WPF ListView Virtualization Grouping

Question

¿Alguien sabe de una implementación de ListView que admita la virtualización de la interfaz de usuario cuando la agrupación está habilitada? Por defecto, VirtualizingStackPanel está deshabilitado cuando se configura la agrupación.

Parece que Microsoft no va a implementar esto en v4.0 de .NET Framework, así que busco soluciones alternativas.

Answer 1

Una opción es echar un vistazo serie de Bea Stollniz en mejorar el rendimiento de un TreeView: Part 1, Part 2 y Part 3. Si bien lo que hace está más orientado a TreeViews, que no tienen virtualización porque se agrupan de manera predeterminada, las lecciones aprendidas definitivamente podrían aplicarse a un ListView personalizado que tiene grupos de virtualización. De hecho, en la parte 3, usa un ListBox como base para crear el árbol de virtualización, que también es un buen comienzo para la agrupación virtualizada. Obviamente, la visualización de los elementos como en un TreeView tiene algunas diferencias, como la selección de los nodos del grupo, desde un ListView con agrupamiento, pero eso podría solucionarse capturando el SelectionChanged.

Answer 2

He localizado un ejemplo en Grouping and Virtualization MSDN Code Sample que convierte el ListView agrupado en una lista plana que admite la virtualización. Sin embargo, no puedo descifrar cómo imitar las acciones de expansión de los encabezados.

Answer 3

WPF/.net 4.5 ahora es compatible con este https://msdn.microsoft.com/en-us/library/system.windows.controls.virtualizingpanel.isvirtualizingwhengrouping(v=vs.110).aspx

Si todavía está apuntando 4.0 se puede establecer con la reflexión por lo que al menos algunos usuarios pueden obtener el beneficio.

Answer 4

Espero que no sea demasiado fuera de tema pero recientemente tuve un problema similar. Como se indicó anteriormente, es solo un problema de .NET 4.0. Incluso estoy de acuerdo en que, en la mayoría de los casos con el cuadro combinado, normalmente no debería necesitar virtualización porque no debería tener tantos elementos y si es necesario agruparlos, se debería implementar algún tipo de solución de detalle maestro. Pero puede haber algunas áreas grises.

El enlace proporcionado por Luke sobre Agrupamiento y virtualización en MSDN me ayudó mucho. En mi caso, ese fue el único enfoque que pude encontrar o encontrar en cualquier lugar que esté en la dirección que necesito. No es compatible con todas las funciones de ListViewCollection. Tuve que anular algunos métodos; de lo contrario, la selección de elementos no funcionaría correctamente. Obviamente hay más trabajo por hacer.

Así que aquí es una solución actualizada de FlatGroupListCollectionView de here:

/// <summary> 
///  Provides a view that flattens groups into a list 
///  This is used to avoid limitation that ListCollectionView has in .NET 4.0, if grouping is used then Virtialuzation would not work 
///  It assumes some appropriate impelmentation in view(XAML) in order to support this way of grouping 
///  Note: As implemented, it does not support nested grouping 
///  Note: Only overriden properties and method behaves correctly, some of methods and properties related to selection of item might not work as expected and would require new implementation 
/// </summary> 
public class FlatGroupListCollectionView : ListCollectionView 
{ 
    /// <summary> 
    /// Initializes a new instance of the <see cref="FlatGroupListCollectionView"/> class. 
    /// </summary> 
    /// <param name="list">A list used in this collection</param> 
    public FlatGroupListCollectionView(IList list) 
     : base(list) 
    { 
    } 

    /// <summary> 
    ///  This currently only supports one level of grouping 
    ///  Returns CollectionViewGroups if the index matches a header 
    ///  Otherwise, maps the index into the base range to get the actual item 
    /// </summary> 
    /// <param name="index">Index from which get an item</param> 
    /// <returns>Item that was found on given index</returns> 
    public override object GetItemAt(int index) 
    { 
     int delta = 0; 
     ReadOnlyObservableCollection<object> groups = this.BaseGroups; 
     if (groups != null) 
     { 
      int totalCount = 0; 
      for (int i = 0; i < groups.Count; i++) 
      { 
       CollectionViewGroup group = groups[i] as CollectionViewGroup; 
       if (group != null) 
       { 
        if (index == totalCount) 
        { 
         return group; 
        } 

        delta++; 
        int numInGroup = group.ItemCount; 
        totalCount += numInGroup + 1; 

        if (index < totalCount) 
        { 
         break; 
        } 
       } 
      } 
     } 

     object item = base.GetItemAt(index - delta); 
     return item; 
    } 

    /// <summary> 
    ///  In the flat list, the base count is incremented by the number of groups since there are that many headers 
    ///  To support nested groups, the nested groups must also be counted and added to the count 
    /// </summary> 
    public override int Count 
    { 
     get 
     { 
      int count = base.Count; 

      if (this.BaseGroups != null) 
      { 
       count += this.BaseGroups.Count; 
      } 

      return count; 
     } 
    } 

    /// <summary> 
    ///  By returning null, we trick the generator into thinking that we are not grouping 
    ///  Thus, we avoid the default grouping code 
    /// </summary> 
    public override ReadOnlyObservableCollection<object> Groups 
    { 
     get 
     { 
      return null; 
     } 
    } 

    /// <summary> 
    ///  Gets the Groups collection from the base class 
    /// </summary> 
    private ReadOnlyObservableCollection<object> BaseGroups 
    { 
     get 
     { 
      return base.Groups; 
     } 
    } 

    /// <summary> 
    ///  DetectGroupHeaders is a way to get access to the containers by setting the value to true in the container style 
    ///  That way, the change handler can hook up to the container and provide a value for IsHeader 
    /// </summary> 
    public static readonly DependencyProperty DetectGroupHeadersProperty = 
     DependencyProperty.RegisterAttached("DetectGroupHeaders", typeof(bool), typeof(FlatGroupListCollectionView), new FrameworkPropertyMetadata(false, OnDetectGroupHeaders)); 

    /// <summary> 
    /// Gets the Detect Group Headers property 
    /// </summary> 
    /// <param name="obj">Dependency Object from which the property is get</param> 
    /// <returns>Value of Detect Group Headers property</returns> 
    public static bool GetDetectGroupHeaders(DependencyObject obj) 
    { 
     return (bool)obj.GetValue(DetectGroupHeadersProperty); 
    } 

    /// <summary> 
    /// Sets the Detect Group Headers property 
    /// </summary> 
    /// <param name="obj">Dependency Object on which the property is set</param> 
    /// <param name="value">Value to set to property</param> 
    public static void SetDetectGroupHeaders(DependencyObject obj, bool value) 
    { 
     obj.SetValue(DetectGroupHeadersProperty, value); 
    } 

    /// <summary> 
    ///  IsHeader can be used to style the container differently when it is a header 
    ///  For instance, it can be disabled to prevent selection 
    /// </summary> 
    public static readonly DependencyProperty IsHeaderProperty = 
     DependencyProperty.RegisterAttached("IsHeader", typeof(bool), typeof(FlatGroupListCollectionView), new FrameworkPropertyMetadata(false)); 

    /// <summary> 
    /// Gets the Is Header property 
    /// </summary> 
    /// <param name="obj">Dependency Object from which the property is get</param> 
    /// <returns>Value of Is Header property</returns> 
    public static bool GetIsHeader(DependencyObject obj) 
    { 
     return (bool)obj.GetValue(IsHeaderProperty); 
    } 

    /// <summary> 
    /// Sets the Is Header property 
    /// </summary> 
    /// <param name="obj">Dependency Object on which the property is set</param> 
    /// <param name="value">Value to set to property</param> 
    public static void SetIsHeader(DependencyObject obj, bool value) 
    { 
     obj.SetValue(IsHeaderProperty, value); 
    } 

    /// <summary> 
    /// Raises the System.Windows.Data.CollectionView.CollectionChanged event. 
    /// </summary> 
    /// <param name="args">The System.Collections.Specialized.NotifyCollectionChangedEventArgs object to pass to the event handler</param> 
    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs args) 
    { 
     switch (args.Action) 
     { 
      case NotifyCollectionChangedAction.Add: 
       { 
        int flatIndex = this.ConvertFromItemToFlat(args.NewStartingIndex, false); 
        int headerIndex = Math.Max(0, flatIndex - 1); 
        object o = this.GetItemAt(headerIndex); 
        CollectionViewGroup group = o as CollectionViewGroup; 
        if ((group != null) && (group.ItemCount == args.NewItems.Count)) 
        { 
         // Notify that a header was added 
         base.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, new object[] { group }, headerIndex)); 
        } 

        base.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, args.NewItems, flatIndex)); 
       } 

       break; 

      case NotifyCollectionChangedAction.Remove: 
       // TODO: Implement this action 
       break; 

      case NotifyCollectionChangedAction.Move: 
       // TODO: Implement this action 
       break; 

      case NotifyCollectionChangedAction.Replace: 
       // TODO: Implement this action 
       break; 

      default: 
       base.OnCollectionChanged(args); 
       break; 
     } 
    } 

    /// <summary> 
    /// Sets the specified item to be the System.Windows.Data.CollectionView.CurrentItem in the view 
    /// This is an override of base method, an item index is get first and its needed to convert that index to flat version which includes groups 
    /// Then adjusted version of MoveCurrentToPosition base method is called 
    /// </summary> 
    /// <param name="item">The item to set as the System.Windows.Data.CollectionView.CurrentItem</param> 
    /// <returns>true if the resulting System.Windows.Data.CollectionView.CurrentItem is within the view; otherwise, false</returns> 
    public override bool MoveCurrentTo(object item) 
    { 
     int index = this.IndexOf(item); 

     int newIndex = this.ConvertFromItemToFlat(index, false); 

     return this.MoveCurrentToPositionBase(newIndex); 
    } 

    /// <summary> 
    /// Sets the item at the specified index to be the System.Windows.Data.CollectionView.CurrentItem in the view 
    /// This is an override of base method, Its called when user selects new item from this collection 
    /// A delta is get of which is the possition shifted because of groups and we shift this position by this delta and then base method is called 
    /// </summary> 
    /// <param name="position">The index to set the System.Windows.Data.CollectionView.CurrentItem to</param> 
    /// <returns>true if the resulting System.Windows.Data.CollectionView.CurrentItem is an item within the view; otherwise, false</returns> 
    public override bool MoveCurrentToPosition(int position) 
    { 
     int delta = this.GetDelta(position); 

     int newPosition = position - delta; 

     return base.MoveCurrentToPosition(newPosition); 
    } 

    private static void OnDetectGroupHeaders(DependencyObject d, DependencyPropertyChangedEventArgs e) 
    { 
     // This assumes that a container will not change between being a header and not 
     // If using ContainerRecycling this may not be the case 
     ((FrameworkElement)d).Loaded += OnContainerLoaded; 
    } 

    private static void OnContainerLoaded(object sender, RoutedEventArgs e) 
    { 
     FrameworkElement element = (FrameworkElement)sender; 
     element.Loaded -= OnContainerLoaded; // If recycling, remove this line 

     // CollectionViewGroup is the type of the header in this sample 
     // Add more types or change the type as necessary 
     if (element.DataContext is CollectionViewGroup) 
     { 
      SetIsHeader(element, true); 
     } 
    } 

    private int ConvertFromItemToFlat(int index, bool removed) 
    { 
     ReadOnlyObservableCollection<object> groups = this.BaseGroups; 
     if (groups != null) 
     { 
      int start = 1; 
      for (int i = 0; i < groups.Count; i++) 
      { 
       CollectionViewGroup group = groups[i] as CollectionViewGroup; 
       if (group != null) 
       { 
        index++; 
        int end = start + group.ItemCount; 

        if ((start <= index) && ((!removed && index < end) || (removed && index <= end))) 
        { 
         break; 
        } 

        start = end + 1; 
       } 
      } 
     } 

     return index; 
    } 

    /// <summary> 
    /// Move <seealso cref="CollectionView.CurrentItem"/> to the item at the given index. 
    /// This is a replacement for base method 
    /// </summary> 
    /// <param name="position">Move CurrentItem to this index</param> 
    /// <returns>true if <seealso cref="CollectionView.CurrentItem"/> points to an item within the view.</returns> 
    private bool MoveCurrentToPositionBase(int position) 
    { 
     // VerifyRefreshNotDeferred was removed 
     bool result = false; 

     // Instead of property InternalCount we use Count property 
     if (position < -1 || position > this.Count) 
     { 
      throw new ArgumentOutOfRangeException("position"); 
     } 

     if (position != this.CurrentPosition || !this.IsCurrentInSync) 
     { 
      // Instead of property InternalCount we use Count property from this class 
      // Instead of InternalItemAt we use GetItemAt from this class 
      object proposedCurrentItem = (0 <= position && position < this.Count) ? this.GetItemAt(position) : null; 

      // ignore moves to the placeholder 
      if (proposedCurrentItem != CollectionView.NewItemPlaceholder) 
      { 
       if (this.OKToChangeCurrent()) 
       { 
        bool oldIsCurrentAfterLast = this.IsCurrentAfterLast; 
        bool oldIsCurrentBeforeFirst = this.IsCurrentBeforeFirst; 

        this.SetCurrent(proposedCurrentItem, position); 

        this.OnCurrentChanged(); 

        // notify that the properties have changed. 
        if (this.IsCurrentAfterLast != oldIsCurrentAfterLast) 
        { 
         this.OnPropertyChanged(PropertySupport.ExtractPropertyName(() => this.IsCurrentAfterLast)); 
        } 

        if (this.IsCurrentBeforeFirst != oldIsCurrentBeforeFirst) 
        { 
         this.OnPropertyChanged(PropertySupport.ExtractPropertyName(() => this.IsCurrentBeforeFirst)); 
        } 

        this.OnPropertyChanged(PropertySupport.ExtractPropertyName(() => this.CurrentPosition)); 
        this.OnPropertyChanged(PropertySupport.ExtractPropertyName(() => this.CurrentItem)); 

        result = true; 
       } 
      } 
     } 

     // Instead of IsCurrentInView we return result 
     return result; 
    } 

    private int GetDelta(int index) 
    { 
     int delta = 0; 
     ReadOnlyObservableCollection<object> groups = this.BaseGroups; 
     if (groups != null) 
     { 
      int totalCount = 0; 
      for (int i = 0; i < groups.Count; i++) 
      { 
       CollectionViewGroup group = groups[i] as CollectionViewGroup; 
       if (group != null) 
       { 
        if (index == totalCount) 
        { 
         break; 
        } 

        delta++; 
        int numInGroup = group.ItemCount; 
        totalCount += numInGroup + 1; 

        if (index < totalCount) 
        { 
         break; 
        } 
       } 
      } 
     } 

     return delta; 
    } 

    /// <summary> 
    /// Helper to raise a PropertyChanged event 
    /// </summary> 
    /// <param name="propertyName">Name of the property</param> 
    private void OnPropertyChanged(string propertyName) 
    { 
     base.OnPropertyChanged(new PropertyChangedEventArgs(propertyName)); 
    } 
} 

parte XAML se queda como está en el código de ejemplo. El modelo View también se mantiene como está, lo que significa usar FlatGroupListCollectionView y configurar GroupDescriptions.

Prefiero esta solución porque separa la lógica de agrupación de mi lista de datos en el modelo de vista. Otra solución sería implementar el soporte de la agrupación en la lista original de elementos en el modelo de vista, lo que significa de alguna manera identificar los encabezados. Para un uso único, debería estar bien, pero podría ser necesario volver a crear la colección con el propósito de agrupar diferentes o no, lo que no es tan agradable.