Wrap platform specific components and let the packager do the work. The packager excludes "*.ios.js" files for Android development and vice versa for iOS development.

Spinner.js

// Spinner.ios.js

var React = require('react-native');

var {
    ActivityIndicatorIOS,
} = React;

class Spinner extends React.Component {
    render() {
        return (
            <ActivityIndicatorIOS
                animating={this.props.isLoading}
                size='large' />
        );
    }
}

module.exports = Spinner;

// Spinner.android.js

var React = require('react-native');

var {
    ProgressBarAndroid,
    View,
} = React;

class Spinner extends React.Component {
    render() {
        if (this.props.isLoading) {
            return (
                <View  style={{alignItems: 'center'}}>
                    <ProgressBarAndroid styleAttr="Large"/>
                </View>
            );
        } else {
            return ( <View /> );
        }
    }
}

module.exports = Spinner;

// MyApp.js

var React = require('react-native');
var Spinner = require('./Spinner');

class MyApp extends React.Component {
  render() {
    return (
      <Spinner animating={true}/>
    ); 
  }
});

module.exports = MyApp;