Category Archives: JavaScript

How to play an Android audio file in React Native

Here’s a quick tutorial to get you started playing an mp3 file in an Android app that was built using React Native. This tutorial was developed using Linux (Ubuntu 16.04), React Native 0.61.5, and the React Native Sound package 0.11.0.

First, make your project directory. Let’s call it AudioTmp. In a terminal window, do this:

npx react-native init AudioTmp
cd AudioTmp

All of the following commands assume you’ve got your terminal window open, and you’re standing in the root directory of your project (in AudioTmp, where App.js is located).

You’ll need some extra packages, so run these install commands:

npm install typescript@
npm install eslint@^5.0.0
npm install react-native-sound

Next, as described in the React Native Sound documentation, use the link command:

react-native link react-native-sound

This “links” the library where needed into the android and ios directories.

Now add an mp3 file to your Android project:

mkdir android/app/src/main/res/raw
cp john_bartmann_04_earning_happiness.mp3 android/app/src/main/res/raw

You can download a copy of John Bartmann’s “Earning Happiness” mp3 file here. It has a public domain license; let’s thank John Bartmann for making his cool “Earning Happiness” music open source (original source at Free Music Archive)!

Next, make the utils directory under your project root:

mkdir utils

Add a new file called SoundPlayer.js in the utils directory. Here’s the source for that:

/**
 * Really simple demo of https://www.npmjs.com/package/react-native-sound
 */
const Sound = require('react-native-sound');
/**
 * Audio credit:
 * https://freemusicarchive.org/music/John_Bartmann/Public_Domain_Soundtrack_Music_Album_One/earning-happiness
 * "Earning Happiness" by John Bartmann licensed under a CC0 1.0 Universal License
 * https://creativecommons.org/publicdomain/zero/1.0/
 */
const FILENAME = 'john_bartmann_04_earning_happiness.mp3';

class SoundPlayer {
    constructor(props) {
        console.log("SoundPlayer constructor");
    };
    stopSound() {
        console.log('SoundPlayer.stopSound');
        this.whoosh.stop();
        this.whoosh.release();
    }
    playSound() {
        console.log('SoundPlayer.playSound');
        let me = this;
        me.whoosh = new Sound(FILENAME, Sound.MAIN_BUNDLE, error => {
            if (error) {
                console.log('failed to load the sound', error);
                return;
            }
            // loaded successfully
            console.log(
                'duration in seconds: ' +
                me.whoosh.getDuration() +
                'number of channels: ' +
                me.whoosh.getNumberOfChannels()
            );
            me.whoosh.play(success => {
                if (success) {
                console.log('successfully finished playing');
                } else {
                console.log('playback failed due to audio decoding errors');
                }
            });
        });
    }
}
export default SoundPlayer;

Replace the code in App.js with this code:

/**
 * Simple demo of https://www.npmjs.com/package/react-native-sound
 * The app plays an included mp3 file when the user clicks on 'Play' button.
 * It stops the playback when the user clicks on the 'Stop' button.
 */
import React from 'react';
import {
  StyleSheet,
  View,
    Text,
  Button
} from 'react-native';

import {
  Colors
} from 'react-native/Libraries/NewAppScreen';

/** Keep all sound functionality in separate utils area */
import SoundPlayer from './utils/SoundPlayer';

class App extends React.Component {
    constructor(props) {
        super(props);
        this.soundPlayer = new SoundPlayer();
    }

    onPlayPress() {
        console.log('onPlayPress method');
        this.soundPlayer.playSound();
    }

    onStopPress() {
        console.log('onPlayPress method');
        this.soundPlayer.stopSound();
    }

    render() {
        return (
        <View style={styles.body}>
            <Text style={styles.sectionTitle}>Click button to Play</Text>
            <View style={styles.buttonStyle}>
                <Button style={styles.buttonStyle}
                    onPress={this.onPlayPress.bind(this)}
                    title="Play"
                />
            </View>
            <Text style={styles.sectionTitle}>Click button to Stop</Text>
            <View style={styles.buttonStyle}>
                <Button buttonStyle={styles.buttonStyle}
                    onPress={this.onStopPress.bind(this)}
                    title="Stop"
                />
            </View>
        </View>
        );
    }
};

const styles = StyleSheet.create({
    body: {
        backgroundColor: Colors.white,
        flex: 1,
        flexDirection: 'column',
        justifyContent: 'center',
        backgroundColor: 'gainsboro'
    },
    sectionTitle: {
        fontSize: 24,
        fontWeight: '600',
        color: Colors.black,
        padding: 20,
        backgroundColor: 'gainsboro' /* See https://reactnative.dev/docs/colors */,
    },
    buttonStyle: {
        padding: 20,
        backgroundColor: 'gainsboro' /* See https://reactnative.dev/docs/colors */,
    }
});

export default App;

Run the Android Emulator:

emulator -no-snapshot -avd Galaxy_Nexus_API_28

Start your Metro server:

npx react-native start

Install the app in your Android Emulator:

npx react-native run-android

I see these warnings at my command line, but everything seems to work okay:

error React Native CLI uses autolinking for native dependencies, but the following modules are linked manually: 
  - react-native-sound (to unlink run: "react-native unlink react-native-sound")
This is likely happening when upgrading React Native from below 0.60 to 0.60 or above. Going forward, you can unlink this dependency via "react-native unlink <dependency>" and it will be included in your app automatically. If a library isn't compatible with autolinking, disregard this message and notify the library maintainers.
Read more about autolinking: https://github.com/react-native-community/cli/blob/master/docs/autolinking.md
info Running jetifier to migrate libraries to AndroidX. You can disable it using "--no-jetifier" flag.
Jetifier found 875 file(s) to forward-jetify. Using 8 workers...

Then it just works! You can hit the Play and Stop buttons to play the audio.

The source code for this project is available at github.

One remark: when I initially created this project, I didn’t use the link command, because the React Native documentation indicated it was not needed for version 0.60. However, that turned out to be untrue! You can save yourself time by doing it the way I’ve done it in this tutorial. When using link, the demo worked immediately, and I didn’t have to make any changes to gradle files.

Here’s a view of the demo app:

Audio Player using React Native

If you found this interesting, click the subscribe button below! I write a new post about once a week.

Adding a map to React Native

This is part of a series on building a movie locator app in React Native. The first post is available here.

In my last post, I’d given the user the ability to navigate to a new page in the app. In this post, I’ll add a map on that new page. It will display the location of the theater that the user picked.

This is the first part of my app build which requires that you do something different depending on whether you’re working with Android or iOS. The installation instructions are here (as of 2020/03/23). I will be following the build configuration on Android, because I’m using an Android emulator (and a Linux OS, not a Mac, for my builds).

There are 6 steps given in the “build config for Android” section. I went through each of them one by one. I created a separate page which describes exactly what I did to prepare to add a map to my React Native app. Take a look there if you want to know about it in more detail.

In my previous post, I listed the code in MapsScreen.js, which is the page where my map should be displayed:

import React, { Component } from 'react';
import { Text, View } from 'react-native';

export default class MapsScreen extends Component {
    render() {
        return (<View style={[{ flex: 1, justifyContent: 'center' },
        { flexDirection: 'row', justifyContent: 'space-around', padding: 10 }]}>
            <Text>A map goes here</Text>
        </View>);
    }
}

But there’s no map there, yet! I need to add the code that will display a map.

Fortunately, the React Native maps project has an example app on GitHub. I took the time to download this example project, and create an entirely separate React Native project which uses its code. This helped me get my feet wet, and I knew where to look for the code that I needed to add a map: StaticMap.js is a page which displays a simple map. I’m going to take some of that code and add it to my own, as follows:

import React, { Component } from 'react';
import { View, Text } from 'react-native';
import MapView from 'react-native-maps';

export default class MapsScreen extends Component {
    render() {
        return (<View style={[{ flex: 1, justifyContent: 'center' },
        { flexDirection: 'row', justifyContent: 'space-around', padding: 10 }]}>
            <MapView
                initialRegion={{
                    latitude: 37.78825,
                    longitude: -122.4324,
                    latitudeDelta: 0.0922,
                    longitudeDelta: 0.0421,
                }}
            />
        </View>);
    }
}

This is almost the same as my previous render method. The only difference is that I’ve added a MapView component. It includes a mocked initialRegion property (latitude and longitude are static). I just want to get maps working, before adding a real initialRegion.

Unfortunately, when I tested this, I just saw an empty display, aside from the Maps title bar. Ugh! No error messages – just nothing! It was frustrating! Since there were no error messages, I wondered if there was something wrong with the display. The StaticMap example uses a MapView that is styled. So I copied that style, which just specified a width and height, and pasted it into my MapView, like this:

<MapView
    style={{width: 250, height: 250}}
    initialRegion={{
        ...

It worked!! I saw a map, but it looked cramped. I changed the style to {{flex:1}}, and the map filled the page! Excellent! Here’s the current behavior:

map of San Francisco in React Native app

The source code for this example – App.js, MoviesScreen.js, and MapsScreen.js – is available online.

If you found this interesting, click the subscribe button below! I write a new post about once a week.

Adding a simple navigation system to React Native – Part II

In my last post, I started the process of adding a navigation system to my app. The idea is to make it easy for the developer to add more pages to the app, and easy for the user to move around from page to page. I’m using the React Navigation community project. I’ve got the right packages installed and imported into my app. It’s clear to me from the documentation that I need to add a new component as a new page.

So I add a rudimentary, blank component. I’ve decided that this page will show a map of the theater that the user picks on the first page, so I name the file MapsScreen.js, with this source:

import React, { Component } from 'react';
import { Text, View } from 'react-native';

export default class MapsScreen extends Component {
    render() {
        return (<View style={[{ flex: 1, justifyContent: 'center' },
        { flexDirection: 'row', justifyContent: 'space-around', padding: 10 }]}>
            <Text>A map goes here</Text>
        </View>);
    }
}

Then I edit App.js to include a route to this component. It looks like this:

import { createAppContainer } from 'react-navigation';
import { createStackNavigator } from 'react-navigation-stack';
import MoviesScreen from './MoviesScreen';
import MapsScreen from './MapsScreen';

const MainNavigator = createStackNavigator({
    Movies: { screen: MoviesScreen },
    Maps: { screen: MapsScreen }
});

const App = createAppContainer(MainNavigator);

export default App;

It was more trouble than I’d like to add the React Navigation packages, but now that they’re in place, it’s super easy to add a new page. According to the documentation, I just need to call a function called navigate in order for a button click to take me to a new page.

I’ve already attached a method handleShowTheaterClick to my “GO” button. Here’s the short navigation code that needs to be added:

handleShowTheaterClick() {
    const { navigate } = this.props.navigation;
    navigate('Maps', {});
}

This is all it takes to make the “GO” button functional. I click the button, and I’m taken to my “Maps” screen, which looks like this:

current app functionality selecting movie and theater

Notice there’s a back arrow in the title bar. I can click that to get back to the previous screen. I hardly needed to do any work at all to get built-in navigation functionality. This is the beauty of code reuse.

The source code for this example – App.js, MoviesScreen.js, and MapsScreen.js – is available online.

If you found this interesting, click the subscribe button below! I write a new post about once a week.

Adding a simple navigation system to React Native – Part I

In previous posts (start here), I’ve written about building a movie app that a person can use to pick a movie from a list, and then click a button to display theaters where the movie is being shown.

Let’s add some code which allows our user to select a specific theater. Upon doing that, a button will appear which will take them to a new page in the app.

I only want the new button to appear when the list of theaters is populated. Here’s how I ensure that:

render() {
    if (this.state && !this.state.isLoading) {
        ...
        var theatersPicker = null;
        var goButton = null;
        if (this.state.nearbyTheaters) {
            ...
            theatersPicker = <Picker>{theaters}</Picker>
            goButton = <Button onPress={this.handleShowTheaterClick.bind(this)} title="Go"></Button>;
        }
        return <View>
        ...
            <Button onPress={this.handleClick.bind(this)} title="Find Movie Near Me"></Button>
            {theatersPicker}
            {goButton}
        </View>;

The goButton has been initialized to null. It will only be set to a Button if the theatersPicker has a list of theaters. I also added a new, empty method called handleShowTheaterClick (not shown here). The new button will call handleShowTheaterClick when clicked, but since it’s empty, it won’t do anything yet. We’ll get to that later.

You’ll find that the {goButton}line doesn’t show anything if goButton is null. That’s the UX that we want.

At this point, I want handleShowTheaterClick to cause the app to display a new page. This behavior is new! So far, everything has been happening in a single page. We’re getting into navigation, also known as “routing”. It’s how users get around the app from one page to another.

Navigation is not built into the default React Native install. You have to install it as a separate package. I followed the instructions for doing this as given in the documentation, but I got a lot of warnings. It took me about 30 minutes to resolve them, ugh! I kept trying and retrying… One of the errors that I saw in the Metro server terminal window looked like this:

error: bundling failed: Error: Unable to resolve module `react-navigation-stack` from `App.js`: react-navigation-stack could not be found within the project.

If you are sure the module exists, try these steps:
 1. Clear watchman watches: watchman watch-del-all
 2. Delete node_modules: rm -rf node_modules and run yarn install
 3. Reset Metro's cache: yarn start --reset-cache
 4. Remove the cache: rm -rf /tmp/metro-*

This turned out to be misleading. I thought that I need to use yarn, so I installed it, and started using it according the instructions. It could be that this works for some setups, but I’m running Ubuntu 16.04, and I’ve been using npm throughout this exercise. I should have stuck with npm. I learned that if a message mentioned yarn, I should just use npm instead. I suggest you do the same, unless you’re already using yarn to run other commands.

When I finally switched back to using npm, I found there were some warnings about “peer dependencies”. Here is one example:

...
npm WARN @typescript-eslint/eslint-plugin@1.13.0 requires a peer of eslint@^5.0.0 but none is installed. You must install peer dependencies yourself.

I wound up installing each peer dependency when I saw that one was missing. Then I had to go back and install the package which required the dependency again (so it seemed). In the end, I did all these installs in my project root (where App.js lives), in the order given:

npm install eslint@^5.0.0
npm install typescript@>=3.7.0-dev
npm install react-native-gesture-handler@*
npm install @react-native-community/masked-view@^0.1.1
npm install react-native-safe-area-context@^0.6.0
npm install react-navigation-stack
npm install react-navigation

Installing all that stuff was kind of a headache, and I was happy when it was finally done.

A quick read of the navigation documentation told me that I should only have navigation code in my App.js file. My existing application logic should go into a separate “screen” page. I’ll call this new page MoviesScreen.

To do this, I copied all the source of App.js into a new file, which I named MoviesScreen.js. Then I replaced HelloWorldApp in that source with MoviesScreen. Really, just replace

export default class HelloWorldApp extends Component {

with

export default class MoviesScreen extends Component {

Next, I replaced all the code in App.js with this simple navigation code:

import { createAppContainer } from 'react-navigation';
import { createStackNavigator } from 'react-navigation-stack';
import MoviesScreen from './MoviesScreen';

const MainNavigator = createStackNavigator({
    Movies: { screen: MoviesScreen }
});

const App = createAppContainer(MainNavigator);

export default App;

After doing this, I ran npx react-native run-android in my project root. I’ve learned that running npx react-native run-android sometimes clears up errors, although I’m not exactly sure when or if it’s required (might be a superstition type thing!).

The net result was that my app looked almost exactly the same as it did before! That’s expected, and good, because I only made infrastructure changes, and no functional ones. The only difference now is that there’s a title bar at the top of the display. It reads “Movies”. It’s not beautiful, but I’ll figure out how to deal with that later.

In testing the latest version of my app, I notice there’s one problem: when I select a theater from my “theaters” Picker, the value doesn’t stay selected. I know how to fix that – the same way that I did it for the “movies” Picker. I add similar code for that in MoviesScreen‘s render method:

...
theatersPicker = <Picker selectedValue={this.state.theater}
    onValueChange={(itemValue, itemIndex) => this.setState({ theater: itemValue })
    }>{theaters}</Picker>
current app functionality selecting movie and theater

Now, my app just works, except for one little problem. The GO button does nothing! Clicking on it should take the user to a new page. I’ll work that out in my next post.

The source code for this new version of App.js and for MoviesScreen are available online.

If you found this interesting, click the subscribe button below! I write a new post about once a week.

How to handle Button clicks in React Native – Part I

I’ve been building an app which lets me find movies “near me” using mocked data. In my last post, I added a progress loader to display when there’s networking latency. I have a dropdown with a list of movie names in my app, and the user can select any movie in the list. Let’s suppose the user wants to see one of these movies, and would like to see a list of closest locations where the movie is showing. We’re going to need a button so that the user can click it to get results.

You might wonder why you shouldn’t just load results when the user selects a movie name from the Picker. Well, it depends on how you feel about UX. Some apps will immediately give you results when an element is selected from a dropdown. However, just because a user selects an item doesn’t mean that is their final choice. Maybe they lifted their finger too soon, and wound up selecting something that they didn’t want. Or, maybe they’re just thinking about their different options.

My personal preference for dealing with a select is to add a confirmation button when the result of selecting an item from the Picker is a resource intensive task, like hitting a database or making a network call. You don’t want to waste resources every time a user selects an item from a dropdown.

So in this example, making a network call should only happen if the user clicks a button. Let’s add a button to our view – just below the Picker:

import { View, Picker, ActivityIndicator, Button } from 'react-native';
...
render() {
    ...
    return <View>
        <Picker...
        </Picker>
        <Button title="Find Movie Near Me"></Button>
    </View>;
    ...
}

You can’t add a Button without a title; you’ll see a warning if you do.

The button is probably the easiest control to add in React Native! Here’s what I see after I add just a couple of lines:

However, clicking the button is a different story. If you’re used to web development, you might expect an “onClick” property. Nope, it’s onPress. And you can’t just add a method like this:

...
handleClick() {
    console.log("Handled");
}
...
    <Button onPress="handleClick" title="Find Movie Near Me"></Button>
...

The app won’t complain if you do this, but if you click the button you’ll see an error: "TypeError: this.props.onPress is not a function. (In 'this.props.onPress(e);, 'this.props.onPress' is "handleClick")".

The method handleClick is actually a function of “this”, the app. So you have to reference it correctly. It doesn’t help to replace onPress="handleClick" with onPress="this.handleClick" because anything in quotes is a “literal”, treated as a string, in JSX. Instead, you have to do as follows:

...
handleClick() {
    console.log("handleClick");
}
...
    <Button onPress={this.handleClick} title="Find Movie Near Me"></Button>
...

Now the button responds to clicks, as you can see below!

If you found this interesting, click the subscribe button below! I write a new post about once a week.

How to add a progress loader in React Native

In my last post, I took a look at how network latency might affect my React Native app, and I didn’t like what I saw: a pure white screen for several seconds. How is a user supposed to “react” to that? (Pun intended!)

It was pretty clear what was happening. My app’s render method just shows a blank view if it has no state. Here’s the render method:

render() {
    if (this.state && !this.state.isLoading) {
        ...
    } else {
        return (<View></View>);
    }
}

That’s my white screen. There’s a super simple fix:

render() {
    if (this.state && !this.state.isLoading) {
        ...
    } else {
        return (<View><Text>waiting...</Text></View>);
    }
}

Now there’s text which reads “waiting…” at the top. That’s okay for a hobby app. You’re probably going to want that to look a little nicer, however. How about a progress indicator? Something that shows “we’re working on this!”. React Native comes with an ActivityIndicator which does what we want. I’m going to grab some of the code from their documentation and add it to my app. In the render method below, I’ve replaced my “waiting” text with an ActivityIndicator (and I’ve shown the import statement to remind you to add that as well).

import { View, Text, Picker, ActivityIndicator } from 'react-native';
...
render() {
    if (this.state && !this.state.isLoading) {
        ...
    } else {
        return (<View><ActivityIndicator size="large" color="#0000ff" /></View>);
    }
}

That helps some, but the progress indicator still doesn’t look very nice. It’s placed at the very top of the screen, like this:

It turns out to be pretty easy to move the ActivityIndicator to the center of the screen. You just have to add some style components to its container View, like this:

render() {
    if (this.state && !this.state.isLoading) {
        ...
    } else {
        return (<View style={[{ flex: 1, justifyContent: 'center' },
            { flexDirection: 'row', justifyContent: 'space-around', padding: 10 }]}>
            <ActivityIndicator size="large" color="#0000ff" />
        </View>);
    }
}

Here’s the result:

With very little trouble at all, I’ve now got a slick little widget to let people know that something is going on. It’s not really enough, though. What if the network request times out, and I get an error? I’ll have to delve into that more deeply later. To remind me to do this, I’ll add a TODO/FIXME comment in the code, and open an issue in my issue tracker.

If you found this interesting, click the subscribe button below! I write a new post about once a week.

What causes “SyntaxError: JSON Parse error: Unrecognized token ‘<'" in React Native?

TL;DR: In React Native, you will get “SyntaxError: JSON Parse error: Unrecognized token ‘<‘” if your URL returns a 404 error, or in general if the content is not a JSON string.

In a recent post, I showed how to display a list of movies that had been fetched from a REST API. It worked great, but I wondered what would happen to my app’s user if their device was offline, or if the REST API ever went down. To mimic this behavior, I changed the URL by adding the number 1 at the end of it, like this: “https://facebook.github.io/react-native/movies.json1”.

And here’s what I saw in the emulator:

SyntaxError: JSON Parse error: Unrecognized token ‘<‘

The red screen says “SyntaxError: JSON Parse error: Unrecognized token ‘<‘”. That may be confusing, although if you work with REST APIs for any time, you’ll soon come to recognize what it means. Meantime, how do we investigate this?

When I load up this test URL in a web browser, I see content which looks like this:

<!DOCTYPE html>
<html>
  <head>
  ...Page not found...
</html>

It’s a fancy 404 error page. That explains why response.json barfs on this; it’s not JSON. Your app expected a JSON string. It tried to parse the string into a JavaScript object, and couldn’t handle a non-JSON string. As a reminder, here’s that fetch call:

componentDidMount() {
    return fetch('https://facebook.github.io/react-native/movies.json1')
        .then((response) => response.json())
        .then((responseJson) => {
...
        })
        .catch((error) => {
            // TODO FIXME replace the red screen with something informative.
            console.error(error);
        });
}

In the longer term, I will want to replace that red screen of death with a nice error page which instructs the user what to do. I’m still developing my application, however, and as a dev, I’d rather see the stack trace for errors like this when they occur.

So to deal with this, I’ll do two things: 1) I’ll add a “TODO FIXME” note in my code. When I’m cleaning up code in the end stages of development, I know to look for these types of comments which indicate work still needs to be done. 2) I’ll open an issue in my issue tracker which will let everyone on my team know that there’s something that still has to be handled in building the application. I’ll bring this to the attention of anyone who needs to know (a project manager, perhaps). The project manager may assign a designer to build a page with some graphics or specific text to display to the user in case of this error.

If you found this interesting, click the subscribe button below! I write a new post about once a week.

How to add a select element in React Native

TL;DR: The React Native Picker component is the equivalent of the HTML select element.

In my last post, I used fetch to download a list of movie titles. However, I only displayed the list by using a debugging tool. My app user will want to see this list, and even better, may want to select something off the list. The commonly used web widget which does this is the HTML <select> element. In React Native, the corresponding widget is a Picker.

You can add a Picker to your render method very easily, like this:

import { Picker } from 'react-native';
...
render() {
    return <View>
        <Picker>
        </Picker>
    </View>;
}

That’s a start, but it doesn’t display my data, and it doesn’t look very interesting. We’ve got a picker with no functionality which looks like this:

Notice the little upside-down triangle in the upper right corner, which indicates the Picker has loaded.

In order to populate the Picker with data, we need to get access to the list of movies inside the render method. Fortunately, I already used setState to make the movie data persist in my app’s state (see previous post). That means I can use it in render, like this:

render() {
    let items = [];
    var length = this.state.dataSource.length;
    for (var i = 0; i < length; i++) {
        var item = this.state.dataSource[i];
        // Really, really important to not put quotes around these braces:
        items.push(<Picker.Item label={item.title} value={item.title} key={item.id} />);
    }
    return <View>
        <Picker>
        </Picker>
    </View>;
}

Uh oh, wait, as soon as I added this code, I saw the red screen of death in the emulator. The error message read “TypeError: null is not an object (evaluating ‘this.state.dataSource’). This error is located at: in HelloWorldApp (at renderApplication.js:40)…”

The problem is that I tried to reference this.state.dataSource before it had been set. The render method might be called before fetch runs. We don’t know when our fetch statement will be called, or even if it is successful. We need to add code in render which handles the case where this data is not yet available.

Here’s the new render code which skips doing anything with dataSource if it is not found:

render() {
    if (this.state && !this.state.isLoading) {
        let items = [];
        var length = this.state.dataSource.length;
        for (var i = 0; i < length; i++) {
            var item = this.state.dataSource[i];
            items.push(<Picker.Item label={item.title} value={item.title} key={item.id} />);
        }
        return <View>
            <Picker>
            </Picker>
        </View>;
    } else {
        return ( <View></View>);
    }
}

That fixes the error, but the Picker is still empty. But in anticipation of needing a list of movies, I’ve already got a list of Picker.Item objects loaded into an array using JSX (items.push(<Picker.Item....). It turns out to be very easy to add these to the Picker. Just add items in braces inside the Picker tags, like this:

<Picker>
{items}
</Picker>

As soon as this is done, like magic, the Picker reflects the changes. A dropdown appears with the text Star Wars selected by default.

Now you can click the dropdown, and try to select a different movie, but what you’ll find is that the dropdown is stuck at Star Wars – you can’t select anything else!

For a finishing touch, let’s make it possible for the user to select a different movie. The Picker documentation tells us how to do this by adding the selectedValue attribute and the onValueChange methods like this:

selectedValue={this.state.movie}
onValueChange={(itemValue, itemIndex) =>
    this.setState({ movie: itemValue })
}

After adding this I can select any movie, and that movie stays selected! Here’s my final code:

import React, { Component } from 'react';
import { View, Picker } from 'react-native';

export default class HelloWorldApp extends Component {
    componentDidMount() {
        return fetch('https://facebook.github.io/react-native/movies.json')
            .then((response) => response.json())
            .then((responseJson) => {
                console.log(responseJson);
                this.setState({
                    isLoading: false,
                    dataSource: responseJson.movies,
                }, function () {

                });

            })
            .catch((error) => {
                console.error(error);
            });
    }

    render() {
        if (this.state && !this.state.isLoading) {
            let items = [];
            var length = this.state.dataSource.length;
            for (var i = 0; i < length; i++) {
                var item = this.state.dataSource[i];
                // Really, really important to not put quotes around these braces:
                items.push(<Picker.Item label={item.title} value={item.title} key={item.id} />);
            }
            return <View>
                <Picker selectedValue={this.state.movie}
                    onValueChange={(itemValue, itemIndex) =>
                        this.setState({ movie: itemValue })
                    }>
                    {items}
                </Picker>
            </View>;
        } else {
            return (<View></View>);
        }
    }
}

If you found this interesting, click the subscribe button below! I write a new post about once a week.

Simple debugging tool in React Native

TL;DR: If you want to debug React Native code really quickly, console.log and console.warn can help.

In my previous post, I described how I ported the React Clock app to React Native. This is the code for my simple app:

import React, { Component } from 'react';
import { Text, View, Button } from 'react-native';
import Clock from './Clock';

export default class HelloWorldApp extends Component {
    render() {
        return (
            <View style={{ flex: 1, justifyContent: "center", alignItems: "center" }}>
                <Text style={{ fontWeight: 'bold', padding: 10 }}>Hello, world!</Text>
                {/* padding does not work with Button!! */}
                <Button style={{ fontWeight: 'bold', padding: 40 }} title="Click me" >Click Me!</Button>
                {/* Since padding does not work with Button, we add an empty text area */}
                <Text>{""}</Text>
                <Clock />
            </View>
        );
    }
}

For my next project, I decided to do something more realistic. I wanted to figure out how to fetch data from a REST API.

I already had the Android emulator started (see previous post). A quick look at the React-Native networking documentation told me that doing a fetch should be a piece of cake. Because I didn’t want to copy their entire sample app, but just reuse their fetch call, I copied the componentDidMount method, and pasted it into my application above the render method:

componentDidMount(){
return fetch('https://facebook.github.io/react-native/movies.json')
    .then((response) => response.json())
    .then((responseJson) => {

    this.setState({
        isLoading: false,
        dataSource: responseJson.movies,
    }, function(){

    });

    })
    .catch((error) =>{
    console.error(error);
    });
}

(The componentDidMount method may be familiar to you from React.js development.)

I didn’t see any errors when I did this, but I also couldn’t tell whether the fetch method had worked! If I had been building this app using JavaScript in a web browser, I could have quickly checked the results by adding a console.log statement to print out responseJson. I tried this, in fact, but nothing noticeable happened onscreen when I made my change. It took a little while, but I finally noticed that my statements were being logged in the terminal window that was running the Metro server (where I’d run the npm start command)! It took me a while before I noticed this because I’m not usually looking at the terminal unless I’m trying to debug a problem.

A quick search also told me that I could use console.warn to display text on the emulator’s screen. I added console.warn(responseJson); just above the setState call, and I could see that the method had succeeded, and I could also see part of the responseJson content in the YellowBox which appeared. Clicking on this YellowBox warning gave me a fullscreen view of the JSON.

Probably it’s a bad idea to display debug messages using console.warn, but if I were debugging on a device without the help of Metro server, I think console.warn would come in handy.

If you found this interesting, click the subscribe button below! I write a new post about once a week.

Invariant violation: Text strings must be rendered within a Text component

TL;DR: To display text in React Native, you must always use a Text tag. This is different from React.js, where text can be displayed in html tags like <div> or <h1>.

For my next mini React Native project, I’m porting the React Clock app to React Native. FYI I’m doing this work on Ubuntu 16.04.

I have to start up my Android emulator first. I list the available ones like this:

emulator -list-avds
Galaxy_Nexus_API_23
Galaxy_Nexus_API_28
Samsung_S4_mIni_API_17

I pick one, and start it:

emulator -avd Galaxy_Nexus_API_28

I didn’t want to have to fire up Android Studio every time I worked on a React Native project, so running the emulator from the command line is super useful. Note the emulator has nothing to do with React Native, per se, and it’s just running in a standalone mode. I can run my emulator commands from any directory; I just open a terminal and do it.

I had already gotten started with React Native. I had a folder called AwesomeProject with a directory structure like this:

...
ios/
android/
App.js

I step into this directory and run the commands npx react-native run-android followed by npm start. I see my project load into the emulator.

I’m using VS Code as my IDE for React Native development. I have no trouble making a small change or two to my App.js file, and I see these changes immediately loaded into the emulator.

Next, I add a file called Clock.js in the same directory as my App.js class.

import React from 'react';
export default class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {date: new Date()};
  }

  componentDidMount() {
    this.timerID = setInterval(
      () => this.tick(),
      1000
    );
  }

  componentWillUnmount() {
    clearInterval(this.timerID);
  }

  tick() {
    this.setState({
      date: new Date()
    });
  }

  render() {
    return (
      <div>
        <h1>Hello, world!</h1>
        <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
      </div>
    );
  }
}

This is almost the same as the Clock.js code in the React.js state and lifecycle demo. I’ve removed the ReactDOM.render method because I don’t want the clock rendering immediately. Also, I’ve exported the class so it can be used by other components.

Now I add the Clock to my App. There’s just one line to import it:

import Clock from './Clock';

And a tag to add it:

<Clock/>

This is what my App.js file looks like:

import React, { Component } from 'react';
import { Text, View, Button } from 'react-native';
import Clock from './Clock';

export default class HelloWorldApp extends Component {
    render() {
        return (
            <View style={{ flex: 1, justifyContent: "center", alignItems: "center" }}>
            <Text style={{ fontWeight: 'bold', padding: 10 }}>Hello, world!</Text>
            {/* padding does not work with Button!! */}
            <Button style={{ fontWeight: 'bold', padding: 40 }} title="Click me" >Click Me!</Button>
            {/* Since padding does not work with Button, we add an empty text area */}
            <Text>{""}</Text>
            <Clock/>
            </View>
        );
    }
}

As soon as I did this, I saw an error message in my emulator:

The error message reads “Invariant violation: Text strings must be rendered within a component. This error is located at: in h1 (at Clock.js:28) in dev (at Clock.js:27)…”

Since the location of the error is very nicely displayed, I can quickly find the problem. The render method of my React.js Clock class is

  render() {
    return (
      <div>
        <h1>Hello, world!</h1>
        <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
      </div>
    );

and the complaint is about the text “Hello, world!” within the <h1> tags. It didn’t take long before I discovered the problem in the React Native documentation for the Text component: “In React Native, we are more strict about it: you must wrap all the text nodes inside of a component. You cannot have a text node directly under a <View>.”

Let me just try using a Text tag instead of h1 and h2 tags. First I add the import statement:

import { Text } from 'react-native';

And then I swap out <h1> and <h2> for <Text>. That seems to work, but now I have a new error: “Invariant violation: View config not found for name div. Make sure to start component names with a capital letter.”

I already know the complaint is about the div tag in Clock.js. Whoops, divs do not live in native apps. I’ll just change my div tag to a Text tag and see what happens. Here’s my render code:

  render() {
    return (
        <Text>
            <Text>Hello, world!</Text>
            <Text>It is {this.state.date.toLocaleTimeString()}.</Text>
        </Text>
    );
  }

And here’s my running clock!

If you found this interesting, click the subscribe button below! I write a new post about once a week.