Use Gradient with Paint object in Flutter Canvas

I am able to draw a semi circle using the following example: Flutter how to draw semicircle (half circle)

However, the Paint object only seems to accept a Color (using Paint.color). I would like to add a RadialGradient or LinearGradient as the Color. Is that possible?

Yes! This is totally possible using Paint.shader.
You can either create a gradient shader directly using dart:ui or convert a Flutter gradient to a shader using Gradient.createShader.

dart:ui gradient

import 'dart:ui' as ui;

// In your paint method
final paint = Paint()
  ..shader = ui.Gradient.linear(
    startOffset,
    endOffset,
    [
      color1,
      color2,
    ],
  );

A real world example can be seen here.

Flutter painting gradient

import 'package:flutter/painting.dart';

// In your paint method
final paint = Paint()
  ..shader = RadialGradient(
    colors: [
      color1,
      color2,
    ],
  ).createShader(Rect.fromCircle(
    center: offset,
    radius: radius,
  ));

An example of this can be found here.

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These two are effectively the same. The Flutter painting version simply converts it to a dart:ui gradient (shader) when you call createShader. The reason it exists is that the painting version is better suited for prebuilt widgets like Container.