usersReducers: Handle 'EVENT_USER_UPDATE' server events

Fixes zulip#3397 Part of zulip#3408

Handles the three types of user data updates possible:
* full name
* avatar
* custom profile field

Extra care is taken not to mutate the state if not necessary.

This is done for two reasons:
 * currently editing values throught he web app causes two events,
   one of which is not necessary (maybe should be fixed)
 * this is likely the largest state, doing an extra `isEqual` can
   save copying all this data

src/users/__tests__/usersReducers-test.js

@@ -1,6 +1,11 @@
 import deepFreeze from 'deep-freeze';
 
-import { REALM_INIT, EVENT_USER_ADD, ACCOUNT_SWITCH } from '../../actionConstants';
+import {
+  REALM_INIT,
+  EVENT_USER_ADD,
+  ACCOUNT_SWITCH,
+  EVENT_USER_UPDATE,
+} from '../../actionConstants';
 import usersReducers from '../usersReducers';
 
 describe('usersReducers', () => {
@@ -96,6 +101,74 @@ describe('usersReducers', () => {
     });
   });
 
+  describe('EVENT_USER_UPDATE', () => {
+    test('updating non existing user does not mutate state', () => {
+      const initialState = deepFreeze([
+        {
+          user_id: 1,
+          full_name: 'Some Guy',
+        },
+      ]);
+      const action = deepFreeze({
+        type: EVENT_USER_UPDATE,
+        person: {
+          user_id: 2,
+          full_name: 'New Name',
+        },
+      });
+
+      const actualState = usersReducers(initialState, action);
+
+      expect(actualState).toBe(initialState);
+    });
+
+    test('updating an existing user mutates state', () => {
+      const initialState = deepFreeze([
+        {
+          user_id: 1,
+          full_name: 'Some Guy',
+        },
+      ]);
+      const action = deepFreeze({
+        type: EVENT_USER_UPDATE,
+        person: {
+          user_id: 1,
+          full_name: 'New Name',
+        },
+      });
+      const expectedState = [
+        {
+          user_id: 1,
+          full_name: 'New Name',
+        },
+      ];
+
+      const actualState = usersReducers(initialState, action);
+
+      expect(actualState).toEqual(expectedState);
+    });
+
+    test('updating existing user with the same values does not mutate state', () => {
+      const initialState = deepFreeze([
+        {
+          user_id: 1,
+          full_name: 'Some Guy',
+        },
+      ]);
+      const action = deepFreeze({
+        type: EVENT_USER_UPDATE,
+        person: {
+          user_id: 1,
+          full_name: 'Some Guy',
+        },
+      });
+
+      const actualState = usersReducers(initialState, action);
+
+      expect(actualState).toBe(initialState);
+    });
+  });
+
   describe('ACCOUNT_SWITCH', () => {
     test('resets state to initial state', () => {
       const initialState = deepFreeze([

src/users/usersReducers.js

@@ -1,5 +1,7 @@
 /* @flow strict-local */
-import type { UsersState, Action } from '../types';
+import isEqual from 'lodash.isequal';
+
+import type { User, UsersState, Action } from '../types';
 import {
   LOGOUT,
   LOGIN_SUCCESS,
@@ -10,6 +12,7 @@ import {
   EVENT_USER_UPDATE,
 } from '../actionConstants';
 import { NULL_ARRAY } from '../nullObjects';
+import { updateUser } from './userHelpers';
 
 const initialState: UsersState = NULL_ARRAY;
 
@@ -29,8 +32,22 @@ export default (state: UsersState = initialState, action: Action): UsersState =>
     case EVENT_USER_REMOVE:
       return state; // TODO
 
-    case EVENT_USER_UPDATE:
-      return state; // TODO
+    case EVENT_USER_UPDATE: {
+      const userIndex = state.findIndex(x => x.user_id === action.person.user_id);
+      const oldUser: User = state[userIndex];
+
+      if (userIndex === -1) {
+        return state;
+      }
+
+      const updatedUser = updateUser(oldUser, action.person);
+
+      if (isEqual(updatedUser, oldUser)) {
+        return state;
+      }
+
+      return [...state.slice(0, userIndex), updatedUser, ...state.slice(userIndex + 1)];
+    }
 
     default:
       return state;