Flutter Future<void> async无法更改外部变量

写猜歌游戏时，遇到的问题。
原代码如下：

class _HomeState extends State<Home> {
  List<dynamic> playlists = [];

  Future<void> getData() async {
    final response = await http.post(
      Uri.parse('https://example.com/api.php'),
      headers: <String, String>{
        'Content-Type': 'application/json; charset=UTF-8'
      }
    );

    if (response.statusCode == 200) {
      // 重点部分
      playlists = jsonDecode(response.body)['data'];
    } else {
      // 重点部分
      setState((){
        playlists = [{"id": 0, "title": "error"}];
      });
      print(response.body);
    }
  }

然后在Scaffold中使用时就会发现playlists仍然是空白（没有被更改）。错误出在async更改变量要setState（所以要extend StatefulWidget)。代码更改后如下

class _HomeState extends State<Home> {
  List<dynamic> playlists = [];

  Future<void> getData() async {
    final response = await http.post(
      Uri.parse('https://example.com/api.php'),
      headers: <String, String>{
        'Content-Type': 'application/json; charset=UTF-8'
      }
    );

    if (response.statusCode == 200) {
      // 重点部分
      setState((){
        playlists = jsonDecode(response.body)['data'];
      });
    }else {
      // 重点部分
      setState((){
        playlists = [{"id": 0, "title": "error"}];
      });
    }
  }