Como: Override the Logical Tree
Although it is not necessary in most cases, advanced control authors have the option to override the logical tree.
Exemplo
This example describes how to subclass StackPanel to override the logical tree, in this case to enforce a behavior that the panel may only have and will only render a single child element. This isn't necessarily a practically desirable behavior, but is shown here as a means of illustrating the scenario for overriding an element's normal logical tree.
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();}
}
}
For more information on the logical tree, see Árvores em WPF.