Add persistent state provider (#1830)

* Add persistent state provider

* Catch write error

* Handle read errors, update error msgs

* Fix lint

* Don't provide initial state to loader

* Remove colorMode from shell state

* Idea: hook into persisted context from other files

* Migrate settings to new hook

* Rework persisted state to split individual contexts

* Tweak persisted schema and validation

---------

Co-authored-by: Paul Frazee <pfrazee@gmail.com>

src/state/persisted/index.ts

@@ -0,0 +1,91 @@
+import EventEmitter from 'eventemitter3'
+import {logger} from '#/logger'
+import {defaults, Schema} from '#/state/persisted/schema'
+import {migrate} from '#/state/persisted/legacy'
+import * as store from '#/state/persisted/store'
+import BroadcastChannel from '#/state/persisted/broadcast'
+
+export type {Schema} from '#/state/persisted/schema'
+export {defaults as schema} from '#/state/persisted/schema'
+
+const broadcast = new BroadcastChannel('BSKY_BROADCAST_CHANNEL')
+const UPDATE_EVENT = 'BSKY_UPDATE'
+
+let _state: Schema = defaults
+const _emitter = new EventEmitter()
+
+/**
+ * Initializes and returns persisted data state, so that it can be passed to
+ * the Provider.
+ */
+export async function init() {
+  logger.debug('persisted state: initializing')
+
+  broadcast.onmessage = onBroadcastMessage
+
+  try {
+    await migrate() // migrate old store
+    const stored = await store.read() // check for new store
+    if (!stored) await store.write(defaults) // opt: init new store
+    _state = stored || defaults // return new store
+  } catch (e) {
+    logger.error('persisted state: failed to load root state from storage', {
+      error: e,
+    })
+    // AsyncStorage failured, but we can still continue in memory
+    return defaults
+  }
+}
+
+export function get<K extends keyof Schema>(key: K): Schema[K] {
+  return _state[key]
+}
+
+export async function write<K extends keyof Schema>(
+  key: K,
+  value: Schema[K],
+): Promise<void> {
+  try {
+    _state[key] = value
+    await store.write(_state)
+    // must happen on next tick, otherwise the tab will read stale storage data
+    setTimeout(() => broadcast.postMessage({event: UPDATE_EVENT}), 0)
+    logger.debug(`persisted state: wrote root state to storage`)
+  } catch (e) {
+    logger.error(`persisted state: failed writing root state to storage`, {
+      error: e,
+    })
+  }
+}
+
+export function onUpdate(cb: () => void): () => void {
+  _emitter.addListener('update', cb)
+  return () => _emitter.removeListener('update', cb)
+}
+
+async function onBroadcastMessage({data}: MessageEvent) {
+  // validate event
+  if (typeof data === 'object' && data.event === UPDATE_EVENT) {
+    try {
+      // read next state, possibly updated by another tab
+      const next = await store.read()
+
+      if (next) {
+        logger.debug(`persisted state: handling update from broadcast channel`)
+        _state = next
+        _emitter.emit('update')
+      } else {
+        logger.error(
+          `persisted state: handled update update from broadcast channel, but found no data`,
+        )
+      }
+    } catch (e) {
+      logger.error(
+        `persisted state: failed handling update from broadcast channel`,
+        {
+          error: e,
+        },
+      )
+    }
+  }
+}