右下にでるアクションボタン。
react-native-action-button

npm install react-native-action-button --save

react-native link

import ActionButton from 'react-native-action-button';

・・・
    render() {
      return (
        <View style={{ flex: 1, alignItems: 'center', justifyContent: 'center' }}>
          <Text>Edit Screen</Text>
          <Button
            title="Go to Details... again2"
            onPress={() => this.props.navigation.push('Details')}
          />
          <Button
            title="Go to Home"
            onPress={() => this.props.navigation.navigate('Home')}
          />
          <Button
            title="Go back"
            onPress={() => this.props.navigation.goBack()}
          />

          <ActionButton
            buttonColor="rgba(231,76,60,1)"
            onPress={() => { console.log("hi")}}
          />

        </View>
      );
    }
・・・