You can inherit from PanAndZoomImage and override the OnZoomChanged method. It's a callback that is called whenever the Zoom property changes and it does so when the user pinches or sets it through code.
You can also take a less optimal approach and create a FrameworElement child class that has a Zoom property. Then you can bind that property to PanAndZoomImage a listen for property changes this way.
All the best,
the Telerik team
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