ZoomExtents on model changed

Hello,

My app is showing a 3d model which is loaded from xaml.

When my HelixViewport3D is Loaded (with event) I execute ZoomExtents, then my model is ocrrectly centred on display.

But my app is also able to change the model at runtime from Binding.
But I can not receive any event which told me when to ZoomExtent.
If I catch PropertyChange on CurrentModel, it seems that my ZoomExtents is done too early.

 <helix:HelixViewport3D x:Name="_helixView" Loaded="HelixViewLoaded" >
            <helix:DefaultLights/>
            <ModelVisual3D x:Name="_helixModel3D" Content="{Binding CurrentModel}"/>
...

What to do to ZoomExtent when a children change ?