Flutterのバージョンアップ時やXcode Cloud上で「Error: The non-abstract class ‘GradientColor’ is missing implementations for these members:」

2025年7月6日

今回はFlutterのバージョンアップ時やXcode Cloud上で下記のようなエラーが起きたときの対処法についてご紹介していこうと思います。

../../.pub-cache/hosted/pub.dev/katana_theme-2.6.8/lib/src/gradient_color.dart:19:7: Error: The non-abstract class 'GradientColor' is missing implementations for these members:
 - Color.toARGB32
Try to either
 - provide an implementation,
 - inherit an implementation from a superclass or mixin,
 - mark the class as abstract, or
 - provide a 'noSuchMethod' implementation.

class GradientColor implements Color {
      ^^^^^^^^^^^^^
org-dartlang-sdk:///flutter/lib/ui/painting.dart:255:7: Context: 'Color.toARGB32' is defined here.
  int toARGB32() {
      ^^^^^^^^
Target kernel_snapshot_program failed: Exception


FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:compileFlutterBuildRelease'.
> Process 'command '/Users/ユーザー名/flutter_development/flutter/bin/flutter'' finished with non-zero exit value 1

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
> Get more help at https://help.gradle.org.

BUILD FAILED in 6s

../../.pub-cache/hosted/pub.dev/katana_theme-2.6.8/lib/src/gradient_color.dart:19:7: Error: The non-abstract class 'GradientColor' is missing implementations for these members:

- Color.toARGB32

Try to either

- provide an implementation,

- inherit an implementation from a superclass or mixin,

- mark the class as abstract, or

- provide a 'noSuchMethod' implementation.

class GradientColor implements Color {

^^^^^^^^^^^^^

org-dartlang-sdk:///flutter/lib/ui/painting.dart:255:7: Context: 'Color.toARGB32' is defined here.

int toARGB32() {

^^^^^^^^

Target kernel_snapshot_program failed: Exception