How to change the width of a control dynamically in UWP？ - xaml

Question

Tabbar is a very common navigation control on iOS and Android. But UWP doesn't seem to have.
I've seen an example XamlPivot(SHORTCUT)①, Use Pivot to make TabBar, The effect is very good, and I tried to modify it, so that the TabBar in the bottom, content in the upper.
My project is MasterDetail, Master part is TabBar(i.e TabsStyle Pivot), Detail part just a blank.
Now I found a big problem, TabBar each item does not automatically divide the width, then I try to use data binding and value converters to dynamically provide the width, the binding source is the ActuallyWidth of the MasterGrid, but the ActuallyWidth does not change with the window size, and when the Window on WideState, the Mater part will become blank.
So, How to change the width of the TabBarItem dynamically?
Various window size effect chart(Remove"()"):
(https:)//i.stack.imgur.com/3GE5t.png
(https:)//i.stack.imgur.com/FyQuX.png
(https:)//i.stack.imgur.com/pChwz.png
(https:)//i.stack.imgur.com/cib1l.png
XAML:
<Pivot x:Name="pivot"
Style="{StaticResource TabsStylePivotStyle}">
<PivotItem>
<PivotItem.Header>
<local:TabHeader Width="{Binding ActualWidth, Converter={StaticResource AutoWidthConverter}, ElementName=pivot, Mode=OneWay}"
Glyph=""
Label="item 1" />
</PivotItem.Header>
<TextBlock Text="Content content content" />
</PivotItem>
<PivotItem>
<PivotItem.Header>
<local:TabHeader Width="{Binding ActualWidth, Converter={StaticResource AutoWidthConverter}, ElementName=pivot, Mode=OneWay}"
Glyph=""
Label="item 2" />
</PivotItem.Header>
<TextBlock Text="Content content content" />
</PivotItem>
<PivotItem>
<PivotItem.Header>
<local:TabHeader Width="{Binding ActualWidth, Converter={StaticResource AutoWidthConverter}, ElementName=pivot, Mode=OneWay}"
Glyph=""
Label="item 3" />
</PivotItem.Header>
<TextBlock Text="Content content content" />
</PivotItem>
</Pivot>
Converter:
public object Convert(object value, Type targetType, object parameter, string language)
{
return (double)value / 3;
}

Answer 1

For the record, ActualWidth is NOT a DependencyProperty in UWP XAML model - so can't participate in bindings properly (it doesn't notify of changes).
So if you wish to do a binding like you're doing, you're going to need to expose ActualWidth in a bindable way. One of the easier ways of doing so is to a create a Behaviour explicitly for that, that attaches to the SizeChange event of the target element (in your case the pivot), and returns it's ActualWidth / ActualHeight / RenderSize as DependencyProperties on the behaviour. Your TabItems would then look to the ActualWidth on the behaviour instead.
(It's not done by default presumably because UWP XAML doesn't have read-only dependency property support, and binding too it can easily lead to circular layout cycles when layout rounding is in play if you're not careful)