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>