Reaction API reference for Reaction in Semantic UI's reactivity system refresh-cw Guide

Reaction

Reaction a core class in Semantic UI’s reactivity system that manages reactive computations. It allows you to create and control reactive contexts which re-run when reactive data sources are updated.

Please see the dedicated section on reactivity to learn more about writing and using reactivity in practice.

Reaction Class

create

Creates a new reaction and runs it immediately. The reaction will re-run whenever any reactive dependencies accessed within the callback change.

Syntax

Reaction.create(callback)

Parameters

NameTypeDescription
callbackFunctionThe function to run reactively. Receives the reaction instance as its argument.

Returns

Reaction - The created reaction instance.

Usage

Basic Usage

import { Reaction, ReactiveVar } from '@semantic-ui/reactivity';


const counter = new ReactiveVar(0);


Reaction.create((reaction) => {
  console.log('Counter value:', counter.get());
});


counter.set(1);


// Console output:
// Counter value: 0
// Counter value: 1

Stopping a Reaction

You can stop a reaction from within its callback or externally using the returned reaction instance.

import { Reaction, ReactiveVar } from '@semantic-ui/reactivity';


const counter = new ReactiveVar(0);


const reaction = Reaction.create((reaction) => {
  console.log('Counter value:', counter.get());
  if (counter.get() >= 2) {
    reaction.stop();
    console.log('Reaction stopped');
  }
});


counter.set(1);
counter.set(2);
counter.set(3); // This won't trigger the reaction


// Console output:
// Counter value: 0
// Counter value: 1
// Counter value: 2
// Reaction stopped

Checking for First Run

The firstRun property allows you to perform specific actions only on the initial execution of the reaction.

import { Reaction, ReactiveVar } from '@semantic-ui/reactivity';


const data = new ReactiveVar({ value: 0 });


Reaction.create((reaction) => {
  const currentData = data.get();


  if (reaction.firstRun) {
    console.log('Initial data:', currentData);
  } else {
    console.log('Updated data:', currentData);
  }
});


data.set({ value: 1 });


// Console output:
// Initial data: { value: 0 }
// Updated data: { value: 1 }

Note: Be cautious when using early returns with firstRun. If you return early on the first run, the reaction won’t establish dependencies on reactive variables accessed after the return statement.

nonreactive

Runs a function without establishing any reactive dependencies.

Syntax

Reaction.nonreactive(func)

Parameters

NameTypeDescription
funcFunctionThe function to run non-reactively

Returns

any - The return value of the provided function.

Usage

import { Reaction, ReactiveVar } from '@semantic-ui/reactivity';


const counter = new ReactiveVar(0);


Reaction.nonreactive(() => {
  console.log('Counter value (non-reactive):', counter.get());
});


counter.set(1); // No console output

guard

Wraps a function to only re-run when its return value changes.

Syntax

Reaction.guard(func)

Parameters

NameTypeDescription
funcFunctionThe function to guard

Callback Parameters

NameTypeDescription
reactionclass instanceThe current running reaction

See individual reactions for more info.

Returns

any - The return value of the guarded function.

Usage

import { Reaction, ReactiveVar } from '@semantic-ui/reactivity';


const counter = new ReactiveVar(0);


Reaction.create(() => {
  const isEven = Reaction.guard(() => counter.get() % 2 === 0);
  console.log('Counter is even:', isEven);
});


counter.set(1); // Console: "Counter is even: false"
counter.set(3); // No console output (value didn't change)

getSource

Returns the stack trace of the current reaction for debugging purposes.

Debugging Reactivity - getSource is designed to be use inside of a breakpoint to determine what reaction is currently running.

Syntax

Reaction.getSource()

Returns

string - The stack trace of the current reaction.

Usage

import { Reaction } from '@semantic-ui/reactivity';


Reaction.create(() => {
  console.log(Reaction.getSource());
});

Individual Reaction

You can control each reaction by returning the result of Reaction.create with the following methods

run

Runs the reaction manually.

Syntax

const reaction = Reaction.create(() => {
  console.log('Reaction ran');
});
reaction.run()
// Console output:
// Reaction ran

Usage

import { Reaction } from '@semantic-ui/reactivity';


const reaction = Reaction.create(() => {
  console.log('Reaction ran');
});


reaction.run(); // Console: "Reaction ran"

invalidate

Marks the reaction as invalid, scheduling it to run again.

Syntax

reaction.invalidate(context)

Parameters

NameTypeDescription
contextany(Optional) Context information for the invalidation

Usage

import { Reaction } from '@semantic-ui/reactivity';


const reaction = Reaction.create(() => {
  console.log('Reaction ran');
});


reaction.invalidate();

stop

Stops the reaction from running again.

Syntax

reaction.stop()

Usage

import { Reaction } from '@semantic-ui/reactivity';


const reaction = Reaction.create(() => {
  console.log('Reaction ran');
});


reaction.stop();

First Run

import { Reaction } from '@semantic-ui/reactivity';


const reaction = Reaction.create((reaction) => {
  console.log(reaction.firstRun); // outputs true
});