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: () {

            },
          ),
        );

如果你使用材质应用程序作为你的主小部件，你总是可以使用材质按钮。

Padding(
  padding: EdgeInsets.symmetric(vertical: 16.0),
  child: Material(
    borderRadius: BorderRadius.circular(30.0),//Set this up for rounding corners.
    shadowColor: Colors.lightBlueAccent.shade100,
    child: MaterialButton(
      minWidth: 200.0,
      height: 42.0,
      onPressed: (){//Actions here//},
      color: Colors.lightBlueAccent,
      child: Text('Log in', style: TextStyle(color: Colors.white),),
    ),
  ),
)

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),
          ),
        ],
      ),
    ),
  ),
);

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),
            ),

试试这个:

SizedBox(
  height: 40.0,
  child: MaterialButton(
    child: Text("Button"),
    color: Colors.blue,
    disabledColor: Colors.blue,
    shape: RoundedRectangleBorder(
      borderRadius: BorderRadius.all(
        Radius.circular(10.0), // Change your border radius here
      ),
    ),
    onPressed: () {},
  ),
),

使用TextButton代替。

据说，自2020年10月以来，FlatButton、RaisedButton和OutlineButton等按钮已被弃用。这是Flutter开发团队为简化并使Flutter API保持一致所做的努力之一，您可以通过使用style属性自定义其样式。

      TextButton(
        child: Padding(
          padding: const EdgeInsets.only(left: 10.0, right: 10.0),
          child: Text('Text here',
              style: TextStyle(
                  color: Colors.teal,
                  fontSize: 14,
                  fontWeight: FontWeight.w500)),
        ),
        style: TextButton.styleFrom(
          primary: Colors.teal,
          onSurface: Colors.yellow,
          side: BorderSide(color: Colors.teal, width: 2),
          shape: const RoundedRectangleBorder(
              borderRadius: BorderRadius.all(Radius.circular(25))),
        ),
        onPressed: () {
          print('Pressed');
        },
      ),