beforeUpdate 函数负责的是在 DOM 更新前发生什么，afterUpdate 则负责更新后发生什么。两个函数用来在 DOM 与数据同步时运行一些代码。
对于一些很难通过纯粹的状态驱动的方式执行的任务，这两个函数会很有用。比如更新一个元素的滚动条位置。
这个 Eliza 聊天机器人用起来有点烦人，因为你每次都需要手动滚动聊天窗口。让我们解决这个问题。
App.svelte
let div;
let autoscroll = false;
beforeUpdate(() => {
if (div) {
const scrollableDistance = div.scrollHeight - div.offsetHeight;
autoscroll = div.scrollTop > scrollableDistance - 20;
}
});
afterUpdate(() => {
if (autoscroll) {
div.scrollTo(0, div.scrollHeight);
}
});要注意的是，beforeUpdate 会在组件初始化之前运行，因此我们需要先检查 div 是否存在，再决定是否读取其属性。
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<script>
import Eliza from 'elizabot';
import {beforeUpdate,
afterUpdate
} from 'svelte';
let div;
beforeUpdate(() => {// determine whether we should auto-scroll
// once the DOM is updated...
});
afterUpdate(() => {// ...the DOM is now in sync with the data
});
const eliza = new Eliza();
const pause = (ms) => new Promise((fulfil) => setTimeout(fulfil, ms));
const typing = { author: 'eliza', text: '...' };let comments = [];
async function handleKeydown(event) { if (event.key === 'Enter' && event.target.value) { const comment = {author: 'user',
text: event.target.value
};
const reply = {author: 'eliza',
text: eliza.transform(comment.text)
};
event.target.value = '';
comments = [...comments, comment];
await pause(200 * (1 + Math.random()));
comments = [...comments, typing];
await pause(500 * (1 + Math.random()));
comments = [...comments, reply].filter(comment => comment !== typing);
}
}
</script>
<div class="container">
<div class="phone">
<div class="chat" bind:this={div}><header>
<h1>Eliza</h1>
<article class="eliza">
<span>{eliza.getInitial()}</span></article>
</header>
{#each comments as comment} <article class={comment.author}> <span>{comment.text}</span></article>
{/each}</div>
<input on:keydown={handleKeydown} /></div>
</div>
<style>
.container {display: grid;
place-items: center;
height: 100%;
}
.phone {display: flex;
flex-direction: column;
width: 100%;
height: 100%;
}
header {display: flex;
flex-direction: column;
height: 100%;
padding: 4em 0 0 0;
box-sizing: border-box;
}
h1 {flex: 1;
font-size: 1.4em;
text-align: center;
}
.chat {height: 0;
flex: 1 1 auto;
padding: 0 1em;
overflow-y: auto;
scroll-behavior: smooth;
}
article {margin: 0 0 0.5em 0;
}
.user {text-align: right;
}
span {padding: 0.5em 1em;
display: inline-block;
}
.eliza span {background-color: var(--bg-1);
border-radius: 1em 1em 1em 0;
color: var(--fg-1);
}
.user span {background-color: #0074d9;
color: white;
border-radius: 1em 1em 0 1em;
word-break: break-all;
}
input {margin: 0.5em 1em 1em 1em;
}
@media (min-width: 400px) { .phone {background: var(--bg-2);
position: relative;
font-size: min(2.5vh, 1rem);
width: auto;
height: 36em;
aspect-ratio: 9 / 16;
border: 0.2em solid #222;
border-radius: 1em;
box-sizing: border-box;
filter: drop-shadow(1px 1px 0px #222) drop-shadow(2px 2px 0px #222) drop-shadow(3px 3px 0px #222)
}
.phone::after {position: absolute;
content: '';
background: #222;
width: 60%;
height: 1em;
left: 20%;
top: 0;
border-radius: 0 0 0.5em 0.5em
}
}
@media (prefers-reduced-motion) { .chat {scroll-behavior: auto;
}
}
</style>