我目前正在为我的应用构建一个验证码屏幕，并使用了一个包含一些positioned（）小部件的堆栈，因为验证码表单也应该放置得很好，但我如何让它水平居中呢？
我的代码如下：

@override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        child: Stack(
          children: [
            Image.asset(verificationImage),
            Positioned(
              left: MediaQuery.of(context).size.width * 0.075,
              top: MediaQuery.of(context).size.height * 0.32,
              width: MediaQuery.of(context).size.width * 0.6,
              child: Text(
                'We have send you an email with a verification code at ${generatedCode[0]}${generatedCode[1]}***',
                style: TextStyle(
                    color: Colors.white,
                    fontWeight: FontWeight.bold,
                    fontSize: MediaQuery.of(context).size.height * 0.025),
              ),
            ),
            //Later the
            // Mail Adress
            Positioned(
              child: Form(
                key: formKey,
                child: CodeFields(
                  length: codeLength,
                  validator: validateCode,
                  fieldWidth: MediaQuery.of(context).size.width * 0.1,
                  fieldHeight: MediaQuery.of(context).size.width * 0.14,
                  inputDecoration: InputDecoration(
                    filled: true,
                    fillColor: Colors.grey.withOpacity(0.2),
                    enabledBorder: OutlineInputBorder(
                        borderRadius: BorderRadius.circular(24.0),
                        borderSide: BorderSide(color: Colors.transparent)),
                    border: OutlineInputBorder(
                      borderRadius: BorderRadius.circular(24.0),
                    ),
                  ),
                ),
              ),
            ),
            SizedBox(height: MediaQuery.of(context).size.height * 0.05),
            FlatButton(
              onPressed: onButtonPressed,
              child: Text('Verify code'),
            ),
          ],
        ),
      ),
    );
  }