Outlined transparent button with gradient border in flutter

You can achieve this by doing just a simple trick

You have to define two Containers. First outer container with a gradient background and the second inner container with white background. and as a child of the inner container, you can place anything e.g. TextField, Text, another button, etc.

final kInnerDecoration = BoxDecoration(
  color: Colors.white,
  border: Border.all(color: Colors.white),
  borderRadius: BorderRadius.circular(32),
);

final kGradientBoxDecoration = BoxDecoration(
  gradient: LinearGradient(colors: [Colors.black, Colors.redAccent]),
  border: Border.all(
    color: kHintColor,
  ),
  borderRadius: BorderRadius.circular(32),
);

Now this is your View

  Container(
    child: Padding(
             padding: const EdgeInsets.all(2.0),
             child: Container(
                      child:Text("Button Title with your style"),
                      decoration: kInnerDecoration,
                    ),
           ),
    height: 66.0,
    decoration: kGradientBoxDecoration,
  ),

Done

I spent about two hours on it :)

how to use:

import 'package:flutter/material.dart';

void main() => runApp(App());

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: Center(
            child: Column(
              mainAxisSize: MainAxisSize.min,
              children: <Widget>[
                UnicornOutlineButton(
                  strokeWidth: 2,
                  radius: 24,
                  gradient: LinearGradient(colors: [Colors.black, Colors.redAccent]),
                  child: Text('OMG', style: TextStyle(fontSize: 16)),
                  onPressed: () {},
                ),
                SizedBox(width: 0, height: 24),
                UnicornOutlineButton(
                  strokeWidth: 4,
                  radius: 16,
                  gradient: LinearGradient(
                    colors: [Colors.blue, Colors.yellow],
                    begin: Alignment.topCenter,
                    end: Alignment.bottomCenter,
                  ),
                  child: Text('Wow', style: TextStyle(fontSize: 16)),
                  onPressed: () {},
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

and the class itself:

class UnicornOutlineButton extends StatelessWidget {
  final _GradientPainter _painter;
  final Widget _child;
  final VoidCallback _callback;
  final double _radius;

  UnicornOutlineButton({
    @required double strokeWidth,
    @required double radius,
    @required Gradient gradient,
    @required Widget child,
    @required VoidCallback onPressed,
  })  : this._painter = _GradientPainter(strokeWidth: strokeWidth, radius: radius, gradient: gradient),
        this._child = child,
        this._callback = onPressed,
        this._radius = radius;

  @override
  Widget build(BuildContext context) {
    return CustomPaint(
      painter: _painter,
      child: GestureDetector(
        behavior: HitTestBehavior.translucent,
        onTap: _callback,
        child: InkWell(
          borderRadius: BorderRadius.circular(_radius),
          onTap: _callback,
          child: Container(
            constraints: BoxConstraints(minWidth: 88, minHeight: 48),
            child: Row(
              mainAxisSize: MainAxisSize.min,
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                _child,
              ],
            ),
          ),
        ),
      ),
    );
  }
}

class _GradientPainter extends CustomPainter {
  final Paint _paint = Paint();
  final double radius;
  final double strokeWidth;
  final Gradient gradient;

  _GradientPainter({@required double strokeWidth, @required double radius, @required Gradient gradient})
      : this.strokeWidth = strokeWidth,
        this.radius = radius,
        this.gradient = gradient;

  @override
  void paint(Canvas canvas, Size size) {
    // create outer rectangle equals size
    Rect outerRect = Offset.zero & size;
    var outerRRect = RRect.fromRectAndRadius(outerRect, Radius.circular(radius));

    // create inner rectangle smaller by strokeWidth
    Rect innerRect = Rect.fromLTWH(strokeWidth, strokeWidth, size.width - strokeWidth * 2, size.height - strokeWidth * 2);
    var innerRRect = RRect.fromRectAndRadius(innerRect, Radius.circular(radius - strokeWidth));

    // apply gradient shader
    _paint.shader = gradient.createShader(outerRect);

    // create difference between outer and inner paths and draw it
    Path path1 = Path()..addRRect(outerRRect);
    Path path2 = Path()..addRRect(innerRRect);
    var path = Path.combine(PathOperation.difference, path1, path2);
    canvas.drawPath(path, _paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => oldDelegate != this;
}