@semantic-ui/reactivity

This is a paired down signal/reactivity library loosely inspired by Tracker from Meteor JS. It is used to control reactive updates from Semantic and is built into its templating system.

ReactiveVar lets you define a variable which will trigger any Reaction or functions called by a reaction to retrigger when it is modified.

Reaction is a reactive context, this will rerun when referenced reactive values are modified.

Reactions are enqueued and then flushed using the microtask queue

• Basic Usage
• Equality
  • Property / Array Mutations
  • Booleans
  • First Run
• Controlling Reactivity
  • Guard
  • Peeking at Current Value
  • Nonreactive
  • Flushing Changes
  • Accessing Computation
• Helper Functions
  • Number Helpers
  • Array Mutation Helpers
  • Array of Objects

Basic Usage

You can create a reaction by simply creating a variable then modifying its value.

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

let reactiveValue = new ReactiveVar('first');
Reaction.create(computation => {
  console.log(reactiveValue.get());
});

// equivalent ways to set value
saying.value = 'second' // option 1
saying.set('second');  // option 2

// outputs first, second

Any computation will receive itself as the first parameter of its callback, which you use to do things like check if its the firstRun or call stop() to stop the computation.

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

let saying = new ReactiveVar('hello');
Reaction.create(comp => {
  if(comp.firstRun) {
    console.log('First run!');
  }
  // stop computation if value is set to goodbye
  if(saying.value == 'goodbye') {
    comp.stop();
  }
});
saying.set('goodbye');

Equality

When declaring a reactive variable you can pass in an equality function that will be used to determine if calculations should be rerun when the value is updated. The default function is isEqual from utils which will handle most common cases like dates, binary data and deep object equality.

Objects

let obj1 = { name: 'Sally', age: 22 };
let obj2 = { name: 'Sally', age: 22 };
let reactiveObj = new ReactiveVar(obj1);
Reaction.create(comp => {
  console.log(reactiveObj.get());
});
reactiveObj.set(obj2);

// outputs {name: 'Sally', age: 22}

let obj1 = { name: 'Sally', age: 22 };
let obj2 = { name: 'Sally', age: 23 };
let reactiveObj = new ReactiveVar(obj1);

Reaction.create(comp => {
  console.log(reactiveObj.get());
});

reactiveObj.set(obj2);
// outputs {name: 'Sally', age: 22}
// outputs {name: 'Sally', age: 23}

// always rerun
const customIsEqual = (a, b) => {
  return false;
}
let reactiveObj = new ReactiveVar({ name: 'Sally', age: 22 }, customIsEqual);

Reaction.create(comp => {
  const obj = reactiveObj.get();
  console.log('Log');
});
reactiveObj.set(reactiveObj.get());
// log runs twice

Property / Array Mutations

You can use the set helper to declaratively update values like arrays and objects, which would not normally trigger reactivity if you simply modify their value.

Objects

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

let person = {
  name: 'Jack',
  age: 32,
}
let reactivePerson = new ReactiveVar(person);
Reaction.create(comp => {
  console.log(reactivePerson.get().name);
});

person.name = 'Jill';

reactivePerson.set(person);
// outputs Jack, Jill

Arrays

let rows = [
  { name: 'Sally', age: 22 },
  { name: 'Jack', age: 32 }
];
let reactiveRows = new ReactiveVar(rows);

Reaction.create(comp => {
  console.log(reactiveRows.get().length);
});

rows.pop();

reactiveRows.set(rows);
// outputs 2, 1

const numbers = new ReactiveVar([10, 20, 30]);

// Add an item to the end
numbers.push(40);
// 10, 20, 30, 40

numbers.unshift(0);
// 0, 10, 20, 30, 40

numbers.splice(0, 2);
// 0, 10

numbers.setIndex(1, 99);
// 0, 99

numbers.removeIndex(1);
// 0

Booleans

Boolean helpers allow you to toggle the state of a ReactiveVar that holds a boolean value.

const isToggled = new ReactiveVar(false);

// Toggle the boolean value
isToggled.toggle();
console.log('Value is now true');

isToggled.toggle();
console.log('Value is now false again');

First Run

You can use firstRun to determine if this calculation is running from an initial value being set. Keep in mind though if you leave the function early on first run it will never set up a reactive reference to unreachable code.

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

let saying = new ReactiveVar('hello');
Reaction.create(comp => {
  if(comp.firstRun) {
    return;
  }
  let saying = saying.get();
  console.log(saying);
  // outputs nothing (early termination on first run prevents reactive var from being referenced)
});

Reaction.create(comp => {
  let saying = saying.get();
  if(comp.firstRun) {
    return;
  }
  console.log(saying);
  // outputs goodbye
});

saying.set('goodbye');

Controlling Reactivity

Guard

You can help fine-tune reactivity by using guard to only pay attention to certain parts of a reactive context

  const userAge = new ReactiveVar(30);
  const userName = new ReactiveVar('John Doe');
  const lastUpdated = new ReactiveVar(new Date()); // Assume this updates frequently
  const getUserInfo = () => {
    return {
      name: userName,
      age: userAge,
      date: lastUpdated,
    };
  };
  Reaction.create((comp) => {
    Reaction.guard(() => {
      let user = getUserInfo(); // we only want to call this function if name/age changes
      return {
        name: user.name,
        age: user.age,
      };
    });
    if(!comp.firstRun) {
      console.log(`User Info Updated: Name: ${userInfo.name}, Age: ${userInfo.age}`);
    }
    // Simulate updates
    setTimeout(() => {
      userName.value = 'Jane Doe'; // This should trigger the reaction
    }, 300);
    setTimeout(() => {
      userAge.value = 31; // This should also trigger the reaction
    }, 1000);
    setTimeout(() => {
      lastUpdated.value = new Date(); // This should NOT trigger the reaction
    }, 2000);
  });

Peeking at Current Value

To get the current value of a ReactiveVar without establishing a reactive dependency, use the peek() method. This is particularly useful when you need to access the value for read-only purposes outside of a reactive computation and do not want to trigger reactivity.

const counter = new ReactiveVar(10);

// Access the value without triggering reactivity
const currentValue = counter.peek();
console.log(`Current value without establishing dependency: ${currentValue}`);

Nonreactive

The Reaction.nonreactive function allows you to perform computations or access reactive variables without establishing a reactive dependency. This is useful when you need to read from a reactive source but don't want the surrounding computation to re-run when the source changes.

const reactiveValue = new ReactiveVar('Initial Value');

// Perform a non-reactive read
Reaction.nonreactive(() => {
  const value = reactiveValue.get();
  console.log(`Read inside nonreactive: ${value}`);
});

reactiveValue.set('Updated Value'); // Does not trigger the console.log inside nonreactive

Flushing Changes

When a ReactiveVar updates an update is enqueued and flushes asynchronously when the microtask queue is processed. This means that intermediary values will not be processed when updating code in a loop.

You can trigger the queue to be immediately flushed to prevent this by using the Reaction.flush() helper.

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

let number = new ReactiveVar(1);
Reaction.create(comp => {
  console.log(number.get());
});

[1,2,3,4,5].forEach(value => number.set(value));

let number = new ReactiveVar(1);
Reaction.create(comp => {
  console.log(number.get());
});

[1,2,3,4,5].forEach(value => {
  number.set(value);
  Reaction.flush();
});
// outputs 1,2,3,4,5

Accessing Computation

You can access the current computation either using the returned value from create or as the first parameter of the callback.

This can be helpful to inspect the listeners or to stop the computation using the stop method.

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

let number = new ReactiveVar(1);
Reaction.create(comp => {
  if(number.get() > 3) {
    comp.stop();
    return;
  }
  console.log(number.get());
});

let comp = Reaction.create(() => {
  if(number.get() > 3) {
    comp.stop();
    return;
  }
  console.log(number.get());
});

[1,2,3,4,5].forEach(value => {
  number.set(value);
  Reaction.flush();
});
// both output 1,2,3

Helper Functions

Numbers

ReactiveVar includes a couple helpers numbers

  let count = new ReactiveVar(0);
  count.increment(); // set to 1

  let count = new ReactiveVar(0);
  count.increment(2); // set to 2

  let count = new ReactiveVar(2);
  count.decrement(); // set to 1

  let count = new ReactiveVar(0);
  count.decrement(2); // set to 1

Date

ReactiveVar includes a helper to make dates asier

  let date = new ReactiveVar(new Date()); // initializes as now
  setTimeout(() => {
    date.now(); // now 1 second later
  }, 1000);

Array Mutation Helpers

ReactiveVar includes a few helpers for some of the most common usecases for manipulating arrays

  let items = new ReactiveVar([0,1,2]);
  items.removeIndex(1); // outputs 0, 2

  let items = new ReactiveVar([0,2,2]);
  items.setIndex(1); // outputs 0, 1, 2

  let items = new ReactiveVar([0,1,2]);
  items.unshift(); // outputs 1, 2

  let items = new ReactiveVar([0,1,2]);
  items.push(3); // outputs 0, 1, 2, 3

Array of Objects

ReactiveVar provides several helpers for manipulating arrays of objects a common data structure when handling structured data.

const tasks = new ReactiveVar([
  { _id: 'task1_uuid', task: 'Implement feature', completed: true }
  { _id: 'task2_uuid', task: 'Write Tests', completed: true }
  { _id: 'task3_uuid', task: 'Write documentation', completed: false },
]);

// sets 'write documentation' to complete
tasks.setProperty('task3_uuid', 'completed', true);

// replaces task 1 with new task
const newTask = { _id: 'tasks1_uuid', task: 'Reimplement feature', completed: false };
tasks.replaceItem('task1_uuid', newTask)

// gets index of id
const index = tasks.getIndex('tasks1_uuid')

// gets id from an item
const id = tasks.getID(tasks.get()[0])

// remove task 2 from list
tasks.removeItem('tasks2_uuid')