svelte/site/content/docs/03-runtime/03-svelte-motion.md

title
svelte/motion

The svelte/motion module exports two functions, tweened and spring, for creating writable stores whose values change over time after set and update, rather than immediately.

tweened

store = tweened(value: any, options)

Tweened stores update their values over a fixed duration. The following options are available:

• delay (number, default 0) — milliseconds before starting
• duration (number | function, default 400) — milliseconds the tween lasts
• easing (function, default t => t) — an easing function
• interpolate (function) — see below

store.set and store.update can accept a second options argument that will override the options passed in upon instantiation.

Both functions return a Promise that resolves when the tween completes. If the tween is interrupted, the promise will never resolve.

Out of the box, Svelte will interpolate between two numbers, two arrays or two objects (as long as the arrays and objects are the same 'shape', and their 'leaf' properties are also numbers).

<script>
	import { tweened } from 'svelte/motion';
	import { cubicOut } from 'svelte/easing';

	const size = tweened(1, {
		duration: 300,
		easing: cubicOut
	});

	function handleClick() {
		// this is equivalent to size.update(n => n + 1)
		$size += 1;
	}
</script>

<button on:click={handleClick} style="transform: scale({$size}); transform-origin: 0 0">
	embiggen
</button>

If the initial value is undefined or null, the first value change will take effect immediately. This is useful when you have tweened values that are based on props, and don't want any motion when the component first renders.

const size = tweened(undefined, {
	duration: 300,
	easing: cubicOut
});

$: $size = big ? 100 : 10;

The interpolate option allows you to tween between any arbitrary values. It must be an (a, b) => t => value function, where a is the starting value, b is the target value, t is a number between 0 and 1, and value is the result. For example, we can use the d3-interpolate package to smoothly interpolate between two colours.

<script>
	import { interpolateLab } from 'd3-interpolate';
	import { tweened } from 'svelte/motion';

	const colors = ['rgb(255, 62, 0)', 'rgb(64, 179, 255)', 'rgb(103, 103, 120)'];

	const color = tweened(colors[0], {
		duration: 800,
		interpolate: interpolateLab
	});
</script>

{#each colors as c}
	<button style="background-color: {c}; color: white; border: none;" on:click={(e) => color.set(c)}>
		{c}
	</button>
{/each}

<h1 style="color: {$color}">{$color}</h1>

spring

store = spring(value: any, options)

A spring store gradually changes to its target value based on its stiffness and damping parameters. Whereas tweened stores change their values over a fixed duration, spring stores change over a duration that is determined by their existing velocity, allowing for more natural-seeming motion in many situations. The following options are available:

• stiffness (number, default 0.15) — a value between 0 and 1 where higher means a 'tighter' spring
• damping (number, default 0.8) — a value between 0 and 1 where lower means a 'springier' spring
• precision (number, default 0.01) — determines the threshold at which the spring is considered to have 'settled', where lower means more precise

All of the options above can be changed while the spring is in motion, and will take immediate effect.

const size = spring(100);
size.stiffness = 0.3;
size.damping = 0.4;
size.precision = 0.005;

As with tweened stores, set and update return a Promise that resolves if the spring settles.

Both set and update can take a second argument — an object with hard or soft properties. { hard: true } sets the target value immediately; { soft: n } preserves existing momentum for n seconds before settling. { soft: true } is equivalent to { soft: 0.5 }.

const coords = spring({ x: 50, y: 50 });
// updates the value immediately
coords.set({ x: 100, y: 200 }, { hard: true });
// preserves existing momentum for 1s
coords.update(
	(target_coords, coords) => {
		return { x: target_coords.x, y: coords.y };
	},
	{ soft: 1 }
);

See a full example on the spring tutorial.

<script>
	import { spring } from 'svelte/motion';

	const coords = spring(
		{ x: 50, y: 50 },
		{
			stiffness: 0.1,
			damping: 0.25
		}
	);
</script>

If the initial value is undefined or null, the first value change will take effect immediately, just as with tweened values (see above).

const size = spring();
$: $size = big ? 100 : 10;