Databinding with kind of join over two lists - wpf

Question

I am familiar with the basics of databinding in wpf. However I now have a problem which I wonder how to solve.
Imagine following use case:
I have a global ObservableCollection called "AItems" of Type A.
I have some Objects of Type B and each has a ObservableCollection "BItems" of type A.
The BItems Collections can contain Objects of the global AItems Collection.
I want to visualize this by a ListView.
Each line should contain an A-Object and a checkbox.
I want the ListView to show all elements of the AItems-Collection. Items which are assigned to the B-Object should be marked with a checked checkbox. All other checkboxes should be unchecked.
My questions are now:
How should I set the datacontext?
How can I make that checking a checkbox inserts its item to the BItems-Collection and unchecking removes it?
I hope anyone can understand my problem.
Thanks for replies.

Answer 1

I'm not clear on the latter part of your question. Partly it's because your naming convention is confusing; I'd expect a collection named BItems to contain objects of type B, not A.
So I'm going to change your nomenclature a bit so that I don't get confused. Instead of A, I'll call the first class User, and instead of B, I'll call the second class Group. A Group contains a collection of User objects, named Users. The global collections look like this:
List<User> Users;
List<Group> Groups;
It's easy to determine if a given User u is in any group:
return Groups.Where(g => g.Users.Contains(u)).Any();
Easy, but computationally expensive if you have many groups and they contain many users. We'll get back to that in a second.
Right away, I see that one of your questions has got a problem:
How can I make that checking a checkbox inserts its item to the BItems-Collection and unchecking removes it?
What should happen if I check an unchecked user? Which group (or groups, since more than one group can contain a user) should it be added to?
Since you say that you want checked items to be "assigned to the B-Object", I'm going to assume that the UI is only looking at one group at a time - we'll call it the SelectedGroup. This is good, because g.Users.Contains(u) is much less expensive than the query I showed above.
If this is so, what you need to do is wrap your User in a class that exposes an IsChecked property. I'd call this class UserViewModel, since that's what it is. The class needs three properties (at a minimum):
public User User { get; set; }
public Group SelectedGroup { get; set; }
public bool IsChecked
{
get { return SelectedGroup.Users.Contains(this.User); }
set
{
if (value != IsChecked)
{
if (IsChecked)
{
SelectedGroup.Users.Remove(this.User);
}
else
{
SelectedGroup.Users.Add(this.User);
}
}
}
}
Your ListView is bound to an ObservableCollection<UserViewModel> named, say, UserViewModels. Whenever SelectedGroup is set, you need to rebuild this collection:
UserViewModels = new ObservableCollection<UserViewModel>(
Users.Select(u => new UserViewModel { User=u, SelectedGroup=SelectedGroup }));
You could avoid rebuilding the collection by implementing INotifyPropertyChanged in the UserViewModel class, and having it raise PropertyChanged for the IsChecked property whenever SelectedGroup changes.
Also, it would probably be responsible to include null-reference checking in the IsChecked property, so that the program doesn't throw an exception if SelectedGroup or SelectedGroup.Users is null.

Answer 2

You can bind a list box to Aitems and using a converter to set the isChecked property
<ListBox ItemsSource="{Binding AItems}">
<ListBox.ItemTemplate>
<DataTemplate>
<CheckBox Content="{Binding}" IsChecked="{Binding Mode=OneTime, Converter={StaticResource BItemCheckConverter}}"></CheckBox>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
public class BItemCheckConverter : IValueConverter
{
public List<Aitems> BItems { get; set; }
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
if (BItems.Contains((value as Aitems)) return true;
else return false
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}
I don't know how's your object model, so take the above code as is.
HTH