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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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