如何在屏幕中心的所有窗口小部件上显示CircularProgressIndicator

Stack是你要找的:

https://docs.flutter.io/flutter/widgets/Stack-class.html

     @override
      Widget build(BuildContext context) {
        return new Scaffold(
            body: Stack(
                      children:<Widget>[ Container(
                  height: MediaQuery.of(context).size.height,
                  decoration: BoxDecoration(
                    color: Colors.white,
                  ),
                  child: new Column(
                    crossAxisAlignment: CrossAxisAlignment.center,
                    mainAxisSize: MainAxisSize.max,
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      new Row(
                        children: <Widget>[
                          new Expanded(
                            child: new Padding(
                              padding: const EdgeInsets.only(left: 40.0),
                              child: new Text(
                                "EMAIL",
                                style: TextStyle(
                                  fontWeight: FontWeight.bold,
                                  color: Colors.redAccent,
                                  fontSize: 15.0,
                                ),
                              ),
                            ),
                          ),
                        ],
                      ),
                      new Container(
                        width: MediaQuery.of(context).size.width,
                        margin:
                            const EdgeInsets.only(left: 40.0, right: 40.0, top: 10.0),
                        alignment: Alignment.center,
                        decoration: BoxDecoration(
                          border: Border(
                            bottom: BorderSide(
                                color: Colors.redAccent,
                                width: 0.5,
                                style: BorderStyle.solid),
                          ),
                        ),
                        padding: const EdgeInsets.only(left: 0.0, right: 10.0),
                        child: new Row(
                          crossAxisAlignment: CrossAxisAlignment.center,
                          mainAxisAlignment: MainAxisAlignment.start,
                          children: <Widget>[
                            new Expanded(
                              child: TextField(
                                controller: emailController,
                                textAlign: TextAlign.left,
                                decoration: InputDecoration(
                                  border: InputBorder.none,
                                  hintText: 'PLEASE ENTER YOUR EMAIL',
                                  hintStyle: TextStyle(color: Colors.grey),
                                ),
                              ),
                            ),
                          ],
                        ),
                      ),
                      Divider(
                        height: 24.0,
                      ),
                      new Container(
                        width: MediaQuery.of(context).size.width,
                        margin:
                            const EdgeInsets.only(left: 30.0, right: 30.0, top: 20.0),
                        alignment: Alignment.center,
                        child: new Row(
                          children: <Widget>[
                            new Expanded(
                              child: new FlatButton(
                                shape: new RoundedRectangleBorder(
                                  borderRadius: new BorderRadius.circular(30.0),
                                ),
                                color: Colors.redAccent,
                                onPressed: () {
                                  setState(() {
                                    isLoading = true;
                                  });
                                  authHandler
                                      .sendPasswordResetEmail(emailController.text)
                                      .then((void nothing) async {
                                    await showDialog(
                                      context: context,
                                      builder: (BuildContext context) {
                                        // return object of type Dialog
                                        return AlertDialog(
                                          content: new Text(
                                              "Password reset email has been sent."),
                                          actions: <Widget>[
                                            // usually buttons at the bottom of the dialog
                                            new FlatButton(
                                              child: new Text("OK"),
                                              onPressed: () {
                                                Navigator.pop(context);
                                              },
                                            ),
                                          ],
                                        );
                                      },
                                    );

                                    Navigator.pop(context);

                                    setState(() {
                                      isLoading = false;
                                    });
                                  }).catchError((e) => print(e));
                                },
                                child: new Container(
                                  padding: const EdgeInsets.symmetric(
                                    vertical: 20.0,
                                    horizontal: 20.0,
                                  ),
                                  child: new Row(
                                    mainAxisAlignment: MainAxisAlignment.center,
                                    children: <Widget>[
                                      new Expanded(
                                        child: Text(
                                          "FORGOT PASSWORD",
                                          textAlign: TextAlign.center,
                                          style: TextStyle(
                                              color: Colors.white,
                                              fontWeight: FontWeight.bold),
                                        ),
                                      ),
                                    ],
                                  ),
                                ),
                              ),
                            ),
                          ],
                        ),
                      ),
                    ],
                  )),

                  isLoading ? Container(
                    color: Colors.black.withOpacity(0.5),
                    child: Center(
                      child: CircularProgressIndicator(),
                    ),
                  ) : Container()
                  ],
            ));
      }