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Questions and answers for Concepts Element Tree
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2. Concepts Element Tree
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| 2.1 How can I
override the Logical Tree ?
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| 2.2 How can I
enumerate all the descendants of a visual object ?
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2.1 How can I override the Logical Tree ?
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Although it is not necessary in most cases, advanced control authors have the
option to override the logical tree. 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.
Normal logical tree can be overridden as follows :
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[C#]
public class SingletonPanel : StackPanel
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();}
}
}
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2.2 How can I enumerate all the descendants of a visual object ?
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If you want to serialize all the rendering information of a visual object
hierarchy, you have to enumerate all the descendants of a visual object as
follows :
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[C#]
// Enumerate all the descendants of the visual object.
static public void EnumVisual(Visual myVisual)
{
for (int i = 0; i < VisualTreeHelper.GetChildrenCount(myVisual); i++)
{
// Retrieve child visual at specified index value.
Visual childVisual = (Visual)VisualTreeHelper.GetChild(myVisual, i);
// Do processing of the child visual object.
// Enumerate children of the child visual object.
EnumVisual(childVisual);
}
}
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