ListView.VirtualMode Property

Definition

Obtiene o establece un valor que indica si proporcionó sus propias operaciones de administración de datos para el control 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

Property Value

Boolean

true si ListView utiliza las operaciones de administración de datos proporcionadas; de lo contrario, false.true if ListView uses data-management operations that you provide; otherwise, false. El valor predeterminado es false.The default is false.

Exceptions

VirtualMode se ha establecido en true y hay una de las siguientes condiciones:VirtualMode is set to true and one of the following conditions exist:

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

  • Items, CheckedItems, or SelectedItems contains items.
  • O bien-or- - Se han realizado modificaciones en Items.- Edits are made to Items.

    Examples

    En este ejemplo se muestra un ListView sencillo cuyo contenido son los primeros 10000 cuadrados.This example illustrates a simple ListView whose contents are the first ten thousand squares. Controla la búsqueda y utiliza una memoria caché para mejorar el rendimiento.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
    

    Remarks

    Establecer la propiedad VirtualMode en true coloca el ListView en modo virtual.Setting the VirtualMode property to true puts the ListView into virtual mode. En el modo virtual, la colección de Items normal no se utiliza.In Virtual mode, the normal Items collection is unused. En su lugar, los objetos ListViewItem se crean dinámicamente a medida que la ListView los requiere.Instead, ListViewItem objects are created dynamically as the ListView requires them.

    El modo virtual puede ser útil en muchas circunstancias.Virtual mode can be useful under many circumstances. Si un objeto de ListView se debe rellenar a partir de una colección muy grande que ya está en la memoria, la creación de un objeto ListViewItem para cada entrada puede ser una pérdida de memoria.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. En el modo virtual, solo se crean los elementos necesarios.In virtual mode, only the items required are created. En otros casos, es posible que sea necesario volver a calcular los valores de los objetos ListViewItem con frecuencia y hacerlo para toda la colección produciría un rendimiento inaceptable.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. En el modo virtual, solo se calculan los elementos necesarios.In virtual mode, only the required items are calculated.

    Para poder usar el modo virtual, debe controlar el evento RetrieveVirtualItem, que se genera cada vez que el ListView requiere un elemento.In order to use virtual mode, you must handle the RetrieveVirtualItem event, which is raised every time the ListView requires an item. Este controlador de eventos debe crear el objeto ListViewItem que pertenece al índice especificado.This event handler should create the ListViewItem object that belongs at the specified index. Además, la propiedad VirtualListSize debe establecerse en el tamaño de la lista virtual.In addition, the VirtualListSize property must be set to the size of the virtual list.

    El control del evento de SearchForVirtualItem permite realizar búsquedas en el modo virtual.Handling the SearchForVirtualItem event enables searching in virtual mode. Si no se controla este evento, los métodos FindItemWithText y FindNearestItem devolverán null.If this event is not handled, the FindItemWithText and FindNearestItem methods will return null.

    Puede controlar el evento CacheVirtualItems para mantener una memoria caché de objetos de ListViewItem.You can handle the CacheVirtualItems event in order to maintain a cache of ListViewItem objects. Si el cálculo o la búsqueda para crear un objeto ListViewItem es caro, el mantenimiento de una memoria caché puede mejorar el rendimiento.If the calculation or lookup to create a ListViewItem object is expensive, maintaining a cache can improve performance.

    Si la propiedad View está establecida en Tile, el valor se cambiará automáticamente a LargeIcon cuando VirtualMode esté establecida en true.If the View property is set to Tile, the value will automatically be changed to LargeIcon when VirtualMode is set to true.

    En el modo virtual, la colección de Items está deshabilitada.In virtual mode, the Items collection is disabled. Al intentar obtener acceso a ella, se produce un InvalidOperationException.Attempting to access it results in an InvalidOperationException. Lo mismo se aplica a la colección de CheckedItems y la colección de SelectedItems.The same is true of the CheckedItems collection and the SelectedItems collection. Si desea recuperar los elementos seleccionados o activados, utilice en su lugar las colecciones SelectedIndices y CheckedIndices.If you want to retrieve the selected or checked items, use the SelectedIndices and CheckedIndices collections instead.

    Applies to