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Sorry to ask just a wide question, but I am having trouble thinking how to approach this problem.

I have a many to many relationship:

Product 1 --- * ProductCategory * --- Category

Category also has:

Category 0..1 --- * Category (As SubCategories)

What I want to to see a tree view, with all categories, and be able to check a checkbox to create the ProductCategory entity. I do not how to approach this with MVVM.

enter image description here

In ASP.NET, when rendering the tree view I would check to see if the CategoryId of the node I was rendering was in my list of 'ProductCategory's if it was, I would check the box, and so setting up the initial state.

Then I would attach event handlers to the nodes (along with maybe some data) so that when the checkbox was checked/unchecked, it would add/remove the appropriate entity from my list.

The problem is I'm trying to move towards MVVM, and sometimes seeing how exactly it helps, and whilst some things a neater, doing anything complicated seems to be a pain! For example, my inital thought was to bind the top level Category (Root) to the tree, and render all entities using the HierarchicalDataTemplate:

                <sdk:TreeView Margin="0,3,30,3" ItemsSource="{Binding Categories}" Height="300">
                    <sdk:TreeView.ItemTemplate>
                        <sdk:HierarchicalDataTemplate ItemsSource="{Binding SubCategories}">
                            <StackPanel Orientation="Horizontal">
                                <CheckBox></CheckBox>
                                <TextBlock Text="{Binding Name}" HorizontalAlignment="Left"></TextBlock>
                            </StackPanel>
                        </sdk:HierarchicalDataTemplate>
                        <!---->
                    </sdk:TreeView.ItemTemplate>

The problem is that I cannot bind my checkbox to anything, because Category has no field I can use to attach it to a product...

Please can an MVVM guru throw some light on this, or should I simply go down the 'event' route?

Many Thanks.

James
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  • Hi, how about creating an IsSelected property for your Category, and bind the IsChecked of your CheckBox to it. Then you can just attach all the selected categories to your product? – Justin XL May 05 '11 at 11:26
  • I dont want to add things that are just for the View to my entities, this doesnt seem like an 'MVVM' way. – James May 05 '11 at 11:46
  • i think Xin means you could add that to you VM – aL3891 May 05 '11 at 16:06

1 Answers1

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One approach is to add a IsChecked property to you category viewmodel, then bind that to the checkbox. When its get set to true the viewmodel can make a call to where ever. It can for example add its own product name to a filter collection for another control.

One of the strong sides of the mvvm pattern is that the viewmodels can have references to each other or other things, and also have properties that you dont want to put in the actual entites

aL3891
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  • So instead of binding Category directly to the tree, I would create CategoryViewModels and bind those instead? How would I handle caching these objects, or would I create the view models (from the entities) each time the page is shown? Also, would I create this view model solely for use on this control? For instance, when editing a category (using the CategoryViewModel) the IsChecked flag would not make sense. – James May 06 '11 at 11:11
  • you would probably not cache the viewmodels, atleast not to begin with, you can always optimize later. yes, you probably would only use this viewmodel for this control, but isnt that usually the case? a 1-1 correlation between view and viewmodel. there might be other ways though :) – aL3891 May 07 '11 at 11:43