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),),
    ),
  ),
)

这里有另一个解决方案:

Container(
    height: MediaQuery.of(context).size.height * 0.10,
    width: MediaQuery.of(context).size.width,
    child: ButtonTheme(
        minWidth: MediaQuery.of(context).size.width * 0.75,
        child: RaisedButton(
            shape: RoundedRectangleBorder(
                borderRadius: new BorderRadius.circular(25.0),
                side: BorderSide(color: Colors.blue)),
                onPressed: () async {
                    // Do something
                },
                color: Colors.red[900],
                textColor: Colors.white,
                child: Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: Text("Button Text,
                    style: TextStyle(fontSize: 24)),
            ),
        ),
    ),
),

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

你也可以使用ButtonTheme():

下面是示例代码

ButtonTheme(
    minWidth: 200.0,
    shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(18.0),
        side: BorderSide(color: Colors.green)),
    child: RaisedButton(
        elevation: 5.0,
        hoverColor: Colors.green,
        color: Colors.amber,
        child: Text(
            "Place Order",
            style: TextStyle(
                     color: Colors.white, fontWeight: FontWeight.bold),
        ),
        onPressed: () {},
    ),
),

2020年9月起，Flutter 1.22.0:

“RaisedButton”和“FlatButton”都已弃用。

最新的解决方案是使用新的按钮:

1. ElevatedButton:

代码:

ElevatedButton(
  child: Text("ElevatedButton"),
  onPressed: () => print("it's pressed"),
  style: ElevatedButton.styleFrom(
    primary: Colors.red,
    onPrimary: Colors.white,
    shape: RoundedRectangleBorder(
      borderRadius: BorderRadius.circular(32.0),
    ),
  ),
)

别忘了，还有一个.icon构造函数可以方便地添加图标:

ElevatedButton.icon(
  icon: Icon(Icons.thumb_up),
  label: Text("Like"),
  onPressed: () => print("it's pressed"),
  style: ElevatedButton.styleFrom(
    shape: RoundedRectangleBorder(
      borderRadius: BorderRadius.circular(32.0),
    ),
  ),
)

2. OutlinedButton:

代码:

OutlinedButton.icon(
  icon: Icon(Icons.star_outline),
  label: Text("OutlinedButton"),
  onPressed: () => print("it's pressed"),
  style: ElevatedButton.styleFrom(
    side: BorderSide(width: 2.0, color: Colors.blue),
    shape: RoundedRectangleBorder(
      borderRadius: BorderRadius.circular(32.0),
    ),
  ),
)

3. TextButton:

如果你不想要轮廓或颜色填充，你总是可以使用TextButton。