ListView.VirtualMode Właściwość

Definicja

Pobiera lub ustawia wartość wskazującą, czy podano własne operacje zarządzania danymi dla ListView kontrolki.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

Wartość właściwości

Boolean

trueListVieww przypadku korzystania z operacji zarządzania danymi, które zapewniasz; w przeciwnym razie false .true if ListView uses data-management operations that you provide; otherwise, false. Wartość domyślna to false.The default is false.

Wyjątki

VirtualMode jest ustawiony na true i jeden z następujących warunków:VirtualMode is set to true and one of the following conditions exist:

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

  • Items, CheckedItems, or SelectedItems contains items.
  • -lub--or-

    -Zmiany są wprowadzane do Items .- Edits are made to Items.

    Przykłady

    Ten przykład ilustruje prostą ListView , której zawartość to pierwsze 10000 kwadratów.This example illustrates a simple ListView whose contents are the first ten thousand squares. Obsługuje wyszukiwanie i używa pamięci podręcznej w celu zwiększenia wydajności.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
    

    Uwagi

    Ustawianie VirtualMode właściwości w celu true umieszczania w ListView trybie wirtualnym.Setting the VirtualMode property to true puts the ListView into virtual mode. W trybie wirtualnym normalna Items kolekcja jest nieużywana.In Virtual mode, the normal Items collection is unused. Zamiast tego ListViewItem obiekty są tworzone dynamicznie, zgodnie z ListView ich wymaganiami.Instead, ListViewItem objects are created dynamically as the ListView requires them.

    Tryb wirtualny może być przydatny w wielu sytuacjach.Virtual mode can be useful under many circumstances. Jeśli ListView obiekt musi zostać wypełniony z bardzo dużej kolekcji już znajdującej się w pamięci, utworzenie ListViewItem obiektu dla każdego wpisu może być wasteful.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. W trybie wirtualnym są tworzone tylko wymagane elementy.In virtual mode, only the items required are created. W innych przypadkach wartości ListViewItem obiektów mogą wymagać ponownego obliczenia, a wykonanie tej operacji dla całej kolekcji spowoduje nieakceptowalną wydajność.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. W trybie wirtualnym są obliczane tylko wymagane elementy.In virtual mode, only the required items are calculated.

    Aby można było używać trybu wirtualnego, należy obsłużyć RetrieveVirtualItem zdarzenie, które jest wywoływane za każdym razem, gdy ListView wymaga elementu.In order to use virtual mode, you must handle the RetrieveVirtualItem event, which is raised every time the ListView requires an item. Ten program obsługi zdarzeń powinien utworzyć ListViewItem obiekt, który należy do określonego indeksu.This event handler should create the ListViewItem object that belongs at the specified index. Ponadto VirtualListSize Właściwość musi być ustawiona na rozmiar listy wirtualnej.In addition, the VirtualListSize property must be set to the size of the virtual list.

    Obsługa SearchForVirtualItem zdarzenia umożliwia wyszukiwanie w trybie wirtualnym.Handling the SearchForVirtualItem event enables searching in virtual mode. Jeśli to zdarzenie nie jest obsługiwane, FindItemWithText metody i FindNearestItem zostaną zwrócone null .If this event is not handled, the FindItemWithText and FindNearestItem methods will return null.

    Możesz obsłużyć CacheVirtualItems zdarzenie, aby zachować pamięć podręczną ListViewItem obiektów.You can handle the CacheVirtualItems event in order to maintain a cache of ListViewItem objects. Jeśli Obliczanie lub wyszukiwanie w celu utworzenia ListViewItem obiektu jest kosztowne, utrzymywanie pamięci podręcznej może zwiększyć wydajność.If the calculation or lookup to create a ListViewItem object is expensive, maintaining a cache can improve performance.

    Jeśli View Właściwość jest ustawiona na Tile , wartość zostanie automatycznie zmieniona na, LargeIcon gdy VirtualMode jest ustawiona na true .If the View property is set to Tile, the value will automatically be changed to LargeIcon when VirtualMode is set to true.

    W trybie wirtualnym Items kolekcja jest wyłączona.In virtual mode, the Items collection is disabled. Próba uzyskania dostępu do niego skutkuje InvalidOperationException .Attempting to access it results in an InvalidOperationException. Ta sama wartość dotyczy CheckedItems kolekcji i SelectedItems kolekcji.The same is true of the CheckedItems collection and the SelectedItems collection. Jeśli chcesz pobrać wybrane lub zaznaczone elementy, użyj SelectedIndices CheckedIndices zamiast nich kolekcji i.If you want to retrieve the selected or checked items, use the SelectedIndices and CheckedIndices collections instead.

    Dotyczy