textfield_gradient_border.dart

class Sample extends StatefulWidget {
  @override
  _Sample createState() => _Sample();
}

class _Sample extends State<Sample> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        fit: StackFit.expand,
        children: [
          Image.asset(
            "images/background.png",
            fit: BoxFit.cover,
          ),
          Positioned(
            left: 0.0,
            right: 0.0,
            top: MediaQuery.of(context).size.height / 2,
            child: Padding(
              padding: const EdgeInsets.all(20.0),
              child: TextFieldWithGradientBorder(),
            ),
          ),
        ],
      ),
    );
  }
}

class TextFieldWithGradientBorder extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return CustomPaint(
      painter: TextFieldBorderPainter(
        LinearGradient(
          colors: [
            Colors.yellow,
            Colors.green,
          ],
          begin: Alignment.centerLeft,
          end: Alignment.centerRight,
        ),
      ),
      child: Container(
        padding: EdgeInsets.all(5),
        child: TextField(
          style: TextStyle(
            color: Colors.white,
          ),
          decoration: InputDecoration(
            border: InputBorder.none,
            contentPadding: EdgeInsets.all(10.0),
          ),
        ),
      ),
    );
  }
}

class TextFieldBorderPainter extends CustomPainter {
  final LinearGradient linearGradient;

  TextFieldBorderPainter(this.linearGradient);

  @override
  void paint(Canvas canvas, Size size) {
    final rect = Rect.fromPoints(Offset.zero, Offset(size.width, size.height));
    final paint = Paint()
      ..strokeWidth = 2.0
      ..style = PaintingStyle.stroke
      ..shader = linearGradient.createShader(rect);
    canvas.drawRect(rect, paint);
  }

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