Speed Up React Native File System (RNFS) on Android

Start writingI had the displeasure of reading and writing a relatively large string to disk using RNFS on a React Native app. It took about 16 seconds to write to a file and 20 seconds to read from the file. I didn’t get enough answers online so I tinkered with the source code.

It boils down to changing the methods in the file

node-modules/react-native-fs/android/source/main/java/com/rnfs/RNFSManager.java

From here...

@ReactMethod
public void readFile(String filepath, Promise promise) {
  try {
    File file = new File(filepath);

    if (file.isDirectory()) {
      rejectFileIsDirectory(promise);
      return;
    }

    if (!file.exists()) {
      rejectFileNotFound(promise, filepath);
      return;
    }

    FileInputStream inputStream = new FileInputStream(filepath);
    byte[] buffer = new byte[(int)file.length()];
    inputStream.read(buffer);

    String base64Content = Base64.encodeToString(buffer, Base64.NO_WRAP);

    promise.resolve(base64Content);
  } catch (Exception ex) {
    ex.printStackTrace();
    reject(promise, filepath, ex);
  }
}

To

@ReactMethod
public void readFile(final String filepath, final Promise promise) {
  new Thread(new Runnable() {
    public void run() {
      try {
        File file = new File(filepath);

        if (file.isDirectory()) {
          rejectFileIsDirectory(promise);
          return;
        }

        if (!file.exists()) {
          rejectFileNotFound(promise, filepath);
          return;
        }

        FileInputStream inputStream = new FileInputStream(filepath);
        byte[] buffer = new byte[(int)file.length()];
        inputStream.read(buffer);
        String str = new String(buffer, "UTF-8");
        promise.resolve(str);
      } catch (Exception ex) {
        ex.printStackTrace();
        reject(promise, filepath, ex);
      }
    }
  }).start();
}

To write to a file, I changed

@ReactMethod
public void writeFile(String filepath, String base64Content, Promise promise) {
  try {
        byte[] bytes = Base64.decode(base64Content, Base64.DEFAULT);
        FileOutputStream outputStream = new FileOutputStream(filepath);
        outputStream.write(bytes);
        outputStream.close();
        promise.resolve(null);
      } catch (Exception ex) {
        ex.printStackTrace();
        reject(promise, filepath, ex);
      }
}

To

@ReactMethod
public void writeFile(final String filepath, final String base64Content, final Promise promise) {
  new Thread(new Runnable() {
    public void run() {
      try {
        byte[] bytes = base64Content.getBytes();
        FileOutputStream outputStream = new FileOutputStream(filepath);
        outputStream.write(bytes);
        outputStream.close();

        promise.resolve(null);
      } catch (Exception ex) {
        ex.printStackTrace();
        reject(promise, filepath, ex);
      }
    }
  }).start();
}

In summary,

1. Move the operation to a new thread
2. Remove the base64 encoding and decoding operation for the string you want to save (This was the major cause)

All the best.