Adding a Gradient Background to Appbar on Flutter

I don't believe you can pass a gradient to an AppBar as it expects a Color rather than a gradient.

You can, however, create your own widget that mimics an AppBar except by using a gradient. Take a look at this example that I've pieced together from the Planets-Flutter tutorial along with the code below it.

import "package:flutter/material.dart";

class Page extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Column(children : <Widget>[GradientAppBar("Custom Gradient App Bar"), Container()],);
  }
}


class GradientAppBar extends StatelessWidget {

  final String title;
  final double barHeight = 50.0;

  GradientAppBar(this.title);

  @override
  Widget build(BuildContext context) {
    final double statusbarHeight = MediaQuery
        .of(context)
        .padding
        .top;

    return new Container(
      padding: EdgeInsets.only(top: statusbarHeight),
      height: statusbarHeight + barHeight,
      child: Center(
        child: Text(
          title,
          style: TextStyle(fontSize: 20.0, color: Colors.white, fontWeight: FontWeight.bold),
        ),
      ),
      decoration: BoxDecoration(
        gradient: LinearGradient(
            colors: [Colors.red, Colors.blue],
            begin: const FractionalOffset(0.0, 0.0),
            end: const FractionalOffset(0.5, 0.0),
            stops: [0.0, 1.0],
            tileMode: TileMode.clamp
        ),
      ),
    );
  }
}

Hope this helps. Let me know if you have any questions.

AppBar does not have that feature by default. But you can make an AppBar like widget which will have a gradient background as follow:

  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new PreferredSize(
        child: new Container(
          padding: new EdgeInsets.only(
            top: MediaQuery.of(context).padding.top
          ),
          child: new Padding(
            padding: const EdgeInsets.only(
              left: 30.0,
              top: 20.0,
              bottom: 20.0
            ),
            child: new Text(
              'Arnold Parge',
              style: new TextStyle(
                fontSize: 20.0,
                fontWeight: FontWeight.w500,
                color: Colors.white
              ),
            ),
          ),
          decoration: new BoxDecoration(
            gradient: new LinearGradient(
              colors: [
                Colors.red,
                Colors.yellow
              ]
            ),
            boxShadow: [
              new BoxShadow(
                color: Colors.grey[500],
                blurRadius: 20.0,
                spreadRadius: 1.0,
              )
            ]
          ),
        ),
        preferredSize: new Size(
          MediaQuery.of(context).size.width,
          150.0
        ),
      ),
      body: new Center(
        child: new Text('Hello'),
      ),
    );
  }

Here boxShadow will give elevated feel.