usersReducer: Handle EVENT_USER_UPDATE a little bit.

I'm wary of suddenly starting to clobber any arbitrary property in
users, so I'm starting small with just `avatar_url`. I chose this in
particular because we're about to use our own new data type here,
the `AvatarURL`. But, in the tests, lay the groundwork for handling
all other documented updates.

Now, when someone updates their avatar, it will be live-updated in
`state.users`; see discussion, where Tim said to check that this is
being done [1].

Note that it's not yet updated in `state.messages`, so the message
list won't be live-updated when someone changes their avatar. See
discussion [2] on implementing this.

[1] https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/user_avatar_url_field_optional/near/909056.
    In retrospect, I think he actually meant to make sure the server
    was behaving correctly -- but we might as well start doing the
    right thing here too.
[2] https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/user_avatar_url_field_optional/near/992469

src/actionTypes.js

@@ -414,9 +414,11 @@ type EventUserRemoveAction = {|
 |};
 
 type EventUserUpdateAction = {|
+  ...ServerEvent,
   type: typeof EVENT_USER_UPDATE,
-  // In reality there's more -- but this will prevent accidentally using
-  // the type before going and adding those other properties here properly.
+  userId: number,
+  // Include only the fields that should be overwritten.
+  person: $Shape<User>,
 |};
 
 type EventMutedTopicsAction = {|

src/events/eventToAction.js

@@ -1,6 +1,7 @@
 /* @flow strict-local */
 import { EventTypes } from '../api/eventTypes';
 
+import * as logging from '../utils/logging';
 import type { GlobalState, EventAction } from '../types';
 import {
   EVENT_ALERT_WORDS,
@@ -31,7 +32,7 @@ import {
   EVENT_SUBSCRIPTION,
   EVENT,
 } from '../actionConstants';
-import { getOwnUser, getOwnUserId } from '../users/userSelectors';
+import { getOwnUser, getOwnUserId, getAllUsersById } from '../users/userSelectors';
 
 const opToActionUserGroup = {
   add: EVENT_USER_GROUP_ADD,
@@ -129,12 +130,37 @@ export default (state: GlobalState, event: $FlowFixMe): EventAction => {
             person: event.person,
           };
 
-        case 'update':
-          // In an upcoming commit, we'll add `person`, with the
-          // fields we wish to update.
+        case 'update': {
+          // We'll use this in an upcoming commit.
+          // eslint-disable-next-line no-unused-vars
+          const existingUser = getAllUsersById(state).get(event.person.user_id);
+          if (!existingUser) {
+            // If we get one of these events and don't have
+            // information on the user, there's nothing to do about
+            // it. But it's probably a bug, so, tell Sentry.
+            logging.warn(
+              "`realm_user` event with op `update` received for a user we don't know about",
+              { userId: event.person.user_id },
+            );
+            return { type: 'ignore' };
+          }
+
           return {
             type: EVENT_USER_UPDATE,
+            id: event.id,
+            userId: event.person.user_id,
+            // Just the fields we want to overwrite.
+            person: {
+              // Note: The `avatar_url` field will be out of sync with
+              // some related, documented properties, but we don't
+              // currently use them: `avatar_source`,
+              // `avatar_url_medium`, and `avatar_version`.
+              ...(event.person.avatar_url !== undefined
+                ? { avatar_url: event.person.avatar_url }
+                : undefined),
+            },
           };
+        }
 
         case 'remove':
           // TODO: Handle this event and properly form this action.

src/users/__tests__/usersReducer-test.js

@@ -2,7 +2,8 @@
 import deepFreeze from 'deep-freeze';
 
 import * as eg from '../../__tests__/lib/exampleData';
-import { EVENT_USER_ADD, ACCOUNT_SWITCH } from '../../actionConstants';
+import type { User } from '../../types';
+import { EVENT_USER_ADD, EVENT_USER_UPDATE, ACCOUNT_SWITCH } from '../../actionConstants';
 import usersReducer from '../usersReducer';
 
 describe('usersReducer', () => {
@@ -46,6 +47,84 @@ describe('usersReducer', () => {
     });
   });
 
+  describe('EVENT_USER_UPDATE', () => {
+    const theUser = eg.makeUser();
+    const prevState = deepFreeze([theUser]);
+
+    /**
+     * Check that an update event with supplied `person` works.
+     *
+     * May omit `user_id` to avoid repetition.
+     */
+    // Tell ESLint to recognize `check` as a helper function that runs
+    // assertions.
+    /* eslint jest/expect-expect: ["error", { "assertFunctionNames": ["expect", "check"] }] */
+    const check = (personMaybeWithoutId: $Shape<User>) => {
+      const person = {
+        user_id: theUser.user_id,
+        ...personMaybeWithoutId,
+      };
+      const action = deepFreeze({
+        id: 1,
+        type: EVENT_USER_UPDATE,
+        userId: person.user_id,
+        person,
+      });
+
+      expect(usersReducer(prevState, action)).toEqual([{ ...theUser, ...person }]);
+    };
+
+    /*
+     * Should match REALM_USER OP: UPDATE in the doc.
+     *
+     * See https://zulip.com/api/get-events#realm_user-update.
+     *
+     * A few properties that we don't handle are commented out.
+     */
+
+    test('When a user changes their full name.', () => {
+      check({ full_name: eg.randString() });
+    });
+
+    test('When a user changes their avatar.', () => {
+      check({
+        avatar_url: eg.randString(),
+        // avatar_source: user1.avatar_source === 'G' ? 'U' : 'G',
+        // avatar_url_medium: eg.randString(),
+        // avatar_version: user1.avatar_version + 1,
+      });
+    });
+
+    test('When a user changes their timezone setting.', () => {
+      check({ timezone: eg.randString() });
+    });
+
+    // Excluded: "When the owner of a bot changes." The `users` state
+    // doesn't include cross-realm bots.
+
+    test('When the role of a user changes.', () => {
+      check({
+        // role: user1.role + 1,
+      });
+    });
+
+    test('When the delivery email of a user changes.', () => {
+      check({
+        // delivery_email: eg.randString(),
+      });
+    });
+
+    test('When the user updates one of their custom profile fields.', () => {
+      check({
+        // custom_profile_field: {
+        //   id: 4,
+        //   value: eg.randString(),
+        //   rendered_value: eg.randString(),
+        // },
+      });
+    });
+  });
+
   describe('ACCOUNT_SWITCH', () => {
     const user1 = eg.makeUser();
 

src/users/usersReducer.js

@@ -30,7 +30,9 @@ export default (state: UsersState = initialState, action: Action): UsersState =>
       return state; // TODO
 
     case EVENT_USER_UPDATE:
-      return state; // TODO
+      return state.map(user =>
+        user.user_id === action.userId ? { ...user, ...action.person } : user,
+      );
 
     default:
       return state;