设置为 true 且存在以下条件之一：

VirtualListSize 大于 0 且未处理 RetrieveVirtualItem。

\- 或 -
Items、 CheckedItems或 SelectedItems 包含项。

\- 或 -
对进行 Items编辑。

示例

此示例演示了一个简单的 ListView ，其内容是前一万个方块。它处理搜索并使用缓存来提高性能。

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

将 VirtualMode 属性设置为 true 将置于 ListView 虚拟模式。在虚拟模式下，常规 Items 集合未使用。相反， ListViewItem 对象是根据需要 ListView 动态创建的。

虚拟模式在许多情况下都很有用。 ListView如果必须从内存中已有的非常大的集合填充对象，则为每个条目创建对象ListViewItem可能会浪费。在虚拟模式下，仅创建所需的项。在其他情况下，可能需要频繁重新计算对象的值 ListViewItem ，并且为整个集合执行此操作会产生不可接受的性能。在虚拟模式下，仅计算所需的项。

若要使用虚拟模式，必须处理 RetrieveVirtualItem 每次需要项时 ListView 引发的事件。此事件处理程序应创建 ListViewItem 属于指定索引处的对象。此外， VirtualListSize 属性必须设置为虚拟列表的大小。

SearchForVirtualItem处理事件可实现在虚拟模式下进行搜索。如果未处理此事件，则 FindItemWithText 和 FindNearestItem 方法将返回 null。

可以处理 CacheVirtualItems 事件以维护对象的缓存 ListViewItem 。如果创建 ListViewItem 对象的计算或查找开销很高，则维护缓存可以提高性能。

如果属性View设置为 Tile，则当设置为 LargeIcontrue时VirtualMode，该值将自动更改为。

在虚拟模式下， Items 集合处于禁用状态。尝试访问它会导致 InvalidOperationException。 CheckedItems集合和SelectedItems集合也是如此。如果要检索选定的或选中的项，请改用 SelectedIndices 和 CheckedIndices 集合。

适用于

ListView.VirtualMode 属性

定义

属性值

例外

示例

注解

适用于

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