Integrate the prototype push notification support.

We're going to move forwards with this for now, as low-utility as it is, so that we can more easily iterate on it in a real-world environment (hi.grimoire.ca).

1 files changed, 141 insertions, 0 deletions

diff --git a/ui/lib/state/local/push.svelte.js b/ui/lib/state/local/push.svelte.js
new file mode 100644
index 0000000..82846b1
--- /dev/null
+++ b/ui/lib/state/local/push.svelte.js
@@ -0,0 +1,141 @@
+import * as api from '$lib/apiServer.js';
+
+// In a few places in this module, I've used suffix to identify which type we're working with:
+// * ...Base64 is a string containing a URL-safe base64 value;
+// * ...Buffer is an ArrayBuffer holding untyped bytes; and
+// * ...Bytes is a Uint8Array holding typed bytes.
+// Working with byte streams in JS is _fun_.
+
+export class Push {
+  static async boot(events) {
+    const serviceWorker = await navigator.serviceWorker.ready;
+    const pushManager = serviceWorker.pushManager;
+    const subscription = await serviceWorker.pushManager.getSubscription();
+
+    const push = new Push(pushManager, subscription);
+
+    for (const event of events) {
+      push.onEvent(event);
+    }
+
+    return push;
+  }
+
+  vapidKey = $state(null);
+  subscription = $state(null);
+
+  constructor(pushManager, subscription) {
+    this.pushManager = pushManager;
+    this.subscription = subscription;
+  }
+
+  async hasPermission() {
+    const vapidKeyBuffer = this.vapidKey;
+    const pushOptions = {
+      userVisibleOnly: true,
+      applicationServerKey: vapidKeyBuffer,
+    };
+
+    const state = await this.pushManager.permissionState(pushOptions);
+    return state === 'granted';
+  }
+
+  async subscribe() {
+    const vapidKeyBuffer = this.vapidKey;
+    const pushOptions = {
+      userVisibleOnly: true,
+      applicationServerKey: vapidKeyBuffer,
+    };
+    this.subscription = await this.pushManager.subscribe(pushOptions);
+
+    const subscriptionJSON = this.subscription.toJSON();
+    const vapidKeyBytes = new Uint8Array(vapidKeyBuffer);
+    const vapidKeyBase64 = vapidKeyBytes.toBase64({ alphabet: 'base64url' });
+    await api.createPushSubscription(subscriptionJSON, vapidKeyBase64);
+  }
+
+  async resubscribe() {
+    if (this.subscription !== null) {
+      // If we have a subscription but it's not for the current VAPID key, then the VAPID key has
+      // been rotated and we need to replace the subscription. The server cannot deliver messages to
+      // subscriptions associated with the old VAPID key after rotation.
+
+      // Per spec, the `options.applicationServerKey` field is either null or an ArrayBuffer,
+      // regardless of the representation  passed into `subscribe` to create the subscription:
+      //
+      // <https://w3c.github.io/push-api/#pushsubscriptionoptions-interface>
+      if (!equalArrayBuffers(this.subscription.options.applicationServerKey, this.vapidKey)) {
+        // We have a subscription, and the server key has rotated, so resubscribe. Destroy the old
+        // subscription first - some UAs (Firefox) want subscriptions within an origin to share a
+        // consistent VAPID key, and we're explicitly changing the VAPID key here.
+        await this.unsubscribe();
+        await this.subscribe();
+      }
+    } else if (await this.hasPermission()) {
+      // If we have permission to create push subscriptions, but no push subscription, then set up
+      // a new subscription. This primarily happens after logging into Pilcrow if the user
+      // previously had a push subscription and has logged out.
+      await this.subscribe();
+    }
+  }
+
+  async unsubscribe() {
+    if (this.subscription !== null) {
+      await this.subscription.unsubscribe();
+      this.subscription = null;
+    }
+  }
+
+  onEvent(event) {
+    switch (event.type) {
+      case 'vapid':
+        return this.onVapidEvent(event);
+    }
+  }
+
+  onVapidEvent(event) {
+    switch (event.event) {
+      case 'changed':
+        return this.onVapidChanged(event);
+    }
+  }
+
+  onVapidChanged(event) {
+    const { key: vapidKeyBase64 } = event;
+
+    // For ease of use later on, parse the key into an ArrayBuffer. This is a little fiddly because
+    // of the APIs involved, but it makes comparing the key to the current subscription's key
+    // (which is also provided as an ArrayBuffer) much easier.
+    const vapidKeyBytes = Uint8Array.fromBase64(vapidKeyBase64, {
+      alphabet: 'base64url',
+    });
+    // In practice, `vapidKeyBytes.buffer` is going to be the same bytes as this slice, but in
+    // principle it could be a subset of the underlying buffer, and there's very little downside to
+    // being meticulous here.
+    this.vapidKey = vapidKeyBytes.buffer.slice(
+      vapidKeyBytes.byteOffset,
+      vapidKeyBytes.byteOffset + vapidKeyBytes.byteLength,
+    );
+
+    // Note that `resubscribe()` is async; this will start the [re]subscription process but will
+    // return to the caller before it completes. I'm not willing to make the entire event handling
+    // chain all the way back up to the EventSource asynchronous, since EventSource isn't designed
+    // to support that (we could make it work), and we can't wait for async functions in a non-async
+    // context, so this is the best we can do.
+    this.resubscribe();
+  }
+}
+
+function equalArrayBuffers(aBuffer, bBuffer) {
+  // You might be thinking, surely there's a way to compare two array buffers for equality. I
+  // certainly expected that there would be. [Nope].
+  //
+  // [Nope]: https://github.com/wbinnssmith/arraybuffer-equal
+  //
+  // The algorithm here is simple enough not to need an external dependency. However, this
+  // comparison is not designed to deal with detached ArrayBuffer instances.
+  const aBytes = new Uint8Array(aBuffer);
+  const bBytes = new Uint8Array(bBuffer);
+
+  return aBytes.length === bBytes.length && aBytes.every((value, index) => value === bBytes[index]);
+}