If all you want is to save an image (example: a .png) to the device, you can easily achieve this with a simple get (http/http.dart) and a File (dart:io).
To do so, you can base yourself in the example below:
var response = await http.get(imgUrl);
Directory documentDirectory = await getApplicationDocumentsDirectory();
File file = new File(join(documentDirectory.path, 'imagetest.png'));
file.writeAsBytesSync(response.bodyBytes); // This is a sync operation on a real
// app you'd probably prefer to use writeAsByte and handle its Future
Note that in the case above I’ve used the ‘path_provider’ package from the dart pub. In overall you would have imported at least these items:
import 'dart:io';
import 'package:http/http.dart' as http;
import 'package:path/path.dart';
import 'package:path_provider/path_provider.dart';
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