The VisiblePeriodRange is updated when the dragging of the slider is completed and the mouse is up. However, RadTimeBar exposes a ActualVisibleRange property that holds the actual range of the slider and it is updated while dragging (panning, zooming) the slider. You can use it to achieve your requirement.
If you want to execute a logic when the property is changed you can create a property in your view model that holds the actual range and bind it in one way to source mode with the ActualVisiblePeriodRange of the time bar. Then call your logic in the custom property's setter.
If this does not help, can you describe what you want to achieve? This way we will better understand your scenario and we will be able to assist you further.
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