如何：重写逻辑树

资深的控件创作者可以选择重写逻辑树，但这在大多数情况下都不是必需的。

示例

本示例介绍如何创建 StackPanel 子类以重写逻辑树，在本示例中重写逻辑树是为了强制该面板只能具有并且仅仅呈现一个子元素的行为。 这不一定是实际需要的行为，但在这里进行演示是为了举例说明重写元素的正常逻辑树的情况。

    Public Class SingletonPanel
        Inherits StackPanel
        'Private _children As UIElementCollection 
        Private _child As FrameworkElement

        Public Sub New()

        End Sub

        Public Property SingleChild() As FrameworkElement

            Get
                Return _child
            End Get
            Set(ByVal value As FrameworkElement)
                If value Is Nothing Then
                     RemoveLogicalChild(_child)
                Else
                     If _child Is Nothing Then
                         _child = value
                     Else
                         ' raise an exception?
                         MessageBox.Show("Needs to be a single element")
                     End If
                End If
            End Set
        End Property
        Public Sub SetSingleChild(ByVal child As Object)
            Me.AddLogicalChild(child)
        End Sub

        Public Shadows Sub AddLogicalChild(ByVal child As Object)
            _child = CType(child, FrameworkElement)
            If Me.Children.Count = 1 Then
                Me.RemoveLogicalChild(Me.Children(0))
                Me.Children.Add(CType(child, UIElement))
            Else
                Me.Children.Add(CType(child, UIElement))
            End If
        End Sub

        Public Shadows Sub RemoveLogicalChild(ByVal child As Object)

            _child = Nothing
            Me.Children.Clear()
        End Sub
        Protected Overrides ReadOnly Property LogicalChildren() As IEnumerator
           Get
           ' cheat, make a list with one member and return the enumerator
           Dim _list As New ArrayList()
           _list.Add(_child)
           Return CType(_list.GetEnumerator(), IEnumerator)
           End Get
        End Property
    End Class

public class SingletonPanel : StackPanel
{
    //private UIElementCollection _children; 
    private FrameworkElement _child;

    public SingletonPanel() {

    }

    public FrameworkElement SingleChild
    {

        get { return _child;}
        set
        {
            if (value==null) {
                 RemoveLogicalChild(_child);
            } else {             
                 if (_child==null) {
                     _child = value;
                 } else {
                     // raise an exception?
                     MessageBox.Show("Needs to be a single element");
                 }
            }
        } 
    }
    public void SetSingleChild(object child)
    {
        this.AddLogicalChild(child);
    }

    public new void AddLogicalChild(object child)
    {
        _child = (FrameworkElement)child;
        if (this.Children.Count == 1)
        {
            this.RemoveLogicalChild(this.Children[0]);
            this.Children.Add((UIElement)child);
        }
        else
        {
            this.Children.Add((UIElement)child);
        }
    }

    public new void RemoveLogicalChild(object child)

    {
        _child = null;
        this.Children.Clear();
    }
    protected override IEnumerator LogicalChildren
    {
       get {
       // cheat, make a list with one member and return the enumerator
       ArrayList _list = new ArrayList();
       _list.Add(_child);
       return (IEnumerator) _list.GetEnumerator();}
    }
}

有关逻辑树的更多信息，请参见 WPF 中的树。