It’s not just variables that can be made reactive — in Svelte, we can also make properties of classes reactive.
Let’s make the width
and height
properties of our Box
class reactive:
App
class Box {
width = $state(0);
height = $state(0);
area = 0;
// ...
}
Now, when we interact with the range inputs or click the ‘embiggen’ button, the box reacts.
We can also use $derived
, so that box.area
updates reactively:
App
class Box {
width = $state(0);
height = $state(0);
area = $derived(this.width * this.height);
// ...
}
In addition to
$state
and$derived
, you can use$state.raw
and$derived.by
to define reactive fields.
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<script>
const MAX_SIZE = 200;
class Box {
width = 0;
height = 0;
area = 0;
constructor(width, height) {
this.width = width;
this.height = height;
}
embiggen(amount) {
this.width += amount;
this.height += amount;
}
}
const box = new Box(100, 100);
</script>
<label>
<input type="range" bind:value={box.width} min={0} max={MAX_SIZE} />
{box.width}
</label>
<label>
<input type="range" bind:value={box.height} min={0} max={MAX_SIZE} />
{box.height}
</label>
<button onclick={() => box.embiggen(10)}>embiggen</button>
<hr>
<div
class="box"
style:width="{box.width}px"
style:height="{box.height}px"
>
{box.area}
</div>
<style>
label {
display: flex;
align-items: center;
}
hr {
margin: 1em 0;
border: none;
border-bottom: 1px solid #888;
}
.box {
background: radial-gradient(at 25% 25%, hsl(15 100 60), hsl(15 100 50)) ;
border-radius: 2px;
filter: drop-shadow(0 0 10px hsl(15 100 50 / 0.3));
display: flex;
align-items: center;
justify-content: center;
overflow: hidden;
}
</style>