flutter_codex_storage_02.dart

// Here we get an instance to Firebase Storage.

FirebaseStorage _storage = FirebaseStorage.instance;

// This async function gets the image from the gallery
// and upload it to Firebase Storage.

Future uploadImage() async {
  var image = await ImagePicker.pickImage(
    source: ImageSource.gallery,
  );

  if (image != null) {
    var ref = _storage.ref().child('user.avatar${extension(image.path)}');
    ref.putFile(image);
    ext = extension(image.path);
  } else {
    print('No image was selected!');
  }
}

// This button launches the process.

RaisedButton(
  child: Text('Choose Avatar'),
  color: Colors.greenAccent,
  onPressed: uploadImage,
)