Support styleString as "Documents directory/Temporary directory" absolute path

[bahung1221]

Hi,

This PR will enable support for using app documents directory or temporary directory as styleString parameter for both Android and iOS.

Use case:

Currently, for dynamic styles, flutter-mapbox-gl only support styleString as raw JSON string and this feature only work on Android, so we doesn't have any way to load dynamic styles on iOS device.

This PR will allow us to save raw JSON that we got from anywhere (API call, generated by code,...) to app storage at runtime and then able to pass their path as styleString parameter.

Sample code:

  ...
  @override
  initState() {
    super.initState();

    getApplicationDocumentsDirectory().then((dir) async {
      String documentDir = dir.path;
      String stylesDir = '$documentDir/styles';
      String styleJSON = '{...}'; // JSON string

      await new Directory(stylesDir).create(recursive: true);

      File styleFile = new File('$stylesDir/style.json');

      await styleFile.writeAsString(styleJSON);

      setState(() {
        styleAbsoluteFilePath = styleFile.path;
      });
    });
  }
  ...
  @override
  Widget build(BuildContext context) {
    if (styleAbsoluteFilePath == null) {
      return Scaffold(
        body: Center(child: Text('Loading local style file...')),
      );
    }

    return new Scaffold(
      body: MapboxMap(
       ...,
        styleString: styleAbsoluteFilePath,
       ...
      )
    );
  }
  ...

Tested on Android 9 simulator, Android 10 real device, and iOS 14.3 simulator.

Thanks for such a great library and sorry for my bad English 😄

[bahung1221]

Hi @tobrun , sorry for troubling you but I need this feature, my deadline is coming 🤣
May you help to review this PR when you have free time?

Cheers 🍺

[tobrun]

Cool workaround!