{#if x}
	{#each things as thing (thing)}
		<div transition:foo></div>
	{/each}
{/if}

<script>
	export default {
		transitions: {
			foo(node, params) {
				return {
					duration: 100,
					tick: t => {
						node.foo = t;
					}
				};
			}
		}
	};
</script>