Signal

Creating a Signal

import { signal } from "kiru"

const userName = signal("bob")

signal takes an initial value and optionally a displayName for debugging. It returns an instance of the Signal class with the following properties:

• value: Gets or sets the signal's current value
• subscribe: Registers a function to be called when the signal's value changes
• peek: Retrieves the value without tracking reactivity
• sneak: Sets the value without emitting a signal change
• notify: Emits a signal change

Reading and Writing Signal Values

You can access a signal's value directly:

console.log(userName.value)  // "bob"
userName.value = "alice"

Subscribing to Signals

You can subscribe to a signal's value changes using the subscribe method:

const unsubscribe = userName.subscribe((newValue) => {
  console.log("Value updated:", newValue)
})

The subscribe function returns an unsubscribe function, allowing you to remove the subscription when needed:

unsubscribe()

Use computed to create a ComputedSignal, which is derived from a getter function. A ComputedSignal is lazily evaluated, meaning that the getter function won't be called until it is observed or subscribed to. Manually assigning a new value to it has no effect.

import { computed } from "kiru"

const userGreeting = computed(() => `Hello, ${userName}!`)

The computed signal will automatically track dependencies (signals that were observed) and update whenever any of them change.

Computed signals also receive the previous value as an argument to the getter function. This can be useful for cleaning up previous resources, or for creating a new resource based on the previous value.

const websocket = computed<WebSocket>((prev) => {
  prev?.close()
  // ^ Websocket | undefined
  return new WebSocket(`ws://localhost:3000/rooms/${userName}`)
})

Use effect create a Effect that will fire the callback whenever observed signals change.

import {effect } from "kiru"

const effectHandle = effect(() => console.log(`Greeting has been updated: ${userGreeting}`))

effectHandle.stop()
effectHandle.start()

The callback provided to effect will fire immediately. When a signal that it observes changes, it will be queued to fire again within a microtask. This allows us to automatically "batch" execution of callbacks.

You can also explicitly pass a list of signals to effect to observe. Their values will be passed to the callback in the order they were passed.

const handle = effect([userName, userGreeting], (newName, newGreeting) => {
  console.log(`Name has been updated: ${newName}`)
  console.log(`Greeting has been updated: ${newGreeting}`)
})

General usage

In Kiru components, reading and writing signals is slightly nuanced but has the capability to provide unmatched performance.

function App() {
  return (
    <div>
      <h1>{userGreeting.value}</h1>
      <input 
        type="text" 
        value={userName.value} 
        oninput={(e) => (userName.value = e.target.value)} 
      />
    </div>
  )
}

In the above example, our userGreeting and userName signals from earlier are observed by the component during render, causing the component to automatically subscribe to them. This means the component and all of its children will be updated whenever their values change.

While this may be the desired effect, signals can be much more performant when used for text or attributes. See the following:

function App() {
  return (
    <div>
      <h1>{userGreeting}</h1>
      <input 
        type="text" 
        value={userName} 
        oninput={(e) => (userName.value = e.target.value)} 
      />
    </div>
  )
}

Because neither of the signals are observed at the time of rendering, when they change, Kiru will only change the things that matter - in this case, the text node inside of our heading that displays the greeting and the value attribute of our input.

Using local signals

signal, computed, and effect can all be created locally in Kiru components by creating them within the setup scope. This will allow them to be persisted across renders and automatically disposed of when the component unmounts.

import { signal, computed, effect } from "kiru"

function App() {
  // Create a local signal
  const userName = signal("bob")
  const userGreeting = computed(() => `Hello, ${userName}!`)

  effect([userName], console.log)

  // return a render function 
  return () => (
    <div>
      <h1>{userGreeting}</h1>
      <input 
        type="text" 
        value={userName} 
        oninput={(e) => (userName.value = e.target.value)} 
      />
    </div>
  )
} 

Signals in style objects

Signals can also be used inside style objects. Kiru will automatically keep track of the signal's value and update the property whenever it changes.

import { signal } from "kiru"

function App() {
  const bgColor = signal("rebeccapurple")
  const toggleColor = () => {
    bgColor.value = bgColor.value === "rebeccapurple" ? "tomato" : "rebeccapurple"
  }

  return () => (
    <button
      style={{
        backgroundColor: bgColor,
        color: "white",
      }}
      onclick={toggleColor}
    >
      Toggle color
    </button>
  )
}

The bind: prefix can be used to create two-way binding for any property that changes via user interaction. When the signal changes, the property is updated, and vice versa.

function App() {
  const userName = signal("bob")

  effect(() => console.log(userName.value))

  return () => (
    <div>
      <input type="text" bind:value={userName} /> 
    </div>
  )
} 

The <For /> component iterates over a signal, producing an automatically-updating list with fine-grained reactivity. This is a great optimization tool for when you would otherwise use signal.value.map((item) => ...) in JSX, causing the component that renders it to update whenever the signal changes.

import { For } from "kiru"

function App() {
  const items = signal([0, 1, 2, 3, 4])
  const doubledItems = computed(() => items.value.map((i) => i * 2))

  return () => (
    <div>
      <ul>
        <For each={doubledItems} fallback={<i>No items</i>}>
          {(item, index) => <li>{item}</li>}
        </For>
      </ul>
      <button onclick={() => (items.value = [...items.value, items.value.length])}>
        Add
      </button>
    </div>
  )
}

Use resource to manage asynchronous data fetching with automatic caching, refetching, and loading states. A Resource is a special signal that holds the result of an async operation and tracks loading and error states.

import { resource } from "kiru"

const userId = signal(1)
const data = resource(userId, async (id, { signal }) => {
  const response = await fetch(`/api/data?id=${id}`)
  return response.json()
})

The resource function takes:

• source: A signal that triggers the resource to refetch when it changes
• callback: An async function that receives the source value and a context object with an AbortSignal

A Resource provides:

• value: The resolved data (or undefined while loading)
• isPending: A signal indicating if the resource is currently loading
• error: A signal containing any error that occurred during fetching
• promise: The underlying promise for the current fetch
• refetch(): Manually trigger a refetch of the data

import { signal, computed, resource, Derive } from "kiru"

interface User {
  id: number
  firstName: string
  lastName: string
}

const search = signal("")

// able to be created globally _or_ within components
const users = resource(search, async (search, { signal }) => {
  const res = await fetch(`https://dummyjson.com/users/search?q=${search}`, {
    signal,
  })
  return res.json() as Promise<{ users: User[] }>
})

function App() {
  return (
    <>
      <input placeholder="search" bind:value={search} />
      <button onclick={() => users.refetch()}>Refetch</button>
      <Derive from={users} fallback={<div>Loading...</div>}>
        {(data, isStale) => (
          <div style={{ opacity: isStale ? 0.5 : 1 }}>
            <ul>
              {data.users.map((user) => (
                <li key={user.id}>
                  {user.firstName} {user.lastName}
                </li>
              ))}
            </ul>
          </div>
        )}
      </Derive>
    </>
  )
}

Using with Derive

The <Derive /> component is perfect for rendering resource data with fallback & pending states:

<Derive from={data} fallback={<div>Loading...</div>}>
  {(result, isStale) => (
    <div className={isStale ? "opacity-50" : ""}>
      {/* render result */}
    </div>
  )}
</Derive>

The second parameter isStale indicates whether the resource is currently refetching while displaying cached data.

Handling Errors

Access the error state through the error signal:

<Derive from={data.error}>
  {(err) => err && <div className="error">{err.message}</div>}
</Derive>

Or use an <ErrorBoundary>:

<ErrorBoundary>
  <Derive from={data}>
    {/* render result */}
  </Derive>
</ErrorBoundary>