Pass control to binding by value - wpf

Question

I have a converter that I use to determine the size of a child element based on the size of its container (parent).
<Setter Property="Width" Value="{Binding ActualWidth, ElementName=rowsContainer,
Converter={StaticResource sizer}}"/>
public class CellSizeConverter : IValueConverter
{
private readonly int _maxNum = 7;
private readonly int _margin = 0;
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value == null)
{
return null;
}
double width = (double)value;
return (width / _maxNum) - _margin;
}
}
This works, and when I resize my window, the size of the components change. Hooray!
I want to factor in the height as well, so I've tried passing in the control itself and getting its properties inside the converter:
<Setter Property="Width" Value="{Binding ElementName=rowsContainer, Converter={StaticResource sizer}}"/>
public class CellSizeConverter : IValueConverter
{
private readonly int _maxNum = 7;
private readonly int _margin = 0;
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value == null)
{
return null;
}
ItemsControl rowsContainer = (ItemsControl)value;
return (rowsContainer.ActualWidth / _maxNum) - _margin;
}
}
However, when I do this the size never changes. It seems like the control gets passed to the converter once but never updates. I imagine it has something to do with passing by reference rather than passing by value. Or something like that.
I know I can pass multiple values into the converter using a MultiBinding and an IMultiValue Converter but since all that values I'd want are already wrapped up in a nice little object (the control) it would be so much cleaner in my xaml to just bind it this way.
How can I pass the control such that it updates?

Answer 1

The converter only gets called when the property that you actually bind to is set to a new value.
This means that you cannot bind to the rowsContainer control itself as it never gets when the ActualWidth or ActualHeight property is updated.
A MultiBinding to both ActualWidth or ActualHeight is the way to go here.
How can I pass the control such that it updates?
You can't for the reason already mentioned.

Answer 2

It seems like the control gets passed to the converter once but never updates. I imagine it has something to do with passing by reference rather than passing by value.
Property updates in WPF aren't magic, code has to explicitly call into WPF to tell it a value has been updated.
When you bind ActualWidth, that's actually a dependency property that has a changed event WPF can hook into and be notified by the code changing it. However, when you bind the control itself, that is not a dependency property, and the control doesn't implement INotifyPropertyChanged, so WPF never knows its properties have changed.
I know I can pass multiple values into the converter using a MultiBinding and an IMultiValue Converter but [...]
A MultiBinding allows you to bind to multiple dependency properties and have WPF listen to all their property change notifications at once. I stopped the quote at "but" because it doesn't matter what comes after, that's what you need to do.