Signal

Creating a Signal

import { signal } from "kaioken"

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 "kaioken"

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

Use watch create a WatchEffect that will fire the callback whenever observed signals change.

import { watch } from "kaioken"

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

watcher.stop()
watcher.start()

The callback provided to watch 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 watch to observe. Their values will be passed to the callback in the order they were passed.

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

General usage

In Kaioken 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, Kaioken 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 watch can all be created locally in Kaioken components via the useSignal, useComputed, and useWatch hooks. This will allow them to be persisted across renders and automatically disposed of when the component unmounts.

import { useSignal, useComputed, useWatch } from "kaioken"

function App() {
  const userName = useSignal("bob")
  const userGreeting = useComputed(() => `Hello, ${userName}!`)

  useWatch([userName], console.log)

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

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 = useSignal("bob")

  useWatch(() => 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 "kaioken"

function App() {
  const items = useSignal([0, 1, 2, 3, 4])
  const doubledItems = useComputed(() => 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>
  )
}

Similar to <For />, <Derive /> allows you to easily create fine-grained reactivity in JSX. The <Derive /> component produces a JSX element that automatically updates whenever the provided from signal changes.

import { Derive } from "kaioken"

function App() {
  const name = useSignal("bob")
  const age = useSignal(42)
  const person = useComputed(() => ({ name: name.value, age: age.value }))

  return (
    <div>
      <input bind:value={name} />
      <input type="number" bind:value={age} />
      <Derive from={person}>
        {(person) => (
          <div>
            {person.name} is {person.age} years old
          </div>
        )}
      </Derive>
      {/* You can also use multiple signals! */}
      <Derive from={[name, age]}>
        {(name, age) => (
          <div>
            {name} is {age} years old
          </div>
        )}
      </Derive>
    </div>
  )
}