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This is invoked when the user takes an action that would cause the filter or command handler to close.
It is passed a Boolean parameter #CanBeCancelled indicating whether it is allowed to cancel the close request.
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A more complicated version could set #Allow to false if there was an error during the save, and in that case, the user would not go to where they clicked , (or the Framework would stay open if they were attempting to close it).
Also see
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