ListView.VirtualMode Propriedade

Definição

Obtém ou define um valor que indica se você forneceu suas próprias operações de gerenciamento de dados para o controle ListView.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

Valor da propriedade

true se ListView usar operações de gerenciamento de dados fornecidos; caso contrário, false.true if ListView uses data-management operations that you provide; otherwise, false. O padrão é false.The default is false.

Exceções

VirtualMode é definido como true e uma das seguintes condições existe:VirtualMode is set to true and one of the following conditions exist:

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

  • Items, CheckedItems, or SelectedItems contains items.
  • -ou--or- - Edições são feitas para Items.- Edits are made to Items.

    Exemplos

    Este exemplo ilustra um ListView simples cujo conteúdo são os primeiros 10000 quadrados.This example illustrates a simple ListView whose contents are the first ten thousand squares. Ele lida com a pesquisa e usa um cache para aumentar o desempenho.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
    

    Comentários

    A definição da propriedade VirtualMode como true coloca a ListView no modo virtual.Setting the VirtualMode property to true puts the ListView into virtual mode. No modo virtual, a coleção de Items normal não é usada.In Virtual mode, the normal Items collection is unused. Em vez disso, ListViewItem objetos são criados dinamicamente à medida que o ListView exige.Instead, ListViewItem objects are created dynamically as the ListView requires them.

    O modo virtual pode ser útil em várias circunstâncias.Virtual mode can be useful under many circumstances. Se um objeto de ListView deve ser preenchido a partir de uma coleção muito grande já na memória, a criação de um objeto de ListViewItem para cada entrada pode ser um desperdício.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. No modo virtual, somente os itens necessários são criados.In virtual mode, only the items required are created. Em outros casos, os valores da ListViewItem objetos podem precisar ser recalculados com frequência e fazer isso para a coleção inteira produziria um desempenho inaceitável.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. No modo virtual, somente os itens necessários são calculados.In virtual mode, only the required items are calculated.

    Para usar o modo virtual, você deve manipular o evento RetrieveVirtualItem, que é gerado toda vez que o ListView requer um item.In order to use virtual mode, you must handle the RetrieveVirtualItem event, which is raised every time the ListView requires an item. Esse manipulador de eventos deve criar o objeto ListViewItem que pertence ao índice especificado.This event handler should create the ListViewItem object that belongs at the specified index. Além disso, a propriedade VirtualListSize deve ser definida como o tamanho da lista virtual.In addition, the VirtualListSize property must be set to the size of the virtual list.

    Manipular o evento de SearchForVirtualItem permite pesquisar no modo virtual.Handling the SearchForVirtualItem event enables searching in virtual mode. Se esse evento não for tratado, os métodos FindItemWithText e FindNearestItem retornarão null.If this event is not handled, the FindItemWithText and FindNearestItem methods will return null.

    Você pode manipular o evento CacheVirtualItems para manter um cache de objetos ListViewItem.You can handle the CacheVirtualItems event in order to maintain a cache of ListViewItem objects. Se o cálculo ou a pesquisa para criar um objeto de ListViewItem for caro, manter um cache poderá melhorar o desempenho.If the calculation or lookup to create a ListViewItem object is expensive, maintaining a cache can improve performance.

    Se a propriedade View for definida como Tile, o valor será automaticamente alterado para LargeIcon quando VirtualMode for definido como true.If the View property is set to Tile, the value will automatically be changed to LargeIcon when VirtualMode is set to true.

    No modo virtual, a coleção de Items está desabilitada.In virtual mode, the Items collection is disabled. A tentativa de acessá-lo resulta em um InvalidOperationException.Attempting to access it results in an InvalidOperationException. O mesmo é verdadeiro para a coleção de CheckedItems e a coleção de SelectedItems.The same is true of the CheckedItems collection and the SelectedItems collection. Se você quiser recuperar os itens selecionados ou marcados, use as coleções SelectedIndices e CheckedIndices em vez disso.If you want to retrieve the selected or checked items, use the SelectedIndices and CheckedIndices collections instead.

    Aplica-se a