ListView.VirtualMode Propriedade

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

Valor de Propriedade

true se ListView utilizar operações de gestão de dados que fornece; caso contrário, false. A predefinição é false.

Exceções

Exemplos

Este exemplo ilustra um simples ListView cujo conteúdo são os primeiros dez mil quadrados. Gere a pesquisa e utiliza uma cache para aumentar o desempenho.

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

Observações

Definir a VirtualMode propriedade para true colocar o ListView em modo virtual. No modo Virtual, a coleção normal Items não é utilizada. Em vez disso, ListViewItem os objetos são criados dinamicamente conforme os ListView necessita.

O modo virtual pode ser útil em muitas circunstâncias. Se um ListView objeto tiver de ser preenchido a partir de uma coleção muito grande já em memória, criar um ListViewItem objeto para cada entrada pode ser um desperdício. No modo virtual, apenas os itens necessários são criados. Noutros casos, os valores dos ListViewItem objetos podem precisar de ser recalculados frequentemente, e fazer isto para toda a coleção produziria um desempenho inaceitável. No modo virtual, apenas os itens necessários são calculados.

Para usar o modo virtual, deve gerir o RetrieveVirtualItem evento, que é ativado sempre que é ListView necessário um item. Este gestor de eventos deve criar o ListViewItem objeto que pertence ao índice especificado. Além disso, a VirtualListSize propriedade deve ser definida para o tamanho da lista virtual.

Gerir o SearchForVirtualItem evento permite a pesquisa em modo virtual. Se este evento não for tratado, os FindItemWithText métodos e FindNearestItem retornarão null.

Podes gerir o CacheVirtualItems evento para manter um cache de ListViewItem objetos. Se o cálculo ou pesquisa para criar um ListViewItem objeto for dispendioso, manter uma cache pode melhorar o desempenho.

Se a View propriedade for definida para Tile, o valor será automaticamente alterado para LargeIcon quando VirtualMode for definida para true.

No modo virtual, a Items coleção está desativada. Tentar aceder a ela resulta num InvalidOperationException. O mesmo se aplica à CheckedItems coleção e à SelectedItems coleção. Se quiseres recuperar os itens selecionados ou verificados, usa as SelectedIndices coleções e CheckedIndices em vez disso.