Its often useful to represent information in a map as key-value pairs. For instance, the location of certain landmarks or areas of impact can be represented in a map to provide an intuitive way to structure data.

What You Need

Node.js
create-react-native-app command line tool

Setup

The following setup is for Android phones. If you use iPhone simulator or load the app in an iPhone you could skip to the code part.

Local Setup

Once the application is created from the project folder open app.json

Create a React Native App with Google Map using Expo.io

Your app.json looks something like this
We are going to install expo command line tools

npm install -g expo

Go to expo.io and create and account.
From terminal

expo login

You will be prompted to enter your username/email address and password.

Create a React Native App with Google Map using Expo.io

You should see a message like above.
We are going to update our app.json with the following

{
   "expo": {
    "name": "Your App Name",
    "icon": "./path/to/your/app-icon.png",
    "version": "1.0.0",
    "slug": "your-app-slug",
    "sdkVersion": "XX.0.0",
    "ios": {
      "bundleIdentifier": "com.yourcompany.yourappname"
    },
    "android": {
      "package": "com.yourcompany.yourappname"
    }
   }
 }

The iOS bundleIdentifier and Android package fields use reverse DNS notation, but don’t have to be related to a domain. Replace "com.yourcompany.yourappname" with whatever makes sense for your app.
You’re probably not surprised that name, icon and version are required.
slug is the url name that your app’s JavaScript is published to. For example: expo.io/@community/native-component-list, where community is my username and native-component-list is the slug.
The sdkVersion tells Expo what Expo runtime version to use, which corresponds to a React Native version. Although "XX.0.0" is listed in the example, you already have an sdkVersion in your app.json and should not change it except when you want to update to a new version of Expo.

With all the info my app.json is the following.

{
   "expo": {
    "name": "map-cuny",
    "icon": "./assets/icon.png",
    "version": "1.0.0",
    "slug": "map-cuny",
    "sdkVersion": "26.0.0",
    "ios": {
      "bundleIdentifier": "com.codingmechanic.mapcuny"
    },
    "android": {
      "package": "com.codingmechanic.mapcuny"
    }
   }
 }

Once the app.json is all set we can then build the android standalone app from terminal.

expo build:android

This will take a few minutes. Get some coffee in the mean time.

Once the Build is completed run expo fetch:android:keystore

This would generate the map-cuny.jks file and return the keystore password . We will need the key store password in a second.
Run keytool -list -v -keystore map-cuny.jks . For your app replace map-cuny.jks with your .jks file.
If you enter the correct keystore password you should see something like this.

What we are interested in is the SHA1 key. We will use that in Google Cloud Platform in a minute.

Google Cloud Platform Setup

Log into Google Cloud Platform

Click on Create

Give your project a meaningful name
Once created you should be taken to your app dashboard.

Go to APIs and Services on the left side bar

Go to Library on the left side bar

Select Google Maps Android API

(If the UI changes over the time and Google Maps Api is not on the screen, You can always search for the Api in the search bar)

Enable Google Maps Android API

The Warning basically asks us to “Create Credentials” for our app to be able to use google maps api.
Click on Create Credentials.

Click on API Key

In the API restrictions tab select Google Maps Android API

Click on create. Copy the api key that was generated.
Go Back to app.json

"android": {
   "package": "com.codingmechanic.mapcuny",
   "config": {
      "googleMaps": { "apiKey": "<Your API Key>" }   
   }
}

Add config to android
Setup is complete

Code

Base Code

We are now ready to start coding. Just to see we have the setup right, we will just put a Map View in the screen. Update your App.js with the following

import React from "react";
import { StyleSheet, Text, View } from "react-native";
import { MapView } from "expo";export default class App extends React.Component {
  render() {
    return (
      <MapView
        style={{
          flex: 1
        }}
        initialRegion={{
          latitude: 37.78825,
          longitude: -122.4324,
          latitudeDelta: 0.0922,
          longitudeDelta: 0.0421
        }}
      />
    );
  }
}

In line 3 we are importing MapView from expo . This MapView is just a wrapper of the React Native Maps library by AirBnB. In Render we render the MapView with style flex:1 So that it takes the entire screen and give it an initial region. This location is of somewhere in SF.

I ran the app in my iPhone 7+ and One Plus 3 device. And as seen above the Map is rendered by Apple MapKit or Google Maps depending on the platform.

This is all fine and working but we would like to show something in this map. Some location info that will be most useful in a map form.

Data

For this demo I will use NYC Bike Data. This data is publicly available and does not require any authentication or token. For our purposes this is the easiest data to work with.

Getting Data

To get data we will be using fetch .

constructor(props) {
    super(props);

    this.state = {
      isLoading: true,
      markers: [],
    };
  }

First we add a constructor above the Render function.
isLoading flag is there to denote the end of fetching.
We will store the bike stop locations in the markers array.
Create a method called fetchMarkerData() and add the following code

fetchMarkerData() {
    fetch('https://feeds.citibikenyc.com/stations/stations.json')
      .then((response) => response.json())
      .then((responseJson) => {
        this.setState({ 
          isLoading: false,
          markers: responseJson.stationBeanList, 
        });
      })
      .catch((error) => {
        console.log(error);
      });
  }

We now call the fetchMarkerData() from life cycle method componentDidMount() (More about life cycle methods on here)
Life Cycle methods are not react native specific and come from react.js
We add the code

componentDidMount() {
    this.fetchMarkerData();
}

Now when the component mounts the fetchMarkerData will be called and when data is received from the api we will update our state
Now its time to render all the markers

<MapView
    style={{ flex: 1 }}
    region={{
      latitude: 40.76727216,
      longitude: -73.99392888,
      latitudeDelta: 0.0922,
      longitudeDelta: 0.0421,
    }}
>
        {this.state.isLoading ? null : this.state.markers.map((marker, index) => {
     const coords = {
         latitude: marker.latitude,
         longitude: marker.longitude,
     };

     const metadata = `Status: ${marker.statusValue}`;

     return (
         <MapView.Marker
            key={index}
            coordinate={coords}
            title={marker.stationName}
            description={metadata}
         />
     );
  })}
</MapView>

What is happening is we are checking if the isLoading flag is true. If so we are returning null else we are mapping over all the marker datas stored in our state and returning a Marker object for each marker in the array.
We can then run the app the same way for Android and iPhone

The App.js code is available in this Github gist.

Google Maps on iOS

You may want to use google maps on you iOS app as well. There are a few benefits from using google maps. They have more data on places outside the US. So if you app targets a global audience google maps may be the better option.

Go to Google Cloud Platform Dashboard and select your project. (It generally opens to the last open project anyway)
Go to APIs and Services
Go to Library
Select Google Maps SDK for iOS
Enable
Go to Credentials
Create Credentials
API Credentials
Click on Restrict Key
Select iOS Application on Application Restrictions
Select Google Maps SDK for iOS on API Restriction
Enter you Bundler ID for iOS app from app.json
Save and Copy the api key
In app.json add googleMapsApiKey

"ios": {
  "bundleIdentifier": "com.codingmechanic.mapcuny",
  "config": {
    "googleMapsApiKey": "YOUR IOS API KEY HERE"
  }
},

In App.js in the MapView pass a new prop called provider and set it to
"google"

<MapView
    style={{ flex: 1 }}
    provider="google"
    region={{
          latitude: 40.76727216,
          longitude: -73.99392888,
          latitudeDelta: 0.0922,
          longitudeDelta: 0.0421
    }}
>