Container(
        width: yourWidth,
        height: yourHeight ,
        decoration: BoxDecoration(
            borderRadius: radius,
            gradient: yourGradient,
            border: yourBorder),
        child: FlatButton(
          onPressed: {} (),
          shape: RoundedRectangleBorder(borderRadius: radius),
    .......

用同样的半径。

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

更新

由于左侧按钮现在已弃用，请使用右侧按钮。

Deprecated    -->   Recommended

RaisedButton  -->   ElevatedButton
OutlineButton -->   OutlinedButton
FlatButton    -->   TextButton

ElevatedButton

Using StadiumBorder ElevatedButton( onPressed: () {}, child: Text('Button'), style: ElevatedButton.styleFrom(shape: StadiumBorder()), ) Using RoundedRectangleBorder ElevatedButton( onPressed: () {}, child: Text('Button'), style: ElevatedButton.styleFrom( shape: RoundedRectangleBorder( borderRadius: BorderRadius.circular(12), // <-- Radius ), ), ) Using CircleBorder ElevatedButton( onPressed: () {}, child: Text('Button'), style: ElevatedButton.styleFrom( shape: CircleBorder(), padding: EdgeInsets.all(24), ), ) Using BeveledRectangleBorder ElevatedButton( onPressed: () {}, child: Text('Button'), style: ElevatedButton.styleFrom( shape: BeveledRectangleBorder( borderRadius: BorderRadius.circular(12) ), ), )

OutlinedButton

Using StadiumBorder OutlinedButton( onPressed: () {}, child: Text('Button'), style: OutlinedButton.styleFrom( shape: StadiumBorder(), ), ) Using RoundedRectangleBorder OutlinedButton( onPressed: () {}, child: Text('Button'), style: OutlinedButton.styleFrom( shape: RoundedRectangleBorder( borderRadius: BorderRadius.circular(12), ), ), ) Using CircleBorder: OutlinedButton( onPressed: () {}, child: Text('Button'), style: OutlinedButton.styleFrom( shape: CircleBorder(), padding: EdgeInsets.all(24), ), ) Using BeveledRectangleBorder OutlinedButton( onPressed: () {}, child: Text('Button'), style: OutlinedButton.styleFrom( shape: BeveledRectangleBorder( borderRadius: BorderRadius.circular(12), ), ), )

TextButton

TextButton也类似于ElevatedButton和OutlinedButton，但是，你只能看到按钮按下的形状。

在新的更新颤振3.0颤振使用材料3指南。

根据它，按钮的默认边框被四舍五入。

默认按钮

  ElevatedButton(
          onPressed: () {}, child: const Text("Default Button ")),

边界半径为零的按钮

 ElevatedButton(
          style: ElevatedButton.styleFrom(
              shape: const RoundedRectangleBorder(
                  borderRadius: BorderRadius.zero)),
          onPressed: () {},
          child: const Text("Border Radius Zero ")),

带有自定义边界半径的按钮

  ElevatedButton(
          style: ElevatedButton.styleFrom(
              shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.circular(50))),
          onPressed: () {},
          child: const Text("Border Radius Custom ")),

注意:你可以对FilledButton, TextButton等使用相同的逻辑。

按钮样式请参考https://m3.material.io/components/all-buttons。

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),
            ),

使用Flutter版本2，请尝试以下操作

ElevatedButton(
    style: ButtonStyle(
        shape: MaterialStateProperty.all<OutlinedBorder>(
            RoundedRectangleBorder(
                side:
                    BorderSide(width: 1.0, color: Colors.red,
                borderRadius:
                    BorderRadius.circular(5.0),),),
        backgroundColor: MaterialStateProperty.all<Color>(Colors.red),
        foregroundColor: MaterialStateProperty.all<Color>(Colors.green),
        elevation:
            MaterialStateProperty.all<double>(8.0),
        padding: MaterialStateProperty.all<EdgeInsetsGeometry>(
            const EdgeInsets.symmetric(
                horizontal: 15.0,
                vertical: 10.0),),),
    onPressed: (){},
    child: Text('Button'),)