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Part 2 / Motion / Springs

The spring function is an alternative to tweened that often works better for values that are frequently changing.

In this example we have two stores — one representing the circle's coordinates, and one representing its size. Let's convert them to springs:

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

	let coords = spring({ x: 50, y: 50 });
	let size = spring(10);
</script>

Both springs have default stiffness and damping values, which control the spring's, well... springiness. We can specify our own initial values:

App.svelte
let coords = spring({ x: 50, y: 50 }, {
	stiffness: 0.1,
	damping: 0.25
});

Waggle your mouse around, and try dragging the sliders to get a feel for how they affect the spring's behaviour. Notice that you can adjust the values while the spring is still in motion.

Next: Transitions

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<script>
	import { writable } from 'svelte/store';
 
	let coords = writable({ x: 50, y: 50 });
	let size = writable(10);
</script>
 
<svg
	on:mousemove={(e) => {
		coords.set({ x: e.clientX, y: e.clientY });
	}}
	on:mousedown={() => size.set(30)}
	on:mouseup={() => size.set(10)}
>
	<circle
		cx={$coords.x}
		cy={$coords.y}
		r={$size}
	/>
</svg>
 
<div class="controls">
	<label>
		<h3>stiffness ({coords.stiffness})</h3>
		<input
			bind:value={coords.stiffness}
			type="range"
			min="0.01"
			max="1"
			step="0.01"
		/>
	</label>
 
	<label>
		<h3>damping ({coords.damping})</h3>
		<input
			bind:value={coords.damping}
			type="range"
			min="0.01"
			max="1"
			step="0.01"
		/>
	</label>
</div>
 
<style>
	svg {
		position: absolute;
		width: 100%;
		height: 100%;
		left: 0;
		top: 0;
	}
 
	circle {
		fill: #ff3e00;
	}
 
	.controls {
		position: absolute;
		top: 1em;
		right: 1em;
		width: 200px;
		user-select: none;
	}
 
	.controls input {
		width: 100%;
	}
</style>
initialising