Add PubSub and ReactiveStore utilities

package.json

@@ -17,7 +17,9 @@
     "./components/view-toggle.js": "./src/components/view-toggle/view-toggle.js",
     "./controllers/computed-value.js": "./src/controllers/computed-values/computed-value.js",
     "./controllers/computed-values.js": "./src/controllers/computed-values/computed-values.js",
-    "./controllers/language-listener.js": "./src/controllers/language-listener/language-listener.js"
+    "./controllers/language-listener.js": "./src/controllers/language-listener/language-listener.js",
+    "./utilities/pub-sub.js": "./src/utilities/pub-sub/pub-sub.js",
+    "./utilities/reactive-store.js": "./src/utilities/reactive-store/reactive-store.js"
   },
   "scripts": {
     "langs:sync": "mfv add-missing && mfv remove-extraneous",
@@ -52,6 +54,7 @@
   },
   "dependencies": {
     "@brightspace-ui/core": "^3",
+    "@lit/context": "^1.1.3",
     "lit": "^3"
   },
   "version": "2.4.1"

src/utilities/pub-sub/README.md

@@ -0,0 +1,76 @@
+# PubSub
+
+A simple class implementation of the publish-subscribe model.
+
+## Simple Example
+
+```js
+import PubSub from '@brightspace-ui/labs/utilites/pub-sub.js';
+
+// Instantiate the PubSub class
+const myPubSub = new PubSub();
+
+// Register subscribers
+const subscriber1 = (message) => console.log('Subscriber 1 received: ', message);
+const subscriber2 = (message) => console.log('Subscriber 2 received: ', message);
+myPubSub.subscribe(subscriber1);
+myPubSub.subscribe(subscriber2);
+
+// Publish messages to subscribers
+myPubSub.publish('Hello!');
+// Console: Subscriber 1 received: Hello!
+// Console: Subscriber 2 received: Hello!
+
+// Unsubscribe
+myPubSub.unsubscribe(subscriber1);
+myPubSub.unsubscribe(subscriber2);
+```
+
+## Instance Methods
+
+### Constructor
+
+The constructor takes no arguments.
+
+### `clear()`
+
+The `clear` method unsubscribes all subscribed callback functions. Subscriber callback functions are not called at this time, just removed.
+
+This method accepts no arguments and returns no values.
+
+### `publish(...args)`
+
+The `publish` method is used to publish messages/data to all subscribed callbacks. All subscribed callback functions are called in subscription order with the same arguments passed to `publish`.
+
+| Parameter Name | Type | Description | Required |
+|---|---|---|---|
+| `...args` | Any | The arguments to be passed to subscriber callback functions when called. | No |
+
+`publish` returns no values.
+
+### `subscribe(callback, initialize = false)`
+
+The `subscribe` method is used to subscribe a callback function for future published messages.
+
+If the callback being subscribed is already subscribed, nothing will change and it will still only be called once when messages are published.
+
+By default, a subscribed callback function won't be called until a message is published, but if the `initialize` argument is set to `true` and the `PubSub` instance has published at least once, then the callback function will be immediately called after subscription with the last published arguments.
+
+| Parameter Name | Type | Description | Required | Default Value |
+|---|---|---|---|---|
+| `callback` | Function | The callback function to be called when messages are published. | True | |
+| `initialize` | Boolean | Whether or not to immedately call the callback function with the last published values. | False | `false` |
+
+`subscribe` returns no values.
+
+### `unsubscribe(callback)`
+
+The `unsubscribe` method is used to remove a callback function from the collection of subscribers so it no longer receives future published messages.
+
+If the callback function passed in does not match a currently subscribed function, nothing happens.
+
+| Parameter Name | Type | Description | Required |
+|---|---|---|---|
+| `callback` | Function | The callback function to remove from the subscribers. | True |
+
+`unsubscribe` returns no values.

src/utilities/pub-sub/pub-sub.js

@@ -0,0 +1,33 @@
+/*
+	A simple pub-sub implementation. It allows for subscribing to the class and publishing to all subscribers.
+*/
+export default class PubSub {
+	constructor() {
+		this._subscribers = new Map();
+		this._hasTriggered = false;
+		this._previousArgs = [];
+	}
+
+	clear() {
+		this._subscribers.clear();
+	}
+
+	publish(...args) {
+		this._subscribers.forEach(callback => callback(...args));
+		this._hasTriggered = true;
+		this._previousArgs = args;
+	}
+
+	// If initialize is true and publish has been called at least once, the callback will be called
+	// immediately with the last published arguments.
+	subscribe(callback, initialize = false) {
+		this._subscribers.set(callback, callback);
+		if (this._hasTriggered && initialize) {
+			callback(...this._previousArgs);
+		}
+	}
+
+	unsubscribe(callback) {
+		this._subscribers.delete(callback);
+	}
+}