It can be useful to know when transitions are beginning and ending. Svelte dispatches events that you can listen to like any other DOM event:

<p
  transition:fly={{ y: 200, duration: 2000 }}
  on:introstart={() => (status = 'intro started')}
  on:outrostart={() => (status = 'outro started')}
  on:introend={() => (status = 'intro ended')}
  on:outroend={() => (status = 'outro ended')}
>
  Flies in and out
</p>