Fine-Tuning Your Reactivity

Harnessing the Power of Signals

🚦

What is reactivity?

What is reactivity?

There are so many definitions for reactivity, but I think the simplest one is

Declaratively express the relationship between values that change over time.

Ryan Carniato

C = A + B

A reactive system will make sure this expression is always up to date

What is reactivity?

Coarse-grain reactivity

Change in data would trigger a large amount of JavaScript to execute.

This meant that fast-changing properties such as animation would cause performance issues.

The framework would eventually reconcile all of the changes into the UI.

< 17

without vapor

State changes only update the part of the UI to which the state is bound to.

Fine-grained reactivity

The hard part is knowing how to listen to property changes in a way that has good DX.

>=17

with vapor

What defines a reactive system?

What defines a reactive system?

Reactive unit (wrapper, holder, subject, signal, etc)

Reactions (effect, watcher, autotun, subscription, etc)

Derivations (memo, computed, thunk, etc)

Scheduler (Concurrent updates, tasks, etc)

Dependency tracking (Automatic, Manual)

 Algorithm (Push, Pull, Hybrid, Lazy, Eager, etc)

Other implementation details

Rutime

vs

Compile time

Rutime Reactivity

In JS we have two mainstream reactive primitives

Reactive Streams (RxJS) - a series/collection of data events that are emitted over time.

const a = 1;
const b = 2;

const c = a + b;
import { map, combineLatest, BehaviorSubject } from 'rxjs';

// reactive wrappers, that always respond with latest value
const a$ = new BehaviorSubject(1);
const b$ = new BehaviorSubject(2);

// derived/computed state using operators
const c$ = combineLatest(a$, b$).pipe(map(([a, b]) => a + b));
// lazy

// effects, subscriptions are explicit
c$.subscribe((c) => console.log(c)); // 3

// push based
a$.next(2);

Observables/Subjects

Rutime Reactivity

Signals

const a = 1;
const b = 2;

const c = a + b;

Signals are fundamentally a simple pub/sub system that holds a value and represents that value over time.

Rutime Reactivity

import { signal, computed, effect } from 'some-signals-library';


const a = signal(1)
const b = signal(2)

// dervied without memo
const c = () => a.value + b.value;

// or with memo
// depending on the reactivity alogorthim
// this could run immediately (eager) or on read
const cachedC = computed(() => a.value + b.value);

effect(() => console.log(c.value));

// update the wrapped value;
a.value += 1;

Signals

Rutime Reactivity

Comp-time Reactivity

In compile-time reactivity systems, the framework can statically analyze your code and convert it to lower-level code that updates the UI.
const a = 1
const b = 2

const c = a + b
// .svelte
<script>
// it looks like js vars, but it is not
let a = 1
let b = 2

// computed/derived state, compiler will transform this to 
// code that keeps this relation alive
$: c = a + b

b += 3

</script>

<p>{c}</p>

< v5

Comp-time Reactivity

Change detection is not always reactive

Perceived Reactivity can be achieved through reactive primitives or through techniques like diffing, hooks, and dirty-checking.

Signals-based reactive systems 

Signals-based reactive systems 

Simple

Composable

Declarative

Signals

under the hood

export function createSignal(value) {
  const read = () => value;
  const write = (nextValue) => value = nextValue;
  return [read, write];
}

Read/Write

Signals under the hood

const [count, setCount] = createSignal(3);
console.log("Initial Read", count()); // 3

setCount(5);
console.log("Updated Read", count()); // 5

setCount(count() * 2);
console.log("Updated Read", count()); // 10

Signals under the hood

const context = [];

function subscribe(running, subscriptions) {
  subscriptions.add(running);
  running.dependencies.add(subscriptions);
}

export function createSignal(value) {
  const subscriptions = new Set();

  const read = () => {
    const running = context[context.length - 1];
    if (running) subscribe(running, subscriptions);
    return value;
  };

  const write = (nextValue) => {
    value = nextValue;

    for (const sub of [...subscriptions]) {
      sub.execute();
    }
  };
  return [read, write];
}

Signal and deps tracking

Signals under the hood

function cleanup(running) {
  for (const dep of running.dependencies) {
    dep.delete(running);
  }
  running.dependencies.clear();
}

export function createEffect(fn) {
  const running = {
    execute,
    dependencies: new Set()
  };
  
  const execute = () => {
    cleanup(running);
    context.push(running);
    try {
      fn();
    } finally {
      context.pop();
    }
  };

  execute();
}

Reactions / Effects

Signals under the hood

console.log("1. Create Signal");
const [count, setCount] = createSignal(0);

console.log("2. Create Reaction");
createEffect(() => console.log("The count is", count()));

console.log("3. Set count to 5");
setCount(5);

console.log("4. Set count to 10");
setCount(10);

Signals under the hood

export function createMemo(fn) {
  const [s, set] = createSignal();
  createEffect(() => set(fn()));
  return s;
}

Computed/memo

Signals under the hood

console.log("1. Create");
const [firstName, setFirstName] = createSignal("John");
const [lastName, setLastName] = createSignal("Smith");
const [showFullName, setShowFullName] = createSignal(true);

const displayName = createMemo(() => {
  if (!showFullName()) return firstName();
  return `${firstName()} ${lastName()}`
});

createEffect(() => console.log("My name is", displayName()));

console.log("2. Set showFullName: false ");
setShowFullName(false);

console.log("3. Change lastName");
setLastName("Legend");

console.log("4. Set showFullName: true");
setShowFullName(true);

Signals under the hood

const [firstName, setFirstName] = createSignal("Salama");
const [lastName, setLastName] = createSignal("Ashoush");
const [showLastName, setShowLastName] = createSignal(false);


const fullName = createMemo(() => {
  if(showLastName()){
    return `${firstName()} ${lastName()}`;
  }
  return firstName();
});

createEffect(() => console.log(fullName())) // ?

setLastName("Salama")

setShowLastName(true)

setLastName("Ashoush")

Dynamic Tracking

Signals under the hood

Signals

state primitives

Nested Reactivity

import { createStore } from "solid-js/store";
// stores are proxy based
const [person, setPerson] = createStore({
  name: "Salama",
  age: 30,
  kids:[
   {
     name: "Omar",
     age: 3,
   },
    {
     name: "Ali",
     age: 0.5,
   },
 ]
});

// update a nested 
setPerson('kids', 0, 'age', 2.5);
// there is also a mutuble version that allows this 
const [person, setPerson] = createMutableStore({...})
kids[0].age = 2.5

Signals state primitives

import { component$, useStore } from '@builder.io/qwik';
export default component$(() => {
  const state = useStore({
    name: "Salama",
    age: 32,
    kids:[
     {
       name: "Omar",
       age: 5,
     },
      {
       name: "Ali",
       age: 2
     },
   ]
  });
  return (
    <>
      <button onClick$={() => state.kids[0].age = 2.5}>Update nested</button>
      <p>Count: {state.kids[0].age}</p>
    </>
  );
});

Signals state primitives

Nested Reactivity

import { createResource } from 'solidjs';
 
// you can pass just a fetch function
const [data, { mutate, refetch }] = createResource(() => fetch("something"));

// or you can pass a signal to be tracked and the fetcher will run when it changes
const [data, { mutate, refetch }] = createResource(someSignal(), () => fetch("something"));

// access the data
data()

// update the data
mutate()

// rerun the fetcher funtion
refetch()

Async Reactivity

Signals state primitives

export default component$(() => {
  const prNumber = useSignal('3576');
  const prTitle = useResource$<string>(async ({ track }) => {
    // it will run first on mount (server), then re-run whenever prNumber changes (client)
    // this means this code will run on the server and the browser
    track(() => prNumber.value);
    const response = await fetch(
      `https://api.github.com/repos/BuilderIO/qwik/pulls/${prNumber.value}`
    );
    const data = await response.json();
    return data.title as string;
  });
 
  return (
    <>
      <input type="number" bind:value={prNumber} />
      <h1>PR#{prNumber}:</h1>
      <Resource
        value={prTitle}
        onPending={() => <p>Loading...</p>}
        onResolved={(title) => <h2>{title}</h2>}
      />
    </>
  );
});

Async Reactivity

Signals state primitives

Signals and Components

import { render } from "solid-js/web";
import { createSignal } from "solid-js";

function Counter() {
  const [count, setCount] = createSignal(1);
  const increment = () => setCount(count() + 1);
  
  // since the components only runs once this will lose reactivity
  const doubleCount = count() * 2

  return (
    <button type="button" onClick={increment}>
      {doubleCount}
    </button>
  );
}

render(() => <Counter />, document.getElementById("app")!);

import { render } from "solid-js/web";
import { createSignal, createMemo } from "solid-js";

function Counter() {
  const [count, setCount] = createSignal(1);
  const increment = () => setCount(count() + 1);
  
  // to fix this you have to ethier wrap it in a thunk
  const doubleCount = () => count() * 2
  
  // or createMemo
  const doubleCount = createMemo(() => count() * 2)


  return (
    <button type="button" onClick={increment}>
      {doubleCount()}
    </button>
  );
}

render(() => <Counter />, document.getElementById("app")!);

Signals and Components

import { component$, useSignal } from '@builder.io/qwik';
 
// this all component will run once on the server and never on the client
// untill user interacts with the button
export default component$(() => {
  // to achieve full resumeabilty qwik tie the reactive graph to the component graph
  // which means you can't use signals outside of components
  const count = useSignal(0);
  // unlike solid this will stay reactive
  // thanks to the qwik compiler
  const doubleCount = count.value * 2
 
  return (
    <button onClick$={() => count.value++}>
      Increment {count.value}
    </button>
  );
});

Signals and Components

Signals everywhere

import { useSignal, useComputed } from "@preact/signals";

function Counter() {
  const count = useSignal(0);
  const double = useComputed(() => count.value * 2);

  return (
    <div>
      <p>{count} x 2 = {double}</p>
      <button onClick={() => count.value++}>click me</button>
    </div>
  );
}
Although it signals, it is not fine-grained reactivity since it still relies on the VDOM and React rendering model.

What about React signals?!

What about React signals?!

// .svelte
<script>
// runes are like compiler macros that add lower level code on compile time
let a = $state(1);
let b = $state(2);

// computed/derived state, compiler will transform this to 
// lower level signal primitives
let c = $derived(a + b)

b += 3

// there is also effects 
effect$(() => {
  console.log(c);
})

</script>

<p>{c}</p>
Runes are similar to compile-time macros, but they are essentially just signal primitives used at runtime.

Svelte v5 Runes

@Component({
  signals: true,
  selector: 'simple-counter',
  template: `
  <!-- count is invoked as a getter! -->
  <p>Count {{ count() }}</p>
  <p>{{ name }}</p> <!-- Not reactive! -->
  <button (click)="increment()">Increment count</button>`,
})
export class SimpleCounter {
  count = signal(0); // WritableSignal<number>
  name = 'Morgan';

  increment() {
    this.count.update(c => c + 1);
  }
}

Angular signals

@Component({
  signals: true,
  selector: 'user-profile',
  template: `
    <p>Name: {{ firstName() }} {{ lastName() }}</p>
    <p>Account suspended: {{ suspended() }}</p>
  `,
})
export class UserProfile {
  // Create an optional input without an initial value.
  firstName = input<string>(); // Signal<string|undefined>

  // Create an input with a default value
  lastName = input('Smith'); // Signal<string>

  // Create an input with options.
  suspended = input<boolean>(false, {
    alias: 'disabled',
  }); // Signal<boolean>
}

Angular signals

@Component({
  signals: true,
  selector: 'form-field',
  template: `
    <field-icon *ngFor="let icon of icons()"> {{ icon }} </field-icon>
    <div class="focus-outline">
      <input #field>
    </div>
  `
})
export class FormField {
  icons = viewChildren(FieldIcon); // Signal<FieldIcon[]>
  input = viewChild<ElementRef>('field'); // Signal<ElementRef>

  someEventHandler() {
    this.input().nativeElement.focus();
  }
}

Angular signals

use leptos::prelude::*;

#[component]
fn App() -> impl IntoView {
    let (count, set_count) = signal(0);

    view! {
        <button
            on:click=move |_| set_count.set(3)
        >
            "Click me: "
            {count}
        </button>
        <p>
            "Double count: "
            {move || count.get() * 2}
        </p>
    }
}

Finally it is not only JS 

Signals are the new v-dom

Thanks

Questions