react native (expo) load markdown files

Question

I'm having some troubles loading markdown files (.md) into my react native (non-detached expo project).

Found this awesome package that allows me to render it. But can't figure out how to load the local .md file as a string.

import react from 'react';
import {PureComponent} from 'react-native';
import Markdown from 'react-native-markdown-renderer';

const copy = `# h1 Heading 8-)

| Option | Description |
| ------ | ----------- |
| data   | path to data files to supply the data that will be passed into templates. |
| engine | engine to be used for processing templates. Handlebars is the default. |
| ext    | extension to be used for dest files. |
`;

export default class Page extends PureComponent {

  static propTypes = {};
  static defaultProps = {};

  render() {
    return (
        <Markdown>{copy}</Markdown>
    );
  }
}

---

BTW: I tried googling, but can't get the suggestions to work

https://forums.expo.io/t/loading-non-media-assets-markdown/522/2?u=norfeldtconsulting

I tried the suggested answers for reactjs on SO, but the problem seems to be that it only accepts .js and .json files

Answer 1

Thanks to @Filipe's response, I got some guidance and got a working example that will fit your needs.

In my case, I had a .md file on the assets/markdown/ folder, the file is called test-1.md

The trick is to get a local url for the file, and then use the fetch API to get its content as a string.

import React from 'react';
import { StyleSheet, Text, View } from 'react-native';
import Markdown from 'react-native-markdown-renderer';
const copy = `# h1 Heading 8-)

| Option | Description |
| ------ | ----------- |
| data   | path to data files to supply the data that will be passed into templates. |
| engine | engine to be used for processing templates. Handlebars is the default. |
| ext    | extension to be used for dest files. |
`;

export default class App extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
      copy: copy
    }
  }

  componentDidMount() {
    this.fetchLocalFile();
  }

  fetchLocalFile = async () => {
    let file = Expo.Asset.fromModule(require("./assets/markdown/test-1.md"))
    await file.downloadAsync() // Optional, saves file into cache
    file = await fetch(file.uri)
    file = await file.text()

    this.setState({copy: file});
  }


  render() {
    return (
        <Markdown>{this.state.copy}</Markdown>
    );
  }
}

EDIT: In order to get get rid of the error

Unable to resolve "./assets/markdown/test-1.md" from "App.js"

you would need to add the packagerOpts part of @Filipe's snippet into your app.json file.

app.json

{
  "expo": {
    ...
    "assetBundlePatterns": [
      "**/*"
    ],
    "packagerOpts": {
      "assetExts": ["md"]
    },
    ...
  }
}

EDIT 2: Answering to @Norfeldt's comment: Although I use react-native init when working on my own projects, and I'm therefore not very familiar with Expo, I got this Expo Snack that might have some answers for you: https://snack.expo.io/Hk8Ghxoqm.

It won't work on the expo snack because of the issues reading non-JSON files, but you can test it locally if you wish.

Using file.downloadAsync() will prevent the app making XHR calls to a server where your file is hosted within that app session (as long as the user does not close and re-open the app).

If you change the file or modify the file (simulated with a call to Expo.FileSystem.writeAsStringAsync()), it should display the updated as long as your component re-renders and re-downloads the file.

This will happen every time your app is closed and re-open, as the file.localUri is not persisted per sessions as far as I'm concerned, so your app will always call file.downloadAsync() at least once every time it's opened. So you should have no problems displaying an updated file.

I also took some time to test the speed of using fetch versus using Expo.FileSystem.readAsStringAsync(), and they were on average the same. Often times Expo.FileSystem.readAsStringAsync was ~200 ms faster, but it 's not a deal breaker in my opinion.

I created three different methods for fetching the same file.

export default class MarkdownRenderer extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
      copy: ""
    }
  }

  componentDidMount() {
    this.fetch()
  }

  fetch = () => {
    if (this.state.copy) {
      // Clear current state, then refetch data
      this.setState({copy: ""}, this.fetch)
      return;
    }
    let asset = Expo.Asset.fromModule(md)
    const id = Math.floor(Math.random()  * 100) % 40;
    console.log(`[${id}] Started fetching data`, asset.localUri)
    let start = new Date(), end;

    const save = (res) => {
      this.setState({copy: res})
      let end = new Date();
      console.info(`[${id}] Completed fetching data in ${(end - start) / 1000} seconds`)
    }

    // Using Expo.FileSystem.readAsStringAsync.
    // Makes it a single asynchronous call, but must always use localUri
    // Therefore, downloadAsync is required
    let method1 = () => {
      if (!asset.localUri) {
        asset.downloadAsync().then(()=>{
          Expo.FileSystem.readAsStringAsync(asset.localUri).then(save)
        })
      } else {
        Expo.FileSystem.readAsStringAsync(asset.localUri).then(save)
      }
    }

    // Use fetch ensuring the usage of a localUri
    let method2 = () => {
      if (!asset.localUri) {
        asset.downloadAsync().then(()=>{
          fetch(asset.localUri).then(res => res.text()).then(save)
        })
      } else {
        fetch(asset.localUri).then(res => res.text()).then(save)
      }
    }

    // Use fetch but using `asset.uri` (not the local file)
    let method3 = () => {
      fetch(asset.uri).then(res => res.text()).then(save)
    }

    // method1()
    // method2()
    method3()
  }

  changeText = () => {
    let asset = Expo.Asset.fromModule(md)
    Expo.FileSystem.writeAsStringAsync(asset.localUri, "Hello World");
  }

  render() {
    return (
        <ScrollView style={{maxHeight: "90%"}}>
          <Button onPress={this.fetch} title="Refetch"/>
          <Button onPress={this.changeText} title="Change Text"/>
            <Markdown>{this.state.copy}</Markdown>
        </ScrollView>
    );
  }
}

Just alternate between the three to see the difference in the logs.

Answer 2

From what I know, this cannot be done within expo. I use react-native and run it on my mobile for development.

react-native use Metro as the default bundler, which also suffers from similar problems. You have to use haul bundler instead.

npm install --save-dev haul

npx haul init

npx haul start --platform android

In a seperate terminal run react-native run-android. This would use haul instead of metro to bundle the files.

To add the markdown file, install raw-loader and edit the haul.config.js file. raw-loader imports any file as a string.

Customise your haul.config.js to look something like this:

import { createWebpackConfig } from "haul";
export default {
 webpack: env => {
  const config = createWebpackConfig({
    entry: './index.js',
  })(env);
  config.module.rules.push({
      test: /\.md$/,
      use: 'raw-loader'
   })
  return config;
 }
};

Now you can import the markdown file by using const example = require('./example.md')

Haul supports webpack configuration so you can add any custom babel transform you want.

Answer 3

I don't know exactly where the problem lies, but I added html files to the project, and I'd imagine it would be very similar.

Inside your app.json, try adding these fields:

"assetBundlePatterns": [
  "assets/**",
],
"packagerOpts": {
  "assetExts": ["md"]
},

The packagerOpts makes it so the standalone will bundle the .md files. I'd imagine you already have an assets folder, but just in case you don't, you will need one.

Then, on AppLoading, loading the assets with Asset.loadAsync might not be needed, but it's a good idea to rule out. Check out the documentation on how to use it.

When importing the file, there are three ways you might want to do so, that change depending on the environment. I'll copy this excerpt from my Medium article:

In the simulator, you can access any file in the project. Thus, source={require(./pathToFile.html)} works. However, when you build a standalone, it doesn’t work quite in the same way. I mean, at least for android it doesn’t. The android webView doesn’t recognise asset:/// uris for some reason. You have to get the file:/// path. Thankfully, that is very easy. The assets are bundled inside file:///android_asset (Careful, don’t write assets), and Expo.Asset.fromModule(require(‘./pathToFile.html')).localUri returns asset:///nameOfFile.html. But that’s not all. For the first few times, this uri will be correct. However, after a while, it changes into another file scheme, and can’t be accessed in the same way. Instead, you’ll have to access the localUri directly. Thus, the complete solution is:

/* Outside of return */
const { localUri } = Expo.Asset.fromModule(require('./pathToFile.html'));
/* On the webView */
source={
  Platform.OS === ‘android’
  ? {
    uri: localUri.includes('ExponentAsset')
      ? localUri
      : ‘file:///android_asset/’ + localUri.substr(9),
  }
  : require(‘./pathToFile.html’)
}

(A constant part of the uri is ExponentAsset, that’s why I chose to check if that was part of it)

That should probably solve your problem. If it doesn't, comment what's going wrong and I'll try to help you further. Cheers!