svelte-development

Svelte 5 开发

使用符文和 SvelteKit 构建现代 Svelte 应用的综合指南。

技术栈概览

工具	用途	版本
Svelte 5	核心框架	5.0+
SvelteKit	全栈框架	2.0+
TypeScript	类型安全	5.0+
Vite	构建工具	5.0+
Vitest	测试	1.0+

---

Svelte 5 符文（核心响应性）

符文系统

符文是在编译时处理、以 $ 为前缀的宏，可提供显式、细粒度的响应性。

符文	用途	替代内容
$state	响应式状态	let 声明
$derived	计算值	$: 响应式语句
$effect	副作用	$: 副作用语句
$props	组件属性	export let
$bindable	双向绑定属性	带 bind: 的 export let

基本组件结构

<script lang="ts">
  // Props with TypeScript
  interface Props {
    title: string
    count?: number
    onUpdate?: (value: number) => void
  }

  let { title, count = 0, onUpdate }: Props = $props()

  // Reactive state
  let localCount = $state(count)
  let items = $state<string[]>([])

  // Derived values (auto-update when dependencies change)
  let doubled = $derived(localCount * 2)
  let total = $derived(items.reduce((sum, i) => sum + i.length, 0))

  // Side effects
  $effect(() => {
    console.log(`Count changed to: ${localCount}`)
    onUpdate?.(localCount)
  })

  // Methods
  function increment() {
    localCount++
  }
</script>

<div class="component">
  <h1>{title}</h1>
  <p>Count: {localCount} (Doubled: {doubled})</p>
  <button onclick={increment}>Increment</button>
</div>

<style>
  .component {
    padding: 1rem;
  }
</style>

状态模式

// Primitives
let count = $state(0);
let name = $state("");

// Objects (deeply reactive)
let user = $state({
  name: "John",
  email: "[email protected]",
  preferences: { theme: "dark" },
});

// Arrays
let items = $state<Item[]>([]);

// Direct mutation works!
user.name = "Jane"; // Reactive
user.preferences.theme = "light"; // Reactive (deep)
items.push({ id: 1, name: "New" }); // Reactive

// Frozen state (shallow reactivity)
let frozenList = $state.frozen([1, 2, 3]);

派生值

// Simple derived
let doubled = $derived(count * 2);

// Complex derived (use $derived.by for multi-line)
let stats = $derived.by(() => {
  const total = items.reduce((sum, i) => sum + i.value, 0);
  const average = items.length ? total / items.length : 0;
  return { total, average, count: items.length };
});

// Derived from multiple sources
let summary = $derived(`${user.name} has ${items.length} items`);

副作用

// Basic effect (runs when dependencies change)
$effect(() => {
  console.log(`Count is now: ${count}`);
});

// Effect with cleanup
$effect(() => {
  const interval = setInterval(() => {
    count++;
  }, 1000);

  // Cleanup function (returned)
  return () => clearInterval(interval);
});

// Pre-effect (runs before DOM updates)
$effect.pre(() => {
  console.log("About to update DOM");
});

// Root effect (doesn't track dependencies)
$effect.root(() => {
  // Manual dependency management
});

---

组件模式

带默认值和展开的属性

<script lang="ts">
  interface Props {
    variant?: 'primary' | 'secondary'
    size?: 'sm' | 'md' | 'lg'
    disabled?: boolean
    children: import('svelte').Snippet
  }

  let {
    variant = 'primary',
    size = 'md',
    disabled = false,
    children,
    ...restProps
  }: Props = $props()
</script>

<button
  class="btn btn-{variant} btn-{size}"
  {disabled}
  {...restProps}
>
  {@render children()}
</button>

使用 $bindable 进行双向绑定

<script lang="ts">
  interface Props {
    value: string
  }

  let { value = $bindable() }: Props = $props()
</script>

<input bind:value />


<script lang="ts">
  let name = $state('')
</script>

<Input bind:value={name} />
<p>Hello, {name}!</p>

片段（替代插槽）

<script lang="ts">
  import type { Snippet } from 'svelte'

  interface Props {
    title: string
    children: Snippet
    footer?: Snippet
  }

  let { title, children, footer }: Props = $props()
</script>

<div class="card">
  <header>{title}</header>
  <main>{@render children()}</main>
  {#if footer}
    <footer>{@render footer()}</footer>
  {/if}
</div>


<Card title="My Card">
  <p>Card content goes here</p>

  {#snippet footer()}
    <button>Action</button>
  {/snippet}
</Card>

带参数的片段

<script lang="ts" generics="T">
  import type { Snippet } from 'svelte'

  interface Props {
    items: T[]
    row: Snippet<[T, number]>
    empty?: Snippet
  }

  let { items, row, empty }: Props = $props()
</script>

{#if items.length === 0}
  {#if empty}
    {@render empty()}
  {:else}
    <p>No items</p>
  {/if}
{:else}
  <ul>
    {#each items as item, index}
      <li>{@render row(item, index)}</li>
    {/each}
  </ul>
{/if}


<List items={users}>
  {#snippet row(user, i)}
    <span>{i + 1}. {user.name}</span>
  {/snippet}

  {#snippet empty()}
    <p>No users found</p>
  {/snippet}
</List>

---

使用 .svelte.ts 文件共享状态

创建共享状态

// lib/stores/counter.svelte.ts
export function createCounter(initial = 0) {
  let count = $state(initial);

  return {
    get count() {
      return count;
    },
    increment() {
      count++;
    },
    decrement() {
      count--;
    },
    reset() {
      count = initial;
    },
  };
}

// Singleton instance
export const counter = createCounter();

基于类的状态

// lib/stores/user.svelte.ts
export class UserStore {
  user = $state<User | null>(null);
  loading = $state(false);
  error = $state<string | null>(null);

  isLoggedIn = $derived(!!this.user);

  async login(email: string, password: string) {
    this.loading = true;
    this.error = null;

    try {
      const response = await fetch("/api/auth/login", {
        method: "POST",
        body: JSON.stringify({ email, password }),
        headers: { "Content-Type": "application/json" },
      });

      if (!response.ok) throw new Error("Login failed");

      this.user = await response.json();
    } catch (e) {
      this.error = e instanceof Error ? e.message : "Unknown error";
      throw e;
    } finally {
      this.loading = false;
    }
  }

  logout() {
    this.user = null;
  }
}

export const userStore = new UserStore();

使用共享状态

<script lang="ts">
  import { counter } from '$lib/stores/counter.svelte'
  import { userStore } from '$lib/stores/user.svelte'
</script>

<p>Count: {counter.count}</p>
<button onclick={counter.increment}>+</button>

{#if userStore.isLoggedIn}
  <p>Welcome, {userStore.user?.name}</p>
  <button onclick={() => userStore.logout()}>Logout</button>
{:else}
  <button onclick={() => userStore.login('[email protected]', 'pass')}>
    Login
  </button>
{/if}

---

SvelteKit（全栈）

项目结构

sveltekit-app/
├── src/
│   ├── routes/              # File-based routing
│   │   ├── +page.svelte     # /
│   │   ├── +page.server.ts  # Server load function
│   │   ├── +layout.svelte   # Root layout
│   │   ├── about/
│   │   │   └── +page.svelte # /about
│   │   ├── users/
│   │   │   ├── +page.svelte # /users
│   │   │   └── [id]/
│   │   │       ├── +page.svelte      # /users/:id
│   │   │       └── +page.server.ts
│   │   └── api/
│   │       └── users/
│   │           └── +server.ts  # /api/users
│   ├── lib/                 # $lib alias
│   │   ├── components/
│   │   └── stores/
│   └── app.html
├── static/
└── svelte.config.js

加载函数

// routes/users/+page.server.ts
import type { PageServerLoad } from './$types'
import { error } from '@sveltejs/kit'

export const load: PageServerLoad = async ({ fetch, params }) => {
  const response = await fetch('/api/users')

  if (!response.ok) {
    throw error(response.status, 'Failed to load users')
  }

  const users = await response.json()

  return { users }
}

// routes/users/+page.svelte
<script lang="ts">
  import type { PageData } from './$types'

  let { data }: { data: PageData } = $props()
</script>

<h1>Users</h1>
{#each data.users as user}
  <p>{user.name}</p>
{/each}

表单操作

// routes/login/+page.server.ts
import type { Actions } from "./$types";
import { fail, redirect } from "@sveltejs/kit";

export const actions: Actions = {
  default: async ({ request, cookies }) => {
    const formData = await request.formData();
    const email = formData.get("email") as string;
    const password = formData.get("password") as string;

    // Validation
    if (!email || !password) {
      return fail(400, { email, missing: true });
    }

    // Authentication
    const user = await authenticate(email, password);

    if (!user) {
      return fail(401, { email, incorrect: true });
    }

    // Set session cookie
    cookies.set("session", user.token, { path: "/" });

    throw redirect(303, "/dashboard");
  },
};

<script lang="ts">
  import type { ActionData } from './$types'
  import { enhance } from '$app/forms'

  let { form }: { form: ActionData } = $props()
</script>

<form method="POST" use:enhance>
  <input name="email" value={form?.email ?? ''} />

  {#if form?.missing}
    <p class="error">All fields are required</p>
  {/if}

  {#if form?.incorrect}
    <p class="error">Invalid credentials</p>
  {/if}

  <input name="password" type="password" />
  <button>Login</button>
</form>

API 路由

// routes/api/users/+server.ts
import type { RequestHandler } from "./$types";
import { json, error } from "@sveltejs/kit";

export const GET: RequestHandler = async ({ url }) => {
  const limit = Number(url.searchParams.get("limit")) || 10;

  const users = await prisma.user.findMany({ take: limit });

  return json(users);
};

export const POST: RequestHandler = async ({ request }) => {
  const body = await request.json();

  if (!body.email || !body.name) {
    throw error(400, "Missing required fields");
  }

  const user = await prisma.user.create({ data: body });

  return json(user, { status: 201 });
};

// routes/api/users/[id]/+server.ts
export const GET: RequestHandler = async ({ params }) => {
  const user = await prisma.user.findUnique({
    where: { id: params.id },
  });

  if (!user) {
    throw error(404, "User not found");
  }

  return json(user);
};

中间件（钩子）

// src/hooks.server.ts
import type { Handle } from "@sveltejs/kit";

export const handle: Handle = async ({ event, resolve }) => {
  // Get session from cookie
  const session = event.cookies.get("session");

  if (session) {
    const user = await validateSession(session);
    event.locals.user = user;
  }

  // Protected routes
  if (event.url.pathname.startsWith("/dashboard") && !event.locals.user) {
    return new Response("Redirect", {
      status: 303,
      headers: { Location: "/login" },
    });
  }

  return resolve(event);
};

---

事件处理（Svelte 5）

DOM 事件（新语法）

<button onclick={() => count++}>Click me</button>

<input
  oninput={(e) => name = e.currentTarget.value}
  onkeydown={(e) => e.key === 'Enter' && submit()}
/>


<button onclick={(e) => {
  e.preventDefault()
  e.stopPropagation()
  handleClick()
}}>
  Submit
</button>


<script>
  function once(fn) {
    return function(e) {
      if (!e.target.dataset.clicked) {
        e.target.dataset.clicked = 'true'
        fn(e)
      }
    }
  }
</script>

<button onclick={once(() => console.log('Once!'))}>
  Click once
</button>

组件事件（回调属性）

<script lang="ts">
  interface Props {
    onSelect?: (id: string) => void
    onClose?: () => void
  }

  let { onSelect, onClose }: Props = $props()
</script>

<button onclick={() => onSelect?.('123')}>Select</button>
<button onclick={onClose}>Close</button>


<Child
  onSelect={(id) => console.log('Selected:', id)}
  onClose={() => console.log('Closed')}
/>

---

使用 Vitest 进行测试

设置

// vitest.config.ts
import { defineConfig } from "vitest/config";
import { svelte } from "@sveltejs/vite-plugin-svelte";

export default defineConfig({
  plugins: [svelte({ hot: !process.env.VITEST })],
  test: {
    globals: true,
    environment: "jsdom",
    include: ["src/**/*.{test,spec}.{js,ts}"],
  },
});

组件测试

// tests/Counter.test.ts
import { describe, it, expect } from "vitest";
import { render, screen, fireEvent } from "@testing-library/svelte";
import Counter from "$lib/components/Counter.svelte";

describe("Counter", () => {
  it("renders initial count", () => {
    render(Counter, { props: { initial: 5 } });

    expect(screen.getByText("Count: 5")).toBeInTheDocument();
  });

  it("increments on click", async () => {
    render(Counter);

    const button = screen.getByRole("button", { name: /increment/i });
    await fireEvent.click(button);

    expect(screen.getByText("Count: 1")).toBeInTheDocument();
  });
});

Store 测试

// tests/stores/counter.test.ts
import { describe, it, expect } from "vitest";
import { counter } from "$lib/stores/counter.svelte";

describe("counter store", () => {
  it("increments count", () => {
    counter.reset();
    expect(counter.count).toBe(0);

    counter.increment();
    expect(counter.count).toBe(1);

    counter.increment();
    expect(counter.count).toBe(2);
  });

  it("decrements count", () => {
    counter.reset();
    counter.decrement();
    expect(counter.count).toBe(-1);
  });
});

---

从 Svelte 4 迁移

Svelte 4	Svelte 5
let count = 0（响应式）	let count = $state(0)
$: doubled = count * 2	let doubled = $derived(count * 2)
$: console.log(count)	$effect(() => console.log(count))
export let value	let { value } = $props()
on:click={handler}	onclick={handler}
<slot />	{@render children()}
createEventDispatcher()	回调属性

迁移脚本

npx sv migrate svelte-5

---

应避免的反模式

反模式	问题	解决方案
使用 $effect 处理派生值	增加不必要的复杂性	使用 $derived
直接导出 $state	会破坏响应性	导出 getter/setter 对象
不使用 TypeScript	缺少类型安全	启用 lang="ts"
使用旧式插槽语法	在 Svelte 5 中已弃用	使用片段
使用 on:event 语法	在 Svelte 5 中已弃用	使用 onevent 属性

---

性能优势

指标	改进
Bundle 大小	比 React/Vue 小 40-60%
运行时性能	无虚拟 DOM 开销
可交互时间	JavaScript 水合量极少
内存使用量	编译后的输出占用更少

---

相关资源

• Svelte 5 文档
• SvelteKit 文档
• Svelte 5 迁移指南
• Svelte 测试文档
• Joy of Code 教程

---

何时使用此技能

• 使用符文构建新的 Svelte 5 应用
• 从 Svelte 4 迁移到 Svelte 5
• 使用 SvelteKit 进行全栈开发
• 创建响应式共享状态
• 实现表单操作和 API 路由
• 使用 Vitest 测试 Svelte 组件