Flutter Listview freezes while it is loading thumbnail images using FutureBuilder

Question

I am creating a file manager in flutter for learning purpose. I have just started learning flutter so i don't have much knowledge about it.

My issue is that I want to display thumbnails for images in a folder, for that i am calling a function written in android native code using a FutureBuilder in ListView. It is working fine but whenever I open a directory with lots of images, it freezes the UI for some time & I am unable to scroll it for that time even though thumbnails are called asynchronously.

Me code of Listview:-

@override
Widget build(BuildContext context) {
    return new ListView.builder(
      itemBuilder: (itemContext, i) {
        if (_filesList != null && i < _filesList.length) {
          String currDir = _filesList[i]['path'];
          return new ListTile(
            title: new Text(_filesList[i]['name']),
            leading: (_filesList[i]['isDir'] == "false")
                ? new FutureBuilder<Uint8List>(
                    future: _getThumbnail(currDir),
                    builder: (BuildContext context,
                        AsyncSnapshot<Uint8List> snapshot) {
                      switch (snapshot.connectionState) {
                        case ConnectionState.waiting:
                          return new Container(
                            decoration: new BoxDecoration(
                              color: Colors.brown,
                              borderRadius: new BorderRadius.circular(20.0),
                            ),
                            width: 30.0,
                            height: 30.0,
                          );
                        default:
                          if (snapshot.hasError)
                            return new Container(
                              decoration: new BoxDecoration(
                                color: Colors.brown,
                                borderRadius: new BorderRadius.circular(20.0),
                              ),
                              width: 30.0,
                              height: 30.0,
                            );
                          else {
                            return new Image.memory(snapshot.data);
                          }
                      }
                    },
                  )
                : new Container(
                    decoration: new BoxDecoration(
                      color: (_filesList[i]['isDir'] == "true")
                          ? Colors.yellow
                          : Colors.brown,
                      borderRadius: new BorderRadius.circular(20.0),
                    ),
                    width: 30.0,
                    height: 30.0),
            onTap: () {
              setState(() {
                _currentDir = currDir;
              });
              _getFilesList(currDir);
            },
          );
        } else {
          return null;
        }
      },
    );
  }

Code for _getThumbnail():-

Future <Uint8List> _getThumbnail(path) async {
    try {
      String thumbnailString =
          await channel.invokeMethod("getThumbnail", {"path": path});
      thumbnailString = thumbnailString.replaceAll('\n', '');
      Uint8List bytes = BASE64.decode(thumbnailString);
      return bytes;
    } on PlatformException catch (e) {
      print("Files Not Found $e{e.message}");
      return null;
    }
  }

Answer 1

Even though calls are asynchronous in Flutter, the code on Android that handles getThumbnail can block the UI thread, leading to frozen loading or choppy scrolling. A solution is to use AsyncTask to build/fetch the thumbnails in doInBackground and return the result in onPostExecute .