What's a simple ParentAdapter implementation look like? - wpf

Question

I'm trying to write a ParentAdapter implementation; I'm interested in providing design-time support for some WPF controls I'm writing and this is how you manage custom logic for reparenting items to different container controls. I started small, with the notion of creating a StackPanel-derived class that would only allow Button elements to be parented at design-time (yes, I'm aware the panel itself needs code to support this as well.) I started with what I figured would be the simplest the ParentAdapter could be:
using System;
using System.Windows;
using System.Windows.Controls;
using Microsoft.Windows.Design.Interaction;
using Microsoft.Windows.Design.Model;
namespace ControlLibrary.Design
{
internal class SimplePanelParentAdapter : ParentAdapter
{
public override bool CanParent(ModelItem parent, Type childType)
{
return (childType == typeof(Button));
}
// moves the child item into the target panel; in this case a SimplePanel
public override void Parent(ModelItem newParent, ModelItem child)
{
using (ModelEditingScope undoContext = newParent.BeginEdit())
{
// is this correct?
//child.Content.SetValue("I'm in a custom panel!");
SimplePanel pnl = newParent.GetCurrentValue() as SimplePanel;
pnl.Children.Add(child.GetCurrentValue() as UIElement);
undoContext.Complete();
}
}
public override void RemoveParent(ModelItem currentParent, ModelItem newParent, ModelItem child)
{
// No special things need to be done, right?
child.Content.SetValue("I was in a custom panel.");
}
}
}
When I work with this at design-time, as soon as I drag a button over my custom panel, a NullReferenceException is thrown from deep within the VS code. My code is not throwing the exception, because I can step all the way through my method; the call stack indicates that code in Microsoft.Windows.Design.Developer.dll is throwing the exception.
Obviously I'm doing something incorrectly, but the documentation provides no examples and my search-fu seems to indicate that either no one is trying this or anyone who is trying it isn't talking about it. Does anyone have suggestions?

Answer 1

I found the answer to my question myself. The problem is caused by editing the model instead of the ModelItem wrapper. What I should have done (and does work) is something like this:
using System;
using System.Windows.Controls;
using Microsoft.Windows.Design.Interaction;
using Microsoft.Windows.Design.Model;
namespace ControlLibrary.Design
{
internal class SimplePanelParentAdapter : ParentAdapter
{
public override bool CanParent(ModelItem parent, Type childType)
{
return (childType == typeof(Button));
}
// moves the child item into the target panel; in this case a SimplePanel
public override void Parent(ModelItem newParent, ModelItem child)
{
using (ModelEditingScope undoContext = newParent.BeginEdit())
{
ModelProperty prop = newParent.Properties["Children"];
ModelItemCollection items = (ModelItemCollection)prop.Value;
items.Add(child);
undoContext.Complete();
}
}
public override void RemoveParent(ModelItem currentParent, ModelItem newParent, ModelItem child)
{
using (ModelEditingScope scope = child.BeginEdit())
{
ModelProperty prop = currentParent.Properties["Children"];
((ModelItemCollection)prop.Value).Remove(child);
scope.Complete();
}
}
}
}
I was confused when I wrote the first code and unsure how I was supposed to call Add() on the Children property; it looks like ModelProperty.Value wraps collections with ModelItemCollection, so unless you go out of your way to make your class use an obtuse interface this should work.