Part 3 / Transitions / Deferred transitions

A particularly powerful feature of Svelte's transition engine is the ability to defer transitions, so that they can be coordinated between multiple elements.

Take this pair of todo lists, in which toggling a todo sends it to the opposite list. In the real world, objects don't behave like that — instead of disappearing and reappearing in another place, they move through a series of intermediate positions. Using motion can go a long way towards helping users understand what's happening in your app.

We can achieve this effect using the crossfade function, as seen in transition.js, which creates a pair of transitions called send and receive. When an element is 'sent', it looks for a corresponding element being 'received', and generates a transition that transforms the element to its counterpart's position and fades it out. When an element is 'received', the reverse happens. If there is no counterpart, the fallback transition is used.

Open TodoList.svelte. First, import the send and receive transitions from transition.js:

<script>
	import { send, receive } from './transition.js';

	export let store;
	export let filter;
</script>

Then, add them to the <label> element, using the todo.id property as a key to match the elements:

<label
	in:receive={{ key: todo.id }}
	out:send={{ key: todo.id }}
>

Now, when you toggle items, they move smoothly to their new location. The non-transitioning items still jump around awkwardly — we can fix that in the next chapter.