main.dart

SharedPreferences sharedPreferences;

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

  loadImages();
}

loadImages() async {
  sharedPreferences = await SharedPreferences.getInstance();
  List<String> photos = sharedPreferences.getStringList("images");
  if (photos == null) {
    return;
  }

  int length = photos.length;

  _galleryItems = List.generate(
    length,
    (index) => GalleryItem(
      isSvg: false,
      id: Uuid().v1(),
      resource: photos[index],
    ),
  );
  _photos = List.generate(
    length,
    (index) => File(
      photos[index],
    ),
  );
  _photosUrls = List.generate(
    length,
    (index) => photos[index],
  );
  _photosStatus = List.generate(
    length,
    (index) => PhotoStatus.LOADED,
  );
  _photosSources = List.generate(
    length,
    (index) => PhotoSource.NETWORK,
  );
  setState(() {});
}

//Update save button
Container(
  margin: EdgeInsets.all(16),
  child: RaisedButton(
    child: Text('Save'),
    onPressed: _onSaveClicked,
  ),
)

//Add listener on save
_onSaveClicked() async {
  try {
    sharedPreferences = await SharedPreferences.getInstance();
    await sharedPreferences.setStringList("images", _photosUrls);
    print('Successfully saved');
  } catch (e) {
    print('Error saving ');
  }
}