appendPostUpdateMessage example

// @codeCoverageIgnoreEnd
                try {
                    $alreadyCompiled += $this->themeService->compileThemeById($theme->getId(), $context);
                } catch (ThemeCompileException $e) {
                    $failedThemes[] = $theme->getName();
                    $alreadyCompiled[] = $theme->getId();
                }
            }

            if (!empty($failedThemes)) {
                $event->appendPostUpdateMessage('Theme(s): ' . implode(', ', $failedThemes) . ' could not be recompiled.');
            }
        }
    }
}