refactor(cli): Reactive useSettingsStore hook

packages/cli/src/config/settings.test.ts

@@ -2512,6 +2512,50 @@ describe('Settings Loading and Merging', () => {
     });
   });
 
+  describe('Reactivity & Snapshots', () => {
+    let loadedSettings: LoadedSettings;
+
+    beforeEach(() => {
+      const emptySettingsFile: SettingsFile = {
+        path: '/mock/path',
+        settings: {},
+        originalSettings: {},
+      };
+
+      loadedSettings = new LoadedSettings(
+        { ...emptySettingsFile, path: getSystemSettingsPath() },
+        { ...emptySettingsFile, path: getSystemDefaultsPath() },
+        { ...emptySettingsFile, path: USER_SETTINGS_PATH },
+        { ...emptySettingsFile, path: MOCK_WORKSPACE_SETTINGS_PATH },
+        true, // isTrusted
+        [],
+      );
+    });
+
+    it('getSnapshot() should return stable reference if no changes occur', () => {
+      const snap1 = loadedSettings.getSnapshot();
+      const snap2 = loadedSettings.getSnapshot();
+      expect(snap1).toBe(snap2);
+    });
+
+    it('setValue() should create a new snapshot reference and emit event', () => {
+      const oldSnapshot = loadedSettings.getSnapshot();
+      const oldUserRef = oldSnapshot.user.settings;
+
+      loadedSettings.setValue(SettingScope.User, 'ui.theme', 'high-contrast');
+
+      const newSnapshot = loadedSettings.getSnapshot();
+
+      expect(newSnapshot).not.toBe(oldSnapshot);
+      expect(newSnapshot.user.settings).not.toBe(oldUserRef);
+      expect(newSnapshot.user.settings.ui?.theme).toBe('high-contrast');
+
+      expect(newSnapshot.system.settings).not.toBe(oldSnapshot.system.settings);
+
+      expect(mockCoreEvents.emitSettingsChanged).toHaveBeenCalled();
+    });
+  });
+
   describe('Security and Sandbox', () => {
     let originalArgv: string[];
     let originalEnv: NodeJS.ProcessEnv;

packages/cli/src/config/settings.ts

@@ -10,6 +10,7 @@ import { platform } from 'node:os';
 import * as dotenv from 'dotenv';
 import process from 'node:process';
 import {
+  CoreEvent,
   FatalConfigError,
   GEMINI_DIR,
   getErrorMessage,
@@ -281,6 +282,20 @@ export function createTestMergedSettings(
   ) as MergedSettings;
 }
 
+/**
+ * An immutable snapshot of settings state.
+ * Used with useSyncExternalStore for reactive updates.
+ */
+export interface LoadedSettingsSnapshot {
+  system: SettingsFile;
+  systemDefaults: SettingsFile;
+  user: SettingsFile;
+  workspace: SettingsFile;
+  isTrusted: boolean;
+  errors: SettingsError[];
+  merged: MergedSettings;
+}
+
 export class LoadedSettings {
   constructor(
     system: SettingsFile,
@@ -300,6 +315,7 @@ export class LoadedSettings {
       : this.createEmptyWorkspace(workspace);
     this.errors = errors;
     this._merged = this.computeMergedSettings();
+    this._snapshot = this.computeSnapshot();
   }
 
   readonly system: SettingsFile;
@@ -311,6 +327,7 @@ export class LoadedSettings {
 
   private _workspaceFile: SettingsFile;
   private _merged: MergedSettings;
+  private _snapshot: LoadedSettingsSnapshot;
   private _remoteAdminSettings: Partial<Settings> | undefined;
 
   get merged(): MergedSettings {
@@ -364,6 +381,36 @@ export class LoadedSettings {
     return merged;
   }
 
+  private computeSnapshot(): LoadedSettingsSnapshot {
+    const cloneSettingsFile = (file: SettingsFile): SettingsFile => ({
+      path: file.path,
+      rawJson: file.rawJson,
+      settings: structuredClone(file.settings),
+      originalSettings: structuredClone(file.originalSettings),
+    });
+    return {
+      system: cloneSettingsFile(this.system),
+      systemDefaults: cloneSettingsFile(this.systemDefaults),
+      user: cloneSettingsFile(this.user),
+      workspace: cloneSettingsFile(this.workspace),
+      isTrusted: this.isTrusted,
+      errors: [...this.errors],
+      merged: structuredClone(this._merged),
+    };
+  }
+
+  // Passing this along with getSnapshot to useSyncExternalStore allows for idiomatic reactivity on settings changes
+  // React will pass a listener fn into this subscribe fn
+  // that listener fn will perform an object identity check on the snapshot and trigger a React re render if the snapshot has changed
+  subscribe(listener: () => void): () => void {
+    coreEvents.on(CoreEvent.SettingsChanged, listener);
+    return () => coreEvents.off(CoreEvent.SettingsChanged, listener);
+  }
+
+  getSnapshot(): LoadedSettingsSnapshot {
+    return this._snapshot;
+  }
+
   forScope(scope: LoadableSettingScope): SettingsFile {
     switch (scope) {
       case SettingScope.User:
@@ -405,6 +452,7 @@ export class LoadedSettings {
     }
 
     this._merged = this.computeMergedSettings();
+    this._snapshot = this.computeSnapshot();
     coreEvents.emitSettingsChanged();
   }
 

packages/cli/src/ui/contexts/SettingsContext.test.tsx

@@ -0,0 +1,167 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type React from 'react';
+import { Component, type ReactNode } from 'react';
+import { renderHook, render } from '../../test-utils/render.js';
+import { act } from 'react';
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { SettingsContext, useSettingsStore } from './SettingsContext.js';
+import {
+  type LoadedSettings,
+  SettingScope,
+  type LoadedSettingsSnapshot,
+  type SettingsFile,
+  createTestMergedSettings,
+} from '../../config/settings.js';
+
+const createMockSettingsFile = (path: string): SettingsFile => ({
+  path,
+  settings: {},
+  originalSettings: {},
+});
+
+const mockSnapshot: LoadedSettingsSnapshot = {
+  system: createMockSettingsFile('/system'),
+  systemDefaults: createMockSettingsFile('/defaults'),
+  user: createMockSettingsFile('/user'),
+  workspace: createMockSettingsFile('/workspace'),
+  isTrusted: true,
+  errors: [],
+  merged: createTestMergedSettings({
+    ui: { theme: 'default-theme' },
+  }),
+};
+
+class ErrorBoundary extends Component<
+  { children: ReactNode; onError: (error: Error) => void },
+  { hasError: boolean }
+> {
+  constructor(props: { children: ReactNode; onError: (error: Error) => void }) {
+    super(props);
+    this.state = { hasError: false };
+  }
+
+  static getDerivedStateFromError(_error: Error) {
+    return { hasError: true };
+  }
+
+  override componentDidCatch(error: Error) {
+    this.props.onError(error);
+  }
+
+  override render() {
+    if (this.state.hasError) {
+      return null;
+    }
+    return this.props.children;
+  }
+}
+
+const TestHarness = () => {
+  useSettingsStore();
+  return null;
+};
+
+describe('SettingsContext', () => {
+  let mockLoadedSettings: LoadedSettings;
+  let listeners: Array<() => void> = [];
+
+  beforeEach(() => {
+    listeners = [];
+
+    mockLoadedSettings = {
+      subscribe: vi.fn((listener: () => void) => {
+        listeners.push(listener);
+        return () => {
+          listeners = listeners.filter((l) => l !== listener);
+        };
+      }),
+      getSnapshot: vi.fn(() => mockSnapshot),
+      setValue: vi.fn(),
+    } as unknown as LoadedSettings;
+  });
+
+  const wrapper = ({ children }: { children: React.ReactNode }) => (
+    <SettingsContext.Provider value={mockLoadedSettings}>
+      {children}
+    </SettingsContext.Provider>
+  );
+
+  it('should provide the correct initial state', () => {
+    const { result } = renderHook(() => useSettingsStore(), { wrapper });
+
+    expect(result.current.settings.merged).toEqual(mockSnapshot.merged);
+    expect(result.current.settings.isTrusted).toBe(true);
+  });
+
+  it('should allow accessing settings for a specific scope', () => {
+    const { result } = renderHook(() => useSettingsStore(), { wrapper });
+
+    const userSettings = result.current.settings.forScope(SettingScope.User);
+    expect(userSettings).toBe(mockSnapshot.user);
+
+    const workspaceSettings = result.current.settings.forScope(
+      SettingScope.Workspace,
+    );
+    expect(workspaceSettings).toBe(mockSnapshot.workspace);
+  });
+
+  it('should trigger re-renders when settings change (external event)', () => {
+    const { result } = renderHook(() => useSettingsStore(), { wrapper });
+
+    expect(result.current.settings.merged.ui?.theme).toBe('default-theme');
+
+    const newSnapshot = {
+      ...mockSnapshot,
+      merged: { ui: { theme: 'new-theme' } },
+    };
+    (
+      mockLoadedSettings.getSnapshot as ReturnType<typeof vi.fn>
+    ).mockReturnValue(newSnapshot);
+
+    // Trigger the listeners (simulate coreEvents emission)
+    act(() => {
+      listeners.forEach((l) => l());
+    });
+
+    expect(result.current.settings.merged.ui?.theme).toBe('new-theme');
+  });
+
+  it('should call store.setValue when setSetting is called', () => {
+    const { result } = renderHook(() => useSettingsStore(), { wrapper });
+
+    act(() => {
+      result.current.setSetting(SettingScope.User, 'ui.theme', 'dark');
+    });
+
+    expect(mockLoadedSettings.setValue).toHaveBeenCalledWith(
+      SettingScope.User,
+      'ui.theme',
+      'dark',
+    );
+  });
+
+  it('should throw error if used outside provider', () => {
+    const onError = vi.fn();
+    // Suppress console.error (React logs error boundary info)
+    const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+
+    render(
+      <ErrorBoundary onError={onError}>
+        <TestHarness />
+      </ErrorBoundary>,
+    );
+
+    expect(onError).toHaveBeenCalledWith(
+      expect.objectContaining({
+        message: 'useSettingsStore must be used within a SettingsProvider',
+      }),
+    );
+
+    consoleSpy.mockRestore();
+  });
+});

packages/cli/src/ui/contexts/SettingsContext.tsx

@@ -4,17 +4,81 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import React, { useContext } from 'react';
-import type { LoadedSettings } from '../../config/settings.js';
+import React, { useContext, useMemo, useSyncExternalStore } from 'react';
+import type {
+  LoadableSettingScope,
+  LoadedSettings,
+  LoadedSettingsSnapshot,
+  SettingsFile,
+} from '../../config/settings.js';
+import { SettingScope } from '../../config/settings.js';
 
 export const SettingsContext = React.createContext<LoadedSettings | undefined>(
   undefined,
 );
 
-export const useSettings = () => {
+export const useSettings = (): LoadedSettings => {
   const context = useContext(SettingsContext);
   if (context === undefined) {
     throw new Error('useSettings must be used within a SettingsProvider');
   }
   return context;
 };
+
+export interface SettingsState extends LoadedSettingsSnapshot {
+  forScope: (scope: LoadableSettingScope) => SettingsFile;
+}
+
+export interface SettingsStoreValue {
+  settings: SettingsState;
+  setSetting: (
+    scope: LoadableSettingScope,
+    key: string,
+    value: unknown,
+  ) => void;
+}
+
+// Components that call this hook will re render when a settings change event is emitted
+export const useSettingsStore = (): SettingsStoreValue => {
+  const store = useContext(SettingsContext);
+  if (store === undefined) {
+    throw new Error('useSettingsStore must be used within a SettingsProvider');
+  }
+
+  // React passes a listener fn into the subscribe function
+  // When the listener runs, it re renders the component if the snapshot changed
+  const snapshot = useSyncExternalStore(
+    (listener) => store.subscribe(listener),
+    () => store.getSnapshot(),
+  );
+
+  const settings: SettingsState = useMemo(
+    () => ({
+      ...snapshot,
+      forScope: (scope: LoadableSettingScope) => {
+        switch (scope) {
+          case SettingScope.User:
+            return snapshot.user;
+          case SettingScope.Workspace:
+            return snapshot.workspace;
+          case SettingScope.System:
+            return snapshot.system;
+          case SettingScope.SystemDefaults:
+            return snapshot.systemDefaults;
+          default:
+            throw new Error(`Invalid scope: ${scope}`);
+        }
+      },
+    }),
+    [snapshot],
+  );
+
+  return useMemo(
+    () => ({
+      settings,
+      setSetting: (scope: LoadableSettingScope, key: string, value: unknown) =>
+        store.setValue(scope, key, value),
+    }),
+    [settings, store],
+  );
+};