Finding a UIElement's true position when within a PivotItem - silverlight

Question

I'd like to get the on-screen position and visibility of a TextBlock which is embedded in a PivotItem - e.g. the on-screen position and visibility of TB2 in:
<phone:Pivot>
<phone:PivotItem Header="first">
<TextBlock x:Name="TB1" Text="Hello 1"></TextBlock>
</phone:PivotItem>
<phone:PivotItem Header="second" >
<TextBlock x:Name="TB2" Text="Hello 2"></TextBlock>
</phone:PivotItem>
<phone:PivotItem Header="three" >
<TextBlock x:Name="TB3" Text="Hello 3"></TextBlock>
</phone:PivotItem>
</phone:Pivot>
For other Xaml controls I have achieved this using code like:
public static Rect Position(this FrameworkElement element)
{
if (element.Visibility == Visibility.Collapsed)
return Rect.Empty;
if (element.Opacity < 0.01)
return Rect.Empty;
// Obtain transform information based off root element
GeneralTransform gt = element.TransformToVisual(Application.Current.RootVisual);
// Find the four corners of the element
Point topLeft = gt.Transform(new Point(0, 0));
Point topRight = gt.Transform(new Point(element.RenderSize.Width, 0));
Point bottomLeft = gt.Transform(new Point(0, element.RenderSize.Height));
Point bottomRight = gt.Transform(new Point(element.RenderSize.Width, element.RenderSize.Height));
var left = Math.Min(Math.Min(Math.Min(topLeft.X, topRight.X), bottomLeft.X), bottomRight.X);
var top = Math.Min(Math.Min(Math.Min(topLeft.Y, topRight.Y), bottomLeft.Y), bottomRight.Y);
var position = new Rect(left, top, element.ActualWidth, element.ActualHeight);
return position;
}
However, for PivotItem children this calculation doesn't provide the expected answers when the PivotItem is off-screen. Instead it provides the answer of the position the item will take on-screen when the PivotItem is swiped back on-screen. During a transition, I can see that the position is correctly calculated using the above method - e.g. during the transition I will see the position correctly swipe in from the left/right of the screen.
I've looked through other properties like Visible and Opacity to see if there is some other mechanism being used here to hide the off-screen PivotItem or one of its Parent, GrandParent, etc - but all of those seem to yield Visible and Opacity == 1.0 results.
I've also tried iterating the VisualTree to see if any element with a solid background is masking the "off-screen" PivotItems - but I can't see any in the tree.
Is there some other property to consider here? How are the PivotItem contents hidden when they are "off-screen"? If I really have to, I know that I can use the SelectedIndex property of the Pivot to try to help with the PivotItem position calculations, but I'm hoping to avoid SelectedIndex if I can - I'd prefer to get the position using general Xaml and VisualTree methods if at all possible.

Answer 1

I looked into it, but I can't find how they are hiding the PivotItems. I guess the position is correct, but the item is just not drawn for unknown (to me) reasons. I'll look into it again, later, but for now I wanted to mention a few other potential issues I noticed.
Shouldn't you be using Math.Max for the bottom right corner? Reason being - ActualHeight and ActualWidth are not rotation-aware so to speak.
RenderSize.Width/Height and ActualWidth/Height are different. I'm not sure it's a good idea to use both of them for the calculation.
Don't you need to check everything in the visual tree above the element to see if it's actually visible?
Edit: I looked at Microsoft.Phone.dll's decompiled source (that's the dll with the Pivot control), and I couldn't find anything about how the PivotItems are hidden. There are some native method calls though.

Answer 2

I have made some Changes to Your code and its giving UIElement's true position
XAML:
<Grid x:Name="LayoutRoot" Background="Transparent">
<!--Pivot Control-->
<phone:Pivot x:Name="MyPivot">
<phone:PivotItem Header="first" x:Name="first">
<TextBlock x:Name="TB1" FontSize="30" Text="Hello 1" Tap="TB1_Tap"></TextBlock>
</phone:PivotItem>
<phone:PivotItem Header="second" x:Name="second">
<TextBlock x:Name="TB2" FontSize="50" Height="66" Width="155" Margin="20" Text="Hello 2" Tap="TB2_Tap"></TextBlock>
</phone:PivotItem>
<phone:PivotItem Header="three" x:Name="three">
<TextBlock x:Name="TB3" Text="Hello 3"></TextBlock>
</phone:PivotItem>
</phone:Pivot>
</Grid>
CS:
public partial class PivotPage1 : PhoneApplicationPage
{
public PivotPage1()
{
InitializeComponent();
}
private void TB1_Tap(object sender, System.Windows.Input.GestureEventArgs e)
{
Rect r = Pos.MyPosition(TB1,first);
string str = "Left" + r.Left.ToString() + "\nTop:" + r.Top.ToString() + "\nRight:" + r.Right.ToString() + "\nBottom:" + r.Bottom.ToString() + "\nHeight:" + r.Height.ToString() + "\nWidth" + r.Width.ToString();
MessageBox.Show(str);
}
private void TB2_Tap(object sender, System.Windows.Input.GestureEventArgs e)
{
Rect r = Pos.MyPosition(TB2,second);
string str = "Left" + r.Left.ToString() + "\nTop:" + r.Top.ToString() + "\nRight:" + r.Right.ToString() + "\nBottom:" + r.Bottom.ToString() + "\nHeight:" + r.Height.ToString() + "\nWidth" + r.Width.ToString();
MessageBox.Show(str);
}
}
public static class Pos
{
public static Rect MyPosition(FrameworkElement child,FrameworkElement parent)
{
if (child.Visibility == Visibility.Collapsed)
return Rect.Empty;
if (child.Opacity < 0.01)
return Rect.Empty;
// Obtain transform information based off root child
GeneralTransform gt = child.TransformToVisual(parent);
// Find the four corners of the child
Point topLeft = gt.Transform(new Point(0, 0));
Point topRight = gt.Transform(new Point(child.RenderSize.Width, 0));
Point bottomLeft = gt.Transform(new Point(0, child.RenderSize.Height));
Point bottomRight = gt.Transform(new Point(child.RenderSize.Width, child.RenderSize.Height));
var left = Math.Min(Math.Min(Math.Min(topLeft.X, topRight.X), bottomLeft.X), bottomRight.X);
var top = Math.Min(Math.Min(Math.Min(topLeft.Y, topRight.Y), bottomLeft.Y), bottomRight.Y);
var position = new Rect(left, top, child.ActualWidth, child.ActualHeight);
return position;
}
}