[DevTools] chore: add useSyncExternalStore examples to shell (#34932)

Few examples of using `useSyncExternalStore` that can be useful for
debugging hook tree reconstruction logic and hook names parsing feature.

packages/react-devtools-shell/src/app/InspectableElements/InspectableElements.js

@@ -20,6 +20,7 @@ import SimpleValues from './SimpleValues';
 import SymbolKeys from './SymbolKeys';
 import UseMemoCache from './UseMemoCache';
 import UseEffectEvent from './UseEffectEvent';
+import UseSyncExternalStore from './UseSyncExternalStore';
 
 // TODO Add Immutable JS example
 
@@ -38,6 +39,7 @@ export default function InspectableElements(): React.Node {
       <SymbolKeys />
       <UseMemoCache />
       <UseEffectEvent />
+      <UseSyncExternalStore />
     </Fragment>
   );
 }

packages/react-devtools-shell/src/app/InspectableElements/UseSyncExternalStore.js

@@ -0,0 +1,133 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import * as React from 'react';
+
+const {useState, useEffect, useSyncExternalStore} = React;
+
+// Create a simple external store for demonstratio
+function createStore<T>(initialValue: T): {
+  subscribe: (cb: () => void) => () => any,
+  getSnapshot: () => T,
+  setValue: (newValue: T) => void,
+} {
+  let value = initialValue;
+  const subscribers = new Set<() => void>();
+
+  return {
+    subscribe(callback) {
+      subscribers.add(callback);
+      return () => subscribers.delete(callback);
+    },
+    getSnapshot() {
+      return value;
+    },
+    setValue(newValue) {
+      value = newValue;
+      subscribers.forEach(callback => callback());
+    },
+  };
+}
+
+const counterStore = createStore(0);
+const themeStore = createStore('light');
+
+export default function UseSyncExternalStore(): React.Node {
+  return (
+    <>
+      <h2>useSyncExternalStore()</h2>
+      <SingleHookCase />
+      <HookTreeCase />
+      <MultipleStoresCase />
+    </>
+  );
+}
+
+function SingleHookCase(): React.Node {
+  const count = useSyncExternalStore(
+    counterStore.subscribe,
+    counterStore.getSnapshot,
+  );
+
+  return (
+    <div>
+      <h3>Single hook case</h3>
+      <p>Count: {count}</p>
+      <button onClick={() => counterStore.setValue(count + 1)}>
+        Increment
+      </button>
+      <button onClick={() => counterStore.setValue(count - 1)}>
+        Decrement
+      </button>
+    </div>
+  );
+}
+
+function useCounter() {
+  const count = useSyncExternalStore(
+    counterStore.subscribe,
+    counterStore.getSnapshot,
+  );
+  const [localState, setLocalState] = useState(0);
+
+  useEffect(() => {
+    // Some effect
+  }, [count]);
+
+  return {count, localState, setLocalState};
+}
+
+function HookTreeCase(): React.Node {
+  const {count, localState, setLocalState} = useCounter();
+
+  return (
+    <div>
+      <h3>Hook tree case</h3>
+      <p>External count: {count}</p>
+      <p>Local state: {localState}</p>
+      <button onClick={() => counterStore.setValue(count + 1)}>
+        Increment External
+      </button>
+      <button onClick={() => setLocalState(localState + 1)}>
+        Increment Local
+      </button>
+    </div>
+  );
+}
+
+function useTheme() {
+  const theme = useSyncExternalStore(
+    themeStore.subscribe,
+    themeStore.getSnapshot,
+  );
+
+  return theme;
+}
+
+function MultipleStoresCase() {
+  const count = useSyncExternalStore(
+    counterStore.subscribe,
+    counterStore.getSnapshot,
+  );
+  const theme = useTheme();
+
+  return (
+    <div style={{background: theme === 'dark' ? '#333' : '#fff'}}>
+      <h3>Multiple stores case</h3>
+      <p>Count: {count}</p>
+      <p>Theme: {theme}</p>
+      <button
+        onClick={() =>
+          themeStore.setValue(theme === 'light' ? 'dark' : 'light')
+        }>
+        Toggle Theme
+      </button>
+    </div>
+  );
+}