Center(
          child: Column(
            children: <Widget>[
              RaisedButton(
                child: Text("aa"),
                onPressed: () {},
              ),
              CircularProgressIndicator()
            ],
          ),
        )

CircularProgressIndicator(
　　valueColor: new AlwaysStoppedAnimation<Color>(Colors.green),
),

loading(BuildContext context) {
  showDialog<Null>(
      context: context, //BuildContext对象
      barrierDismissible: false,
      builder: (BuildContext context) {
        return LoadingDialog(
          //调用对话框
          text: '正在加载请稍后...',
        );
      });
}

class LoadingDialog extends Dialog {
  final String text;

  LoadingDialog({Key key, @required this.text}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
        onWillPop: () => Future.value(false), //禁止返回
        child: Material(
          //创建透明层
          type: MaterialType.transparency, //透明类型
          child: Center(
            //保证控件居中效果
            child: SizedBox(
              width: 200.0,
              height: 200.0,
              child: Container(
                // decoration: ShapeDecoration(
                //   color: Color(0xffffffff),
                //   // color: Color(0xffffffff),
                //   shape: RoundedRectangleBorder(
                //     borderRadius: BorderRadius.all(
                //       Radius.circular(8.0),
                //     ),
                //   ),
                // ),
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  crossAxisAlignment: CrossAxisAlignment.center,
                  children: <Widget>[
                    CircularProgressIndicator(
                      valueColor: AlwaysStoppedAnimation<Color>(Colors.grey),
                    ),
                    Padding(
                      padding: const EdgeInsets.only(
                        top: 20.0,
                      ),
                      child: Text(
                        text,
                        style: TextStyle(fontSize: 16.0, color: Colors.white),
                      ),
                    ),
                  ],
                ),
              ),
            ),
          ),
        ));
  }
}

LinearProgressIndicator()