是不是遇到过这种问题：

原因：

[Scaffold.of], to obtain the current [ScaffoldState], which manages the
display and animation of snack bars.
[ScaffoldState.showSnackBar], which displays a [SnackBar].
[ScaffoldState.removeCurrentSnackBar], which abruptly hides the currently
displayed snack bar, if any, and allows the next to be displayed.

解决方案：

  var _scaffoldkey = new GlobalKey<ScaffoldState>();

   return new Scaffold(
      key: _scaffoldkey,
      ........
      ),
      .......
      onPressed: () {
     var snackBar = SnackBar(content: Text('显示snackBar'));
     _scaffoldkey.currentState.showSnackBar(snackBar);
              }
       .......

已有项目集成到Flutter代码已经上传到我的GITHUB

知乎日报Flutter版代码已经上传到我的GITHUB

基础学习过程中的代码都放在GITHUB

每天学一点，学到Flutter发布正式版！