[RN] LoadingIndicator on WelcomePage

It will replace the "Join" text button while appNavigate lasts.

Note about the implementation: when appNavigate completes the component
may have been unmounted and thus we cannot touch its state. In order to
avoid this problem I added a 'mounted' instance variable which gets set
and reset in componentWillMount / Unmount respectively. This is to avoid
using isMounted, which is highly discouraged.

react/features/welcome/components/AbstractWelcomePage.js

         this.state = {
             animateTimeoutId: null,
             generatedRoomname: '',
+            joining: false,
             room: '',
             roomPlaceholder: '',
             updateTimeoutId: null
     }
 
     /**
-     * This method is executed when component receives new properties.
+     * Implements React's {@link Component#componentWillMount()}. Invoked
+     * immediately before mounting occurs.
+     *
+     * @inheritdoc
+     */
+    componentWillMount() {
+        this._mounted = true;
+    }
+
+    /**
+     * Implements React's {@link Component#componentWillReceiveProps()}. Invoked
+     * before this mounted component receives new props.
      *
      * @inheritdoc
      * @param {Object} nextProps - New props component will receive.
     }
 
     /**
-     * This method is executed when method will be unmounted from DOM.
+     * Implements React's {@link Component#componentWillUnmount()}. Invoked
+     * immediately before this component is unmounted and destroyed.
      *
      * @inheritdoc
      */
     componentWillUnmount() {
         this._clearTimeouts();
+        this._mounted = false;
     }
 
     /**
      * otherwise, false.
      */
     _isJoinDisabled() {
-        return !isRoomValid(this.state.room);
+        return this.state.joining || !isRoomValid(this.state.room);
     }
 
     /**
     _onJoin() {
         const room = this.state.room || this.state.generatedRoomname;
 
-        room && this.props.dispatch(appNavigate(room));
+        if (room) {
+            this.setState({ joining: true });
+
+            // By the time the Promise of appNavigate settles, this component
+            // may have already been unmounted.
+            const onAppNavigateSettled = () => {
+                this._mounted && this.setState({ joining: false });
+            };
+
+            this.props.dispatch(appNavigate(room))
+                .then(onAppNavigateSettled, onAppNavigateSettled);
+        }
     }
 
     /**

react/features/welcome/components/WelcomePage.native.js

 
 import { translate } from '../../base/i18n';
 import { MEDIA_TYPE } from '../../base/media';
-import { Link, Text } from '../../base/react';
+import { Link, LoadingIndicator, Text } from '../../base/react';
 import { ColorPalette } from '../../base/styles';
 import { createDesiredLocalTracks } from '../../base/tracks';
 
     static propTypes = AbstractWelcomePage.propTypes;
 
     /**
-     * Creates a video track if not already available.
+     * Implements React's {@link Component#componentWillMount()}. Invoked
+     * immediately before mounting occurs. Creates a local video track if none
+     * is available.
      *
      * @inheritdoc
      * @returns {void}
      */
     componentWillMount() {
+        super.componentWillMount();
+
         this.props.dispatch(createDesiredLocalTracks(MEDIA_TYPE.VIDEO));
     }
 
     /**
-     * Renders a prompt for entering a room name.
+     * Implements React's {@link Component#render()}. Renders a prompt for
+     * entering a room name.
      *
+     * @inheritdoc
      * @returns {ReactElement}
      */
     render() {
                         style = { styles.textInput }
                         underlineColorAndroid = 'transparent'
                         value = { this.state.room } />
-                    <TouchableHighlight
-                        accessibilityLabel = { 'Tap to Join.' }
-                        disabled = { this._isJoinDisabled() }
-                        onPress = { this._onJoin }
-                        style = { styles.button }
-                        underlayColor = { ColorPalette.white }>
-                        <Text style = { styles.buttonText }>
-                            { t('welcomepage.join') }
-                        </Text>
-                    </TouchableHighlight>
+                    {
+                        this._renderJoinButton()
+                    }
                 </View>
                 {
                     this._renderLegalese()
         );
     }
 
+    /**
+     * Renders the join button.
+     *
+     * @private
+     * @returns {ReactElement}
+     */
+    _renderJoinButton() {
+        let children;
+
+        /* eslint-disable no-extra-parens */
+
+        if (this.state.joining) {
+            // TouchableHighlight is picky about what its children can be, so
+            // wrap it in a native component, i.e. View to avoid having to
+            // modify non-native children.
+            children = (
+                <View>
+                    <LoadingIndicator />
+                </View>
+            );
+        } else {
+            children = (
+                <Text style = { styles.buttonText }>
+                    { this.props.t('welcomepage.join') }
+                </Text>
+            );
+        }
+
+        /* eslint-enable no-extra-parens */
+
+        return (
+            <TouchableHighlight
+                accessibilityLabel = { 'Tap to Join.' }
+                disabled = { this._isJoinDisabled() }
+                onPress = { this._onJoin }
+                style = { styles.button }
+                underlayColor = { ColorPalette.white }>
+                {
+                    children
+                }
+            </TouchableHighlight>
+        );
+    }
+
     /**
      * Renders legal-related content such as Terms of service/use, Privacy
      * policy, etc.

react/features/welcome/components/WelcomePage.web.js

     }
 
     /**
-     * This method is executed when comonent is mounted.
+     * Implements React's {@link Component#componentDidMount()}. Invoked
+     * immediately after this component is mounted.
      *
      * @inheritdoc
      * @returns {void}