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"),
),

如果有人正在寻找完整的圆形按钮，那么我实现它的方式:

Center(
            child: SizedBox.fromSize(
              size: Size(80, 80), // Button width and height
              child: ClipOval(
                child: Material(
                  color: Colors.pink[300], // Button color
                  child: InkWell(
                    splashColor: Colors.yellow, // splash color
                    onTap: () {}, // Button pressed
                    child: Column(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: <Widget>[
                        Icon(Icons.linked_camera), // Icon
                        Text("Picture"), // Text
                      ],
                    ),
                  ),
                ),
              ),
            ),
          )

您可以简单地使用RaisedButton

Padding(
  padding: EdgeInsets.only(left: 150.0, right: 0.0),
  child: RaisedButton(
    textColor: Colors.white,
    color: Colors.black,
    child: Text("Search"),
    onPressed: () {},
    shape: new RoundedRectangleBorder(
      borderRadius: new BorderRadius.circular(30.0),
    ),
  ),
)

输出:

更多信息:RSCoder

要在按钮中使用任何形状，请确保执行button小部件内的所有代码:

 **shape: RoundedRectangleBorder(
        borderRadius: new BorderRadius.circular(18.0),
        side: BorderSide(color: Colors.red) ),**

如果你想让它变成正方形，使用BorderRadius.circular(0.0)，它会自动把它变成正方形。

按钮是这样的:

下面是给出UI屏幕的所有源代码:

 Scaffold(
    backgroundColor: Color(0xFF8E44AD),
    body: new Center(
      child: Column(
        children: <Widget>[
          Container(
            margin: EdgeInsets.fromLTRB(90, 10, 20, 0),
            padding: new EdgeInsets.only(top: 92.0),
            child: Text(
              "Currency Converter",
              style: TextStyle(
                fontSize: 48,
                fontWeight: FontWeight.bold,
                color: Colors.white,
              ),
            ),
          ),
          Container(
            margin: EdgeInsets.only(),
            padding: EdgeInsets.all(25),
            child: TextFormField(
              decoration: new InputDecoration(
                filled: true,
                fillColor: Colors.white,
                labelText: "Amount",
                border: OutlineInputBorder(
                  borderRadius: BorderRadius.circular(10),
                ),
              ),
            ),
          ),
          Container(
            padding: EdgeInsets.all(25),
            child: TextFormField(
              decoration: new InputDecoration(
                filled: true,
                fillColor: Colors.white,
                labelText: "From",
                border: OutlineInputBorder(
                  borderRadius: BorderRadius.circular(10),
                ),
              ),
            ),
          ),
          Container(
            padding: EdgeInsets.all(25),
            child: TextFormField(
              decoration: new InputDecoration(
                  filled: true,
                  fillColor: Colors.white,
                  labelText: "To",
                  border: OutlineInputBorder(
                    borderRadius: BorderRadius.circular(10),
                  )),
            ),
          ),
          SizedBox(height: 20.0),
          MaterialButton(
            height: 58,
            minWidth: 340,
            shape: RoundedRectangleBorder(
                borderRadius: new BorderRadius.circular(12)),
            onPressed: () {},
            child: Text(
              "CONVERT",
              style: TextStyle(
                fontSize: 24,
                color: Colors.black,
              ),
            ),
            color: Color(0xFFF7CA18),
          ),
        ],
      ),
    ),
  ),
);

在Flutter中，Container()小部件用于设置小部件的样式。使用Container()小部件，您可以设置任何小部件的边框或圆角。

如果您想设置任何类型的样式和装饰，请将该小部件放到Container()小部件中。这为装饰提供了许多属性。

Container(
  width: 100,
  padding: EdgeInsets.all(10),
  alignment: Alignment.center,
  decoration: BoxDecoration(
          color: Colors.blueAccent,
          borderRadius: BorderRadius.circular(30)), // Make rounded corner
  child: Text("Click"),
)

你可以使用这个样式为你的elevatedButton使它成为圆形

style: ButtonStyle(
              elevation: MaterialStateProperty.all(8.0),
              backgroundColor:
                  MaterialStateProperty.all(Constants().orangeColor),
              textStyle: MaterialStateProperty.all(
                TextStyle(
                  fontSize: 16.0,
                ),
              ),
              shape: MaterialStateProperty.all<CircleBorder>(
                CircleBorder(),
              ),
              shadowColor: MaterialStateProperty.all(Constants().orangeColor),
            ),