不仅仅是变量可以具有响应性 - 在 Svelte 中，我们还可以使类的属性具有响应性。
¥It’s not just variables that can be made reactive — in Svelte, we can also make properties of classes reactive.
让我们将 Box 类的 width 和 height 属性变为响应式：
¥Let’s make the width and height properties of our Box class reactive:
App
class Box {
width = $state(0);
height = $state(0);
area = 0;
// ...
}现在，当我们与范围输入交互或单击 ‘embiggen’ 按钮时，框会做出反应。
¥Now, when we interact with the range inputs or click the ‘embiggen’ button, the box reacts.
我们还可以使用 $derived，以便 box.area 被动更新：
¥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);
// ...
}除了
$state和$derived，你还可以使用$state.raw和$derived.by来定义反应字段。¥[!NOTE] In addition to
$stateand$derived, you can use$state.rawand$derived.byto 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>