了解 Svelte 中的上下文（从 React 上下文转换）
Understanding Context in Svelte (convert from React Context)
我有一个使用 ContextAPI 来管理身份验证的 React 应用程序，我正在尝试在 Svelte 中实现类似的功能。 [Web 开发简化][1]
在 Authenticate.js 我有这个：
import React, { useContext, useState, useEffect } from "react"
import { auth } from "../firebase"
const AuthCt = React.createContext()
export function Auth() {
return useContext(AuthCt)
}
export function AuthComp({ children }) {
const [currentUser, setCurrentUser] = useState()
const [loading, setLoading] = useState(true)
function login(email, password) {
return auth.signInWithEmailAndPassword(email, password)
}
function logout() {
return auth.signOut()
}
useEffect(() => {
const unmount = auth.onAuthStateChanged(user => {
setCurrentUser(user)
setLoading(false)
})
return unmount
}, [])
const value = {
currentUser,
login,
signup
}
return (
<AuthCt.Provider value={value}>
{!loading && children}
</AuthCt.Provider>
)
}
此上下文在其他 Login.js 组件中使用，如下所示：
import { Auth } from "./Authenticate"
const Login = () => {
const { currentUser, login } = Auth()
在 App.js 我有：
import { AuthComp } from "./Authenticate";
function App() {
return (
<AuthComp>
<div> All others go here </div>
</AuthComp>
);
}
如何在 Svelte 中实现这一点，尤其是 Authenticate 上下文？
我无法在 Svelte 中做很多事情，因为我不知道如何从这里开始。到目前为止，我有 AuthComp.svelte。我不知道我做的对不对。
<script>
import { getContext, setContext } from 'svelte';
import { auth } from '../firebase';
import { writable } from 'svelte/store';
let Auth = getContext('AuthCt')
setContext('Auth', Auth)
let currentUser;
let loading = true;
const unmount = auth.onAuthStateChanged(user => {
currentUser = user;
loading = false
});
function login(email, password) {
return auth.signInWithEmailandPassWord(email,password)
}
function logout() {
return auth.signOut()
}
const value = { currentUser, login, signUp }
</script>
<slot value={value}></slot>
在 Svelte 中，在父组件中使用 setContext(key, value) 设置上下文，子组件可以使用 getContext(key) 访问 value 对象。有关详细信息，请参阅 the docs。
在您的情况下，上下文将像这样使用：
<script>
import { getContext, setContext } from 'svelte';
import { auth } from '../firebase';
import { writable } from 'svelte/store';
// you can initialize this to something else if you want
let currentUser = writable(null)
let loading = true
// maybe you're looking for `onMount` or `onDestroy`?
const unmount = auth.onAuthStateChanged(user => {
currentUser.set(user)
loading = false
});
function login(email, password) {
return auth.signInWithEmailandPassWord(email,password)
}
function logout() {
return auth.signOut()
}
const value = { currentUser, login, signUp }
setContext('Auth', value)
</script>
{#if !loading}
<slot></slot>
{/if}
此处，currentUser、login 和 signup（不确定这是从哪里来的？）被设置为 setContext() 的上下文。要使用此上下文，您可能会有这样的内容：
<!-- App -->
<AuthComp>
<!-- Some content here -->
<Component />
</AuthComp>
<!-- Component.svelte -->
<script>
import { getContext } from 'svelte'
const { currentUser, login, signup } = getContext('Auth')
// you can subscribe to currentUser with $currentUser
</script>
<div>some content</div>
如文档中所写，上下文 不是反应性的 ，因此 currentUser 首先转换为 store so it can be subscribed to in a child. As for the useEffect, Svelte has lifecycle functions，您可以将其用于 运行 不同点的代码，例如 onMount 或 onDestroy.
如果您是 Svelte 的新手，他们的 tutorial 非常全面，有大量示例供您参考。
希望对您有所帮助！
Migrating from React Context to Svelte
Context in Svelte and React may seem similar, but they are actually used differently. Because at the core, Svelte's context is much more limited. But that's ok. In fact, it actually will make your code simpler to write and understand.
In Svelte, you have more tools at your disposal for passing data round your app (and keeping it in sync) than just context. Each one does pretty much one thing (making everything predictable), and they do it well. Of these, you have:
- Context
- Stores
- Props
As someone who's recently switched from React to Svelte, I think I can help explain some of the differences between each of these and help you avoid some of my conceptual mistakes. I'll also go over some differences in life cycle methods, because if you used to use useEffect, you might feel very lost since Svelte doesn't have an equivalent API. Yet combining everything together in Svelte will make everything simple.
Context
Context in Svelte does one thing: pass data from a parent component to any children (not necessarily direct children). Unlike in React, context is not reactive. It is set once when the component mounts, and then will not be updated again. We'll get to "reactive context" in a second.
<!-- parent.svelte -->
<script>
import { setContext } from 'svelte'
setContext('myContext', true)
</script>
<!-- child.svelte -->
<script>
import { getContext } from 'svelte'
const myContext = getContext('myContext')
</script>
Notice that context involves two things, a key and a value. Context is set to a specific key, then the value can be retrieved using that key. Unlike React, you do not need to export functions to retrieve the context. Both the key and value for the context can be anything. If you can save it to a variable, you can set it to context. You can even use an object as a key!
Stores
If you have data that needs to stay in sync in multiple places across your app, stores are the way to go. Stores are reactive, meaning they can be updated after they're created. Unlike context in either React or Svelte, stores don't simply pass data to their children. Any part of your app can create a store, and any part of your app can read the store. You can even create stores outside of Svelte components in separate JavaScript files.
// mystore.ts
import { writable } from 'svelte/store'
// 0 is the initial value
const writableStore = writable(0)
// set the new value to 1
writableStore.set(1)
// use `update` to set a new value based on the previous value
writableStore.update((oldValue) => oldValue + 1)
export { writableStore }
Then inside a component, you can subscribe to the store.
<script>
import { writableStore } from './mystore'
</script>
{$writableStore}
The dollar sign subscribes to the store. Now, whenever the store is updated, the component will rerender automatically.
Using stores with context
Now that we have stores and context, we can create "reactive context"(a term I just made up, but it works). Stores are great because they're reactive, and context is great to pass data down to the children components. But we can actually pass a store down through context. This makes the context reactive and the store scoped.
<!-- parent.svelte -->
<script>
import { setContext } from 'svelte'
import { writable } from 'svelte/store'
const writableStore = writable(0)
setContext('myContext', writableStore)
</script>
<!-- child.svelte -->
<script>
import { getContext } from 'svelte'
const myContext = getContext('myContext')
</script>
{$myContext}
Now, whenever the store updates in the parent, the child will also update. Stores can of course do much more than this, but if you were looking to replicate React context, this is the closest you can get in Svelte. It's also a lot less boilerplate!
Using "reactive context" with "useEffect"
Svelte does not have an equivalent of useEffect. Instead, Svelte has reactive statements. There's a lot on these in the docs/tutorial, so I'll keep this brief.
// doubled will always be twice of single. If single updates, doubled will run again.
$: doubled = single * 2
// equivalent to this
let single = 0
const [doubled, setDoubled] = useState(single * 2)
useEffect(() => {
setDoubled(single * 2)
}, [single])
Svelte is smart enough to figure out the dependencies and only 运行 each reactive statement as needed. And if you create a dependency cycle, the compiler will yell at you.
This means that you can use reactive statements to update stores (and hence update the context). Here, the valueStore will be update on every keystroke to the input. Since this store is passed down through context, any child can then get the current value of the input.
<script>
import { setContext } from 'svelte'
import { writable } from 'svelte/store'
// this value is bound to the input's value. When the user types, this variable will always update
let value
const valueStore = writable(value)
setContext('inputContext', valueStore)
$: valueStore.set(value)
</script>
<input type='text' bind:value />
Props
For the most part, props function exactly the same in React and Svelte. There are a few differences because Svelte props can take advantage of two-way binding (not necessary, but possible). That's really a different conversation though, and the tutorial is really good at teaching two-way binding with props.
Authentication in Svelte
Ok, now after all of that, let's look at how you'd create an authentication wrapper component.
- Create an auth store
- Pass the auth store down via context
- Use Firebase's
onAuthStateChanged to listen to changes in auth state
- Subscribe to the auth store in the child
- Unsubscribe from
onAuthStateChanged when the parent is destroyed to prevent memory leaks
<!-- parent.svelte -->
<script>
import { writable } from 'svelte/store'
import { onDestroy, setContext } from 'svelte'
import { auth } from '../firebase'
const userStore = writable(null)
const firebaseUnsubscribe = auth.onAuthStateChanged((user) => {
userStore.set(user)
})
const login = (email, password) => auth.signInWithEmailandPassWord(email,password)
const logout = () => auth.signOut()
setContext('authContext', { user: userStore, login, logout })
onDestroy(() => firebaseUnsubscribe())
</script>
<slot />
<!-- child.svelte -->
<script>
import { getContext } from 'svelte'
const { login, logout, user } = getContext('authContext')
</script>
{$user?.displayName}
我有一个使用 ContextAPI 来管理身份验证的 React 应用程序，我正在尝试在 Svelte 中实现类似的功能。 [Web 开发简化][1]
在 Authenticate.js 我有这个：
import React, { useContext, useState, useEffect } from "react"
import { auth } from "../firebase"
const AuthCt = React.createContext()
export function Auth() {
return useContext(AuthCt)
}
export function AuthComp({ children }) {
const [currentUser, setCurrentUser] = useState()
const [loading, setLoading] = useState(true)
function login(email, password) {
return auth.signInWithEmailAndPassword(email, password)
}
function logout() {
return auth.signOut()
}
useEffect(() => {
const unmount = auth.onAuthStateChanged(user => {
setCurrentUser(user)
setLoading(false)
})
return unmount
}, [])
const value = {
currentUser,
login,
signup
}
return (
<AuthCt.Provider value={value}>
{!loading && children}
</AuthCt.Provider>
)
}
此上下文在其他 Login.js 组件中使用，如下所示：
import { Auth } from "./Authenticate"
const Login = () => {
const { currentUser, login } = Auth()
在 App.js 我有：
import { AuthComp } from "./Authenticate";
function App() {
return (
<AuthComp>
<div> All others go here </div>
</AuthComp>
);
}
如何在 Svelte 中实现这一点，尤其是 Authenticate 上下文？
我无法在 Svelte 中做很多事情，因为我不知道如何从这里开始。到目前为止，我有 AuthComp.svelte。我不知道我做的对不对。
<script>
import { getContext, setContext } from 'svelte';
import { auth } from '../firebase';
import { writable } from 'svelte/store';
let Auth = getContext('AuthCt')
setContext('Auth', Auth)
let currentUser;
let loading = true;
const unmount = auth.onAuthStateChanged(user => {
currentUser = user;
loading = false
});
function login(email, password) {
return auth.signInWithEmailandPassWord(email,password)
}
function logout() {
return auth.signOut()
}
const value = { currentUser, login, signUp }
</script>
<slot value={value}></slot>
在 Svelte 中，在父组件中使用 setContext(key, value) 设置上下文，子组件可以使用 getContext(key) 访问 value 对象。有关详细信息，请参阅 the docs。
在您的情况下，上下文将像这样使用：
<script>
import { getContext, setContext } from 'svelte';
import { auth } from '../firebase';
import { writable } from 'svelte/store';
// you can initialize this to something else if you want
let currentUser = writable(null)
let loading = true
// maybe you're looking for `onMount` or `onDestroy`?
const unmount = auth.onAuthStateChanged(user => {
currentUser.set(user)
loading = false
});
function login(email, password) {
return auth.signInWithEmailandPassWord(email,password)
}
function logout() {
return auth.signOut()
}
const value = { currentUser, login, signUp }
setContext('Auth', value)
</script>
{#if !loading}
<slot></slot>
{/if}
此处，currentUser、login 和 signup（不确定这是从哪里来的？）被设置为 setContext() 的上下文。要使用此上下文，您可能会有这样的内容：
<!-- App -->
<AuthComp>
<!-- Some content here -->
<Component />
</AuthComp>
<!-- Component.svelte -->
<script>
import { getContext } from 'svelte'
const { currentUser, login, signup } = getContext('Auth')
// you can subscribe to currentUser with $currentUser
</script>
<div>some content</div>
如文档中所写，上下文 不是反应性的 ，因此 currentUser 首先转换为 store so it can be subscribed to in a child. As for the useEffect, Svelte has lifecycle functions，您可以将其用于 运行 不同点的代码，例如 onMount 或 onDestroy.
如果您是 Svelte 的新手，他们的 tutorial 非常全面，有大量示例供您参考。
希望对您有所帮助！
Migrating from React Context to Svelte
Context in Svelte and React may seem similar, but they are actually used differently. Because at the core, Svelte's context is much more limited. But that's ok. In fact, it actually will make your code simpler to write and understand.
In Svelte, you have more tools at your disposal for passing data round your app (and keeping it in sync) than just context. Each one does pretty much one thing (making everything predictable), and they do it well. Of these, you have:
- Context
- Stores
- Props
As someone who's recently switched from React to Svelte, I think I can help explain some of the differences between each of these and help you avoid some of my conceptual mistakes. I'll also go over some differences in life cycle methods, because if you used to use useEffect, you might feel very lost since Svelte doesn't have an equivalent API. Yet combining everything together in Svelte will make everything simple.
Context
Context in Svelte does one thing: pass data from a parent component to any children (not necessarily direct children). Unlike in React, context is not reactive. It is set once when the component mounts, and then will not be updated again. We'll get to "reactive context" in a second.
<!-- parent.svelte -->
<script>
import { setContext } from 'svelte'
setContext('myContext', true)
</script>
<!-- child.svelte -->
<script>
import { getContext } from 'svelte'
const myContext = getContext('myContext')
</script>
Notice that context involves two things, a key and a value. Context is set to a specific key, then the value can be retrieved using that key. Unlike React, you do not need to export functions to retrieve the context. Both the key and value for the context can be anything. If you can save it to a variable, you can set it to context. You can even use an object as a key!
Stores
If you have data that needs to stay in sync in multiple places across your app, stores are the way to go. Stores are reactive, meaning they can be updated after they're created. Unlike context in either React or Svelte, stores don't simply pass data to their children. Any part of your app can create a store, and any part of your app can read the store. You can even create stores outside of Svelte components in separate JavaScript files.
// mystore.ts
import { writable } from 'svelte/store'
// 0 is the initial value
const writableStore = writable(0)
// set the new value to 1
writableStore.set(1)
// use `update` to set a new value based on the previous value
writableStore.update((oldValue) => oldValue + 1)
export { writableStore }
Then inside a component, you can subscribe to the store.
<script>
import { writableStore } from './mystore'
</script>
{$writableStore}
The dollar sign subscribes to the store. Now, whenever the store is updated, the component will rerender automatically.
Using stores with context
Now that we have stores and context, we can create "reactive context"(a term I just made up, but it works). Stores are great because they're reactive, and context is great to pass data down to the children components. But we can actually pass a store down through context. This makes the context reactive and the store scoped.
<!-- parent.svelte -->
<script>
import { setContext } from 'svelte'
import { writable } from 'svelte/store'
const writableStore = writable(0)
setContext('myContext', writableStore)
</script>
<!-- child.svelte -->
<script>
import { getContext } from 'svelte'
const myContext = getContext('myContext')
</script>
{$myContext}
Now, whenever the store updates in the parent, the child will also update. Stores can of course do much more than this, but if you were looking to replicate React context, this is the closest you can get in Svelte. It's also a lot less boilerplate!
Using "reactive context" with "useEffect"
Svelte does not have an equivalent of useEffect. Instead, Svelte has reactive statements. There's a lot on these in the docs/tutorial, so I'll keep this brief.
// doubled will always be twice of single. If single updates, doubled will run again.
$: doubled = single * 2
// equivalent to this
let single = 0
const [doubled, setDoubled] = useState(single * 2)
useEffect(() => {
setDoubled(single * 2)
}, [single])
Svelte is smart enough to figure out the dependencies and only 运行 each reactive statement as needed. And if you create a dependency cycle, the compiler will yell at you.
This means that you can use reactive statements to update stores (and hence update the context). Here, the valueStore will be update on every keystroke to the input. Since this store is passed down through context, any child can then get the current value of the input.
<script>
import { setContext } from 'svelte'
import { writable } from 'svelte/store'
// this value is bound to the input's value. When the user types, this variable will always update
let value
const valueStore = writable(value)
setContext('inputContext', valueStore)
$: valueStore.set(value)
</script>
<input type='text' bind:value />
Props
For the most part, props function exactly the same in React and Svelte. There are a few differences because Svelte props can take advantage of two-way binding (not necessary, but possible). That's really a different conversation though, and the tutorial is really good at teaching two-way binding with props.
Authentication in Svelte
Ok, now after all of that, let's look at how you'd create an authentication wrapper component.
- Create an auth store
- Pass the auth store down via context
- Use Firebase's
onAuthStateChangedto listen to changes in auth state - Subscribe to the auth store in the child
- Unsubscribe from
onAuthStateChangedwhen the parent is destroyed to prevent memory leaks
<!-- parent.svelte -->
<script>
import { writable } from 'svelte/store'
import { onDestroy, setContext } from 'svelte'
import { auth } from '../firebase'
const userStore = writable(null)
const firebaseUnsubscribe = auth.onAuthStateChanged((user) => {
userStore.set(user)
})
const login = (email, password) => auth.signInWithEmailandPassWord(email,password)
const logout = () => auth.signOut()
setContext('authContext', { user: userStore, login, logout })
onDestroy(() => firebaseUnsubscribe())
</script>
<slot />
<!-- child.svelte -->
<script>
import { getContext } from 'svelte'
const { login, logout, user } = getContext('authContext')
</script>
{$user?.displayName}