如果你想使用MaterialButton，

=======================================

你可以添加RoundedRectangleBorder，就像这样，

MaterialButton(
  onPressed: () {},
  minWidth: MediaQuery.of(context).size.width * 0.4,
  height: 34,
  color: colorWhite,
  highlightColor: colorSplash,
  splashColor: colorSplash,
  visualDensity: VisualDensity.compact,
  shape: RoundedRectangleBorder(
    borderRadius: BorderRadius.circular(4),
    side: BorderSide(
      color: colorGrey,
      width: 0.6,
    ),
  ),
  child: Text("CANCEL"),
),

您可以使用ElevatedButton小部件。提升的按钮小部件有一个shape属性，您可以使用该属性，如下面的代码片段所示。

ElevatedButton(
      style: ButtonStyle(
          shape: MaterialStateProperty.all<RoundedRectangleBorder>(
              RoundedRectangleBorder(
                  borderRadius: BorderRadius.circular(18.0),
                  side: BorderSide(
                      color: Colors.teal, 
                      width: 2.0,
                  ),
              ),
          ),
      ),
      child: Text('Submit'),
      onPressed: () {},
),

在容器小部件中包装一个TextButton，如下所示:

Container(
  decoration: BoxDecoration(
    borderRadius: BorderRadius.circular(5),
    border: Border.all(color: Colors.black),
  ),
  child: TextButton(
    onPressed: () {
      // To do
    },
    child: Text("Go to Change Language Screen "),
  ),
)

您可以使用下面的代码创建一个渐变颜色的圆形按钮。

 Container(
          width: 130.0,
          height: 43.0,
          decoration: BoxDecoration(
            borderRadius: BorderRadius.circular(30.0),
            gradient: LinearGradient(
              // Where the linear gradient begins and ends
              begin: Alignment.topRight,
              end: Alignment.bottomLeft,
              // Add one stop for each color. Stops should increase from 0 to 1
              stops: [0.1, 0.9],
              colors: [
                // Colors are easy thanks to Flutter's Colors class.
                Color(0xff1d83ab),
                Color(0xff0cbab8),
              ],
            ),
          ),
          child: FlatButton(
            child: Text(
              'Sign In',
              style: TextStyle(
                fontSize: 16.0,
                fontFamily: 'Righteous',
                fontWeight: FontWeight.w600,
              ),
            ),
            textColor: Colors.white,
            color: Colors.transparent,
            shape:
                RoundedRectangleBorder(borderRadius: BorderRadius.circular(30.0)),
            onPressed: () {

            },
          ),
        );

1. 解决方案概述

FlatButton和RaisedButton已弃用。

因此，你可以使用形状放在样式属性，为TextButton和ElevatedButton。

自Flutter 2.0以来有一些变化:

style:属性类型已更改为ButtonStyle shape:属性类型已更改为MaterialStateProperty<T>

2. 圆形的按钮

在style属性中存在shape属性:

style: ButtonStyle(
  shape: MaterialStateProperty.all<RoundedRectangleBorder>(
    RoundedRectangleBorder(
      borderRadius: BorderRadius.circular(18.0),
      side: BorderSide(color: Colors.red)
    )
  )
)

方形按钮

对于方形按钮，您可以使用ElevatedButton或添加其他方式:

style: ButtonStyle(
  shape: MaterialStateProperty.all<RoundedRectangleBorder>(
    RoundedRectangleBorder(
      borderRadius: BorderRadius.zero,
      side: BorderSide(color: Colors.red)
    )
  )
)

完整的示例

Row(
  mainAxisAlignment: MainAxisAlignment.end,
  children: [
    TextButton(
      child: Text(
        "Add to cart".toUpperCase(),
        style: TextStyle(fontSize: 14)
      ),
      style: ButtonStyle(
        padding: MaterialStateProperty.all<EdgeInsets>(EdgeInsets.all(15)),
        foregroundColor: MaterialStateProperty.all<Color>(Colors.red),
        shape: MaterialStateProperty.all<RoundedRectangleBorder>(
          RoundedRectangleBorder(
            borderRadius: BorderRadius.circular(18.0),
            side: BorderSide(color: Colors.red)
          )
        )
      ),
      onPressed: () => null
    ),
    SizedBox(width: 10),
    ElevatedButton(
      child: Text(
        "Buy now".toUpperCase(),
        style: TextStyle(fontSize: 14)
      ),
      style: ButtonStyle(
        foregroundColor: MaterialStateProperty.all<Color>(Colors.white),
        backgroundColor: MaterialStateProperty.all<Color>(Colors.red),
        shape: MaterialStateProperty.all<RoundedRectangleBorder>(
          RoundedRectangleBorder(
            borderRadius: BorderRadius.zero,
            side: BorderSide(color: Colors.red)
          )
        )
      ),
      onPressed: () => null
    )
  ]
)

下面是解决问题的代码。你只需要在盒子装饰中取一个有边界半径的简单容器。

new Container(
    alignment: Alignment.center,
    decoration: BoxDecoration(
        borderRadius: BorderRadius.all(Radius.circular(15.0)),
        color: Colors.blue,
    ),

    child: Row(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
            Padding(
                padding: const EdgeInsets.all(10.0),
                child: new Text(
                    "Next",
                    style: new TextStyle(
                        fontWeight: FontWeight.w500,
                        color: Colors.white,
                        fontSize: 15.0,
                    ),
                ),
            ),
        ],
    ),
),