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Use third-party native libraries in Expo

Learn how to create a simple wrapper around two separate native libraries using Expo Modules.


Expo modules make it possible to easily use native, external libraries built for Android and iOS in React Native projects. This tutorial focuses on utilizing the Expo Modules API to create radial charts using two similar libraries accessible on both native platforms.

  • MPAndroidChart by PhilJay
  • Charts by Daniel Cohen Gindi

The iOS library is inspired by the Android library, so they both have a very similar API and functionality. This makes them a good example for this tutorial.

1

Create a new module

You can start by creating a new empty Expo module. We're creating a separate project for this tutorial. However, you can create a new module inside your existing project.

Start with a new project

To create an empty Expo module that can be published on npm and utilized in any Expo application, run the following command:

Terminal
npx create-expo-module expo-radial-chart

Tip: If you aren't going to ship this library, press return for all of the prompts to accept the default values in the terminal window.

Now, open the newly created expo-radial-chart directory to start editing the native code.

Start with an existing project

Alternatively, you can use the new module as a view inside the existing project. Run the following command in your project's directory:

Terminal
npx create-expo-module --local expo-radial-chart

Now, open the newly created modules/expo-radial-chart directory to start editing the native code.

2

Run the example project

To verify that everything is functioning correctly, let's run the example project. In the terminal window, start the TypeScript compiler to watch for changes and rebuild the module JavaScript.

Terminal
# Run this in the root of the project to start the TypeScript compiler
npm run build

In another terminal window, compile and run the example app:

Terminal
cd example
# Run the example app on iOS
npx expo run:ios
# Run the example app on Android
npx expo run:android

3

Add native dependencies

Now, add the native dependencies to the module by editing the android/build.gradle and ios/ExpoRadialChart.podspec files:

android/build.grade
dependencies {
  implementation project(':expo-modules-core')
  implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:${getKotlinVersion()}"
+ implementation 'com.github.PhilJay:MPAndroidChart:v3.1.0'
}
ios/ExpoRadialChart.podspec
    s.static_framework = true

    s.dependency 'ExpoModulesCore'
+   s.dependency 'Charts', '~> 4.1.0'

    # Swift/Objective-C compatibility

4

Define an API

To use the module in the app without mistakes, define the types for the props. This module accepts a list of series — each with a color and a percentage value.

src/ExpoRadialChart.types.ts
import { ViewStyle } from 'react-native/types';

export type ChangeEventPayload = {
  value: string;
};

type Series = {
  color: string;
  percentage: number;
};

export type ExpoRadialChartViewProps = {
  style?: ViewStyle;
  data: Series[];
};

Since we won't implement the module on the web in this example, let's replace the src/ExpoRadialChartView.web.tsx file:

src/ExpoRadialChartView.web.tsx
import * as React from 'react';

export default function ExpoRadialChartView() {
  return <div>Not implemented</div>;
}

5

Implement the module on Android

Now you can implement the native functionality by editing the placeholder files with the following changes:

  • Create a PieChart instance and set its layoutParams to match the parent view. Then, add it to the view hierarchy using the addView function.
  • Define a setChartData function that accepts a list of Series objects. You can iterate over the list, create a PieEntry for each series and store the colors in a separate list. Then, create a PieDataSet, use it to create a PieData object, and set it as data on the PieChart instance.
android/src/main/java/expo/modules/radialchart/ExpoRadialChartView.kt
package expo.modules.radialchart

import android.content.Context
import android.graphics.Color
import androidx.annotation.ColorInt
import com.github.mikephil.charting.charts.PieChart
import com.github.mikephil.charting.data.PieData
import com.github.mikephil.charting.data.PieDataSet
import com.github.mikephil.charting.data.PieEntry
import expo.modules.kotlin.AppContext
import expo.modules.kotlin.records.Field
import expo.modules.kotlin.records.Record
import expo.modules.kotlin.views.ExpoView


class Series : Record {
  @Field
  val color: String = "#ff0000"

  @Field
  val percentage: Float = 0.0f
}

class ExpoRadialChartView(context: Context, appContext: AppContext) : ExpoView(context, appContext) {
  internal val chartView = PieChart(context).also {
    it.layoutParams = LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT)
    addView(it)
  }

  fun setChartData(data: ArrayList<Series>) {
    val entries: ArrayList<PieEntry> = ArrayList()
    val colors: ArrayList<Int> = ArrayList()
    for (series in data) {
      entries.add(PieEntry(series.percentage))
      colors.add(Color.parseColor(series.color))
    }
    val dataSet = PieDataSet(entries, "DataSet");
    dataSet.colors = colors;
    val pieData = PieData(dataSet);
    chartView.data = pieData;
    chartView.invalidate();

  }
}

You also need to use the Prop function to define the data prop and call the native setChartData function when the prop changes:

android/src/main/java/expo/modules/radialchart/ExpoRadialChartModule.kt
package expo.modules.radialchart

import expo.modules.kotlin.modules.Module
import expo.modules.kotlin.modules.ModuleDefinition

class ExpoRadialChartModule : Module() {
  override fun definition() = ModuleDefinition {
    Name("ExpoRadialChart")

    View(ExpoRadialChartView::class) {
      Prop("data") { view: ExpoRadialChartView, prop: ArrayList<Series> ->
        view.setChartData(prop);
      }
    }
  }
}

6

Implement the module on iOS

Now you can implement the native functionality by editing the placeholder files with the following changes:

  • Create a new PieChartView instance and use the addSubview function to add it to the view hierarchy.
  • Set the clipsToBounds property and override the layoutSubviews function to make sure the chart view is always the same size as the parent view.
  • Finally, create a setChartData function that accepts a list of series, creates a PieChartDataSet instance with the data, and assigns it to the data property of the PieChartView instance.
ios/ExpoRadialChartView.swift
import ExpoModulesCore
import Charts

struct Series: Record {
  @Field
  var color: UIColor = UIColor.black

  @Field
  var percentage: Double = 0
}

class ExpoRadialChartView: ExpoView {
  let chartView = PieChartView()

  required init(appContext: AppContext? = nil) {
    super.init(appContext: appContext)
    clipsToBounds = true
    addSubview(chartView)
  }

  override func layoutSubviews() {
    chartView.frame = bounds
  }

  func setChartData(data: [Series]) {
    let set1 = PieChartDataSet(entries: data.map({ (series: Series) -> PieChartDataEntry in
      return PieChartDataEntry(value: series.percentage)
    }))
    set1.colors = data.map({ (series: Series) -> UIColor in
      return series.color
    })
    let chartData: PieChartData = [set1]
    chartView.data = chartData
  }
}

You also need to use the Prop function to define the data prop and call the native setChartData function when the prop changes:

ios/ExpoRadialChartModule.swift
import ExpoModulesCore

public class ExpoRadialChartModule: Module {
  public func definition() -> ModuleDefinition {
    Name("ExpoRadialChart")

    View(ExpoRadialChartView.self) {
      Prop("data") { (view: ExpoRadialChartView, prop: [Series]) in
        view.setChartData(data: prop)
      }
    }
  }
}

7

Write an example app to use the module

You can update the app inside the example directory to test the module. Use the ExpoRadialChartView component to render a pie chart with three slices:

example/App.tsx
import { ExpoRadialChartView } from 'expo-radial-chart';
import { StyleSheet } from 'react-native';

export default function App() {
  return (
    <ExpoRadialChartView
      style={styles.container}
      data={[
        {
          color: '#ff0000',
          percentage: 0.5,
        },
        {
          color: '#00ff00',
          percentage: 0.2,
        },
        {
          color: '#0000ff',
          percentage: 0.3,
        },
      ]}
    />
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
  },
});

Tip: If you created the module inside an existing application, make sure to import it directly from your modules directory by using a relative import: import { ExpoRadialChartView } from '../modules/expo-radial-chart';

8

Rebuild and launch your application

To make sure your app builds successfully on both platforms, rerun the build commands from step 2. After the app is successfully built on any of the platform you'll see a pie chart with three slices:

A PieChart module on Android and iOS

Next steps

Congratulations! You've created your first simple wrapper around two separate third-party native libraries using Expo Modules!

Creating a native module

Expo Module API reference

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