presence: move all presence-reporting logic into new HeartbeatComponent

Centralize all presence-reporting logic into a new zero-display React
component, currently (and somewhat arbitrarily) located under
AppStateHandlers.

This fixes at least two issues:

* If the user logs in and out, or otherwise performs an additional
  "initial fetch" without killing the app, the presence timer will
  be started multiple times. (This was previously concealed by
  throttling logic in usersActions.reportPresence, which was itself
  removed in a previous patch in this set.)

* If the user goes idle (e.g. by hitting the Home button), the
  presence timer will still run, and will still attempt to send
  `active` notifications. (These attempts are often suppressed while
  the app is in the background, but there's no guarantee of this.)

Fixes zulip#3699 (properly), and is work towards zulip#3659. Unfortunately,
there is no user-visible change yet: modifications will be needed on
the Zulip server side as well.

src/boot/AppEventHandlers.js

@@ -11,13 +11,8 @@ import type { Dispatch, Orientation as OrientationT } from '../types';
 import { connect } from '../react-redux';
 import { getUnreadByHuddlesMentionsAndPMs } from '../selectors';
 import { handleInitialNotification, NotificationListener } from '../notification';
-import {
-  appOnline,
-  appOrientation,
-  appState,
-  initSafeAreaInsets,
-  reportPresence,
-} from '../actions';
+import { appOnline, appOrientation, appState, initSafeAreaInsets } from '../actions';
+import PresenceHeartbeat from '../presence/PresenceHeartbeat';
 
 /**
  * Part of the interface from react-native-netinfo.
@@ -90,7 +85,6 @@ class AppEventHandlers extends PureComponent<Props> {
   /** For the type, see docs: https://facebook.github.io/react-native/docs/appstate */
   handleAppStateChange = (state: 'active' | 'background' | 'inactive') => {
     const { dispatch, unreadCount } = this.props;
-    dispatch(reportPresence(state === 'active'));
     dispatch(appState(state === 'active'));
     if (state === 'background' && Platform.OS === 'android') {
       NativeModules.BadgeCountUpdaterModule.setBadgeCount(unreadCount);
@@ -131,7 +125,12 @@ class AppEventHandlers extends PureComponent<Props> {
   }
 
   render() {
-    return <View style={styles.wrapper}>{this.props.children}</View>;
+    return (
+      <>
+        <PresenceHeartbeat />
+        <View style={styles.wrapper}>{this.props.children}</View>
+      </>
+    );
   }
 }
 

src/message/fetchActions.js

@@ -23,7 +23,6 @@ import { tryUntilSuccessful } from '../utils/async';
 import { initNotifications } from '../notification/notificationActions';
 import { addToOutbox, sendOutbox } from '../outbox/outboxActions';
 import { realmInit } from '../realm/realmActions';
-import { reportPresence } from '../users/usersActions';
 import { startEventPolling } from '../events/eventActions';
 import { logout } from '../account/accountActions';
 
@@ -219,9 +218,6 @@ const fetchTopMostNarrow = () => async (dispatch: Dispatch, getState: GetState)
  * we want to do when starting up, or regaining a network connection.
  */
 export const doInitialFetch = () => async (dispatch: Dispatch, getState: GetState) => {
-  dispatch(reportPresence());
-  setInterval(() => dispatch(reportPresence()), 60 * 1000);
-
   dispatch(initialFetchStart());
   const auth = getAuth(getState());
 

src/presence/PresenceHeartbeat.js

@@ -0,0 +1,71 @@
+// @flow strict-local
+import { PureComponent } from 'react';
+import { AppState } from 'react-native';
+import type { Auth, Dispatch } from '../types';
+
+import { connect } from '../react-redux';
+import { tryGetAuth } from '../account/accountsSelectors';
+import { reportPresence } from '../actions';
+import Heartbeat from './heartbeat';
+
+type Props = $ReadOnly<{|
+  dispatch: Dispatch,
+  auth: Auth | void,
+|}>;
+
+/**
+ * Component providing a recurrent `presence` signal.
+ */
+// Note that "PureComponent" is of questionable veracity: this component's
+// entire purpose is to emit network calls for their observable side effects
+// as a side effect of being rendered.
+//
+// However, it is at least true that there is never a need to call `render()`
+// (and therefore `componentDidUpdate()`) if the props haven't changed.
+class PresenceHeartbeat extends PureComponent<Props> {
+  /** Callback for Heartbeat object. */
+  onHeartbeat = (state: boolean) => {
+    // N.B.: If `auth` changes, we do not send out a final `false` presence
+    // status for the previous `auth`. It's the responsibility of our logout
+    // handler to determine whether that's necessary.
+    //
+    // (TODO: ensure that our logout handlers actually do that.)
+    if (this.props.auth) {
+      this.props.dispatch(reportPresence(state));
+    }
+  };
+
+  heartbeat: Heartbeat = new Heartbeat(this.onHeartbeat, 1000 * 60);
+
+  componentDidMount() {
+    this.updateHeartbeatState(); // conditional start
+    AppState.addEventListener('change', this.updateHeartbeatState);
+  }
+
+  componentWillUnmount() {
+    AppState.removeEventListener('change', this.updateHeartbeatState);
+    this.heartbeat.stop(); // unconditional stop
+  }
+
+  // React to any state change.
+  updateHeartbeatState = () => {
+    // heartbeat.toState is idempotent