From 9573f06c36cf8fa553aa26809908188f3664bc1b Mon Sep 17 00:00:00 2001
From: Adam Simon <adamosimoni@gmail.com>
Date: Wed, 27 May 2026 13:22:08 +0200
Subject: [PATCH] Fix useFeatureFlag to re-evaluate on default user change

---
 src/ConfigCatContext.tsx    |  3 +-
 src/ConfigCatHooks.test.tsx | 41 +++++++++++++++++++++--
 src/ConfigCatProvider.tsx   | 65 ++++++++++++++++++++++++++++++-------
 3 files changed, 95 insertions(+), 14 deletions(-)

diff --git a/src/ConfigCatContext.tsx b/src/ConfigCatContext.tsx
index 55100b2..3d0c483 100644
--- a/src/ConfigCatContext.tsx
+++ b/src/ConfigCatContext.tsx
@@ -1,8 +1,9 @@
-import type { IConfigCatClient } from "@configcat/sdk";
+import type { IConfigCatClient, IUser } from "@configcat/sdk";
 import React from "react";
 
 export interface ConfigCatContextData {
   client: IConfigCatClient;
+  defaultUser?: IUser | null;
   lastUpdated?: Date;
 }
 
diff --git a/src/ConfigCatHooks.test.tsx b/src/ConfigCatHooks.test.tsx
index d543e71..06080ba 100644
--- a/src/ConfigCatHooks.test.tsx
+++ b/src/ConfigCatHooks.test.tsx
@@ -1,5 +1,5 @@
-import { PollingMode } from "@configcat/sdk";
-import { render, screen } from "@testing-library/react";
+import { IUser, PollingMode } from "@configcat/sdk";
+import { fireEvent, render, screen } from "@testing-library/react";
 import React, { useEffect, useState } from "react";
 import { vi } from "vitest";
 import { useConfigCatClient, useFeatureFlag } from "./ConfigCatHooks";
@@ -92,6 +92,43 @@ it("useFeatureFlag Manual poll with forceRefresh should work", async () => {
   await screen.findByText("Feature flag value: Cat", void 0, { timeout: 2000 });
 });
 
+it("useFeatureFlag should pick up changed default user", async () => {
+  const defaultUser: IUser = { identifier: "0", email: "test@configcat.com" };
+
+  const TestComponent = () => {
+    const client = useConfigCatClient();
+    const [user, setUser] = useState<IUser | null>(defaultUser);
+    useEffect(() => user ? client.setDefaultUser(user) : client.clearDefaultUser(), [client, user]);
+    const { value: featureFlag } = useFeatureFlag("stringContainsDogDefaultCat", "NOT_CAT");
+    return (
+      <>
+        <div>Feature flag value: {featureFlag}</div>
+        <button onClick={() => setUser(defaultUser)}>Set default user</button>
+        <button onClick={() => setUser(null)}>Clear default user</button>
+      </>
+    );
+  };
+
+  await render(<ConfigCatProvider sdkKey={sdkKey} options={{ defaultUser }}><TestComponent /></ConfigCatProvider>);
+  const flagValueDiv = await screen.findByText("Feature flag value: Dog", void 0, { timeout: 2000 });
+
+  let button = screen.getByText("Clear default user");
+  fireEvent.click(button);
+
+  // Allow the component to update.
+  await new Promise<void>(resolve => setTimeout(() => resolve(), 0));
+
+  expect(flagValueDiv.textContent).toBe("Feature flag value: Cat");
+
+  button = screen.getByText("Set default user");
+  fireEvent.click(button);
+
+  // Allow the component to update.
+  await new Promise<void>(resolve => setTimeout(() => resolve(), 0));
+
+  expect(flagValueDiv.textContent).toBe("Feature flag value: Dog");
+});
+
 it("useFeatureFlag with invalid providerId should fail", () => {
   const spy = vi.spyOn(console, "error");
   spy.mockImplementation(() => { });
diff --git a/src/ConfigCatProvider.tsx b/src/ConfigCatProvider.tsx
index 2866cee..b0ec32f 100644
--- a/src/ConfigCatProvider.tsx
+++ b/src/ConfigCatProvider.tsx
@@ -30,6 +30,8 @@ type AugmentedConfigCatClient = IConfigCatClient & {
 
 class ConfigCatProvider extends Component<PropsWithChildren<ConfigCatProviderProps>, ConfigCatProviderState, {}> {
   private configChangedHandler?: (newConfig: Config) => void;
+  private originalClearDefaultUser?: () => void;
+  private originalSetDefaultUser?: (defaultUser: IUser) => void;
 
   constructor(props: ConfigCatProviderProps) {
     super(props);
@@ -42,31 +44,72 @@ class ConfigCatProvider extends Component<PropsWithChildren<ConfigCatProviderPro
     const providers = client._configCatReactSdkProviders ??= new Set();
     providers.add(this);
 
-    this.state = { client };
+    this.state = { client, defaultUser: this.props.options?.defaultUser };
   }
 
   componentDidMount(): void {
+    const { client } = this.state;
+
+    // Monkey-patch client's `clearDefaultUser` and `setDefaultUser` methods to detect default user changes.
+
+    // eslint-disable-next-line @typescript-eslint/unbound-method
+    this.originalClearDefaultUser = client.clearDefaultUser;
+    client.clearDefaultUser = () => {
+      if (this.originalClearDefaultUser && this.state.client === client) {
+        this.originalClearDefaultUser.call(client);
+        this.setState({ defaultUser: void 0 });
+      }
+    };
+
+    // eslint-disable-next-line @typescript-eslint/unbound-method
+    this.originalSetDefaultUser = client.setDefaultUser;
+    client.setDefaultUser = defaultUser => {
+      if (this.originalSetDefaultUser && this.state.client === client) {
+        this.originalSetDefaultUser.call(client, defaultUser);
+        this.setState({ defaultUser });
+      }
+    };
+
+    // Wire up config data change detection.
+
     this.configChangedHandler = newConfig => this.reactConfigChanged(newConfig);
 
-    this.state.client.waitForReady().then(() => {
-      if (!this.configChangedHandler) {
-        // If the component was unmounted before client initialization finished, we have nothing left to do.
-        return;
+    client.waitForReady().then(() => {
+      // If the component was unmounted before client initialization finished, we have nothing left to do.
+      if (this.configChangedHandler && this.state.client === client) {
+        client.on("configChanged", this.configChangedHandler);
+        this.clientReady();
       }
-      this.state.client.on("configChanged", this.configChangedHandler);
-      this.clientReady();
     });
   }
 
   componentWillUnmount(): void {
+    const { client } = this.state;
+
+    // Stop config data change detection.
+
     if (this.configChangedHandler) {
-      this.state.client.off("configChanged", this.configChangedHandler);
-      delete this.configChangedHandler;
+      client.off("configChanged", this.configChangedHandler);
+      this.configChangedHandler = void 0;
+    }
+
+    // Restore monkey-patched client methods.
+
+    if (this.originalClearDefaultUser) {
+      client.clearDefaultUser = this.originalClearDefaultUser!;
+      this.originalClearDefaultUser = void 0;
+    }
+
+    if (this.originalSetDefaultUser) {
+      client.setDefaultUser = this.originalSetDefaultUser!;
+      this.originalSetDefaultUser = void 0;
     }
 
-    const providers = (this.state.client as AugmentedConfigCatClient)._configCatReactSdkProviders;
+    // Dispose client if no longer in use.
+
+    const providers = (client as AugmentedConfigCatClient)._configCatReactSdkProviders;
     if (providers?.delete(this) && !providers.size) {
-      this.state.client.dispose();
+      client.dispose();
     }
   }