Reactive Dict

A ReactiveDict stores an arbitrary set of key-value pairs. Use it to manage internal state in your components, ie. like the currently selected item in a list. Each key is individully reactive such that calling set for a key will invalidate any Computations that called get with that key, according to the usual contract for reactive data sources.

That means if you call ReactiveDict#get('currentList') from inside a Blaze template helper, the template will automatically be rerendered whenever ReactiveDict#set('currentList', x) is called.

To use ReactiveDict, add the reactive-dict package to your project by running in your terminal:

bash

meteor add reactive-dict

If you provide a name to its constructor, its contents will be saved across Hot Code Push client code updates.

Example:

import { ReactiveDict } from "meteor/reactive-dict";
import { Tracker } from "meteor/tracker";
import { Meteor } from "meteor/meteor";

const state = new ReactiveDict();
state.set("currentRoomId", "random");

Tracker.autorun(() => {
  Meteor.subscribe("chatHistory", { room: state.get("currentRoomId") });
});

// Causes the function passed to `Tracker.autorun` to be rerun, so that the
// 'chatHistory' subscription is moved to the room 'general'.
state.set("currentRoomId", "general");

ReactiveDict.set can also be called with an object of keys and values, which is equivalent to calling ReactiveDict.set individually on each key/value pair.

import { ReactiveDict } from "meteor/reactive-dict";
const state = new ReactiveDict();
state.set({
  a: "foo",
  b: "bar",
});

This is useful in initialization code, to avoid re-initializing your state every time a new version of your app is loaded.

Example in Blaze:

::: code-group

html

<template name="main">
  <p>We've always been at war with {{theEnemy}}.</p>
  <button class="change-enemy">Change Enemy</button>
</template>

Template.main.onCreated(function () {
  this.state = new ReactiveDict();
  this.state.set("enemy", "Eastasia");
});
Template.main.helpers({
  theEnemy() {
    const inst = Template.instance();
    return inst.state.get("enemy");
  },
});
Template.main.events({
  "click .change-enemy"(event, inst) {
    inst.state.set("enemy", "Eurasia");
  },
});

// Clicking the button will change the page to say "We've always been at war with Eurasia"

:::

If value is a scalar, then these two expressions do the same thing:

import { ReactiveDict } from "meteor/reactive-dict";
const state = new ReactiveDict();
// ...
state.get("key") === value;
state.equals("key", value);

However, the second is recommended, as it triggers fewer invalidations (template redraws), making your program more efficient.