implement sessionAtom function (same api as persistenAtom, just used sessionStorage instead of localStorage)

website/src/user_pattern_utils.mjs

@@ -1,5 +1,5 @@
-import { atom } from 'nanostores';
-import { persistentAtom, setPersistentEngine, windowPersistentEvents } from '@nanostores/persistent';
+import {atom, onMount} from 'nanostores';
+import { persistentAtom, windowPersistentEvents } from '@nanostores/persistent';
 import { useStore } from '@nanostores/react';
 import { logger } from '@strudel/core';
 import { nanoid } from 'nanoid';
@@ -21,14 +21,58 @@ export const patternFilterName = {
   user: 'user',
 };
 
+export let sessionAtom = persistentAtom;
+
 if (typeof sessionStorage !== 'undefined') {
-  // NB: this switches storageEngine of all persistent atoms
-  // ideally we would switch the engine per atom
-  // (like in this abandoned pr: https://github.com/nanostores/persistent/pull/30)
-  setPersistentEngine(sessionStorage, windowPersistentEvents);
+  const identity = a => a;
+  const eventsEngine = windowPersistentEvents;
+  sessionAtom = (name, initial = undefined, opts = {}) => {
+    let encode = opts.encode || identity
+    let decode = opts.decode || identity
+
+    let store = atom(initial)
+
+    let set = store.set
+    store.set = newValue => {
+      if (typeof newValue === 'undefined') {
+        delete sessionStorage[name]
+      } else {
+        sessionStorage[name] = encode(newValue)
+      }
+      set(newValue)
+    }
+
+    function listener(e) {
+      if (e.key === name) {
+        if (e.newValue === null) {
+          set(undefined)
+        } else {
+          set(decode(e.newValue))
+        }
+      } else if (!sessionStorage[name]) {
+        set(undefined)
+      }
+    }
+
+    function restore() {
+      store.set(sessionStorage[name] ? decode(sessionStorage[name]) : initial)
+    }
+
+    onMount(store, () => {
+      restore()
+      if (opts.listen !== false) {
+        eventsEngine.addEventListener(name, listener, restore)
+        return () => {
+          eventsEngine.removeEventListener(name, listener, restore)
+        }
+      }
+    })
+
+    return store
+  }
 }
 
-export let $viewingPatternData = persistentAtom(
+export let $viewingPatternData = sessionAtom(
   'viewingPatternData',
   {
     id: '',
@@ -75,7 +119,7 @@ export async function loadDBPatterns() {
 }
 
 // reason: https://github.com/tidalcycles/strudel/issues/857
-const $activePattern = persistentAtom('activePattern', '', { listen: false });
+const $activePattern = sessionAtom('activePattern', '', { listen: false });
 
 export function setActivePattern(key) {
   $activePattern.set(key);