readonly this 绑定适用于每个元素（和组件），并允许你获取对渲染元素的引用。 例如，我们可以获得对 <canvas> 元素的引用：

英The readonly this binding applies to every element (and component) and allows you to obtain a reference to rendered elements. For example, we can get a reference to a <canvas> element:

<canvas bind:this={canvas} width={32} height={32} />

请注意，在组件安装之前，canvas 的值将是 undefined，因此我们将逻辑放在 onMount 生命周期函数 中。

英Note that the value of canvas will be undefined until the component has mounted, so we put the logic inside the onMount lifecycle function.