Creating a custom Icon Set Provider 

An icon set provider is responsible for providing icons and writing the necessary stylesheet. The class must implement \DanielHaring\IconHub\Imaging\IconSet\IconSetProviderInterface, but typically extends \DanielHaring\IconHub\Imaging\IconSet\AbstractIconSetProvider instead.

If the abstract class is used, you must specify the icon provider, which will later be responsible for rendering the icons. You can use existing core classes or your own implementations for this purpose.

EXT:my_ext/Classes/Imaging/IconSet/CustomIconSetProvider.php 
declare(strict_types=1);
namespace Vendor\MyExt\Imaging\IconSet;

use DanielHaring\IconHub\Imaging\IconSet\AbstractIconSetProvider;
use TYPO3\CMS\Core\Imaging\IconProvider\SvgIconProvider;

class CustomIconSetProvider extends AbstractIconSetProvider
{
    protected function getProviderName(): string
    {
        return SvgIconProvider::class;
    }
}
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Providing icons 

The actual icons are provided via the generator method provideIcons(). This method is expected to yield instances of \DanielHaring\IconHub\Imaging\IconDefinition.

If the recommended abstract class is used, icons can be created using the helper method createIcon().

EXT:my_ext/Classes/Imaging/IconSet/CustomIconSetProvider.php 
declare(strict_types=1);
namespace Vendor\MyExt\Imaging\IconSet;

use DanielHaring\IconHub\Imaging\IconSet\AbstractIconSetProvider;

class CustomIconSetProvider extends AbstractIconSetProvider
{
    public function provideIcons(): \Generator
    {
        yield $this->createIcon('tx-myext-dummy', 'Dummy');
    }
}
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Write the stylesheet 

The provideIcons() method is also primarily used to generate the stylesheet.

The required styles can be written using $this->styles->write(). This method can be used as often as needed to accumulate styles.

Although not strictly necessary, it is highly recommended to check in advance whether stylesheet writing is enabled to avoid unnecessary computing. $this->styles->isEnabled() can be used for this purpose.

EXT:my_ext/Classes/Imaging/IconSet/CustomIconSetProvider.php 
declare(strict_types=1);
namespace Vendor\MyExt\Imaging\IconSet;

use DanielHaring\IconHub\Imaging\IconSet\AbstractIconSetProvider;

class CustomIconSetProvider extends AbstractIconSetProvider
{
    public function provideIcons(): \Generator
    {
        if ($this->styles->isEnabled()) {
            $this->styles->write('/* Base styles to write. */');
            $this->styles->write('/* Icon specific styles to write. */');
        }
        yield $this->createIcon('tx-myext-dummy', 'Dummy');
    }
}
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Full example 

This is what a simplified, typical icon set provider looks like:

EXT:my_ext/Classes/Imaging/IconSet/CustomIconSetProvider.php 
declare(strict_types=1);
namespace Vendor\MyExt\Imaging\IconSet;

use DanielHaring\IconHub\Imaging\IconSet\AbstractIconSetProvider;
use TYPO3\CMS\Core\Imaging\IconProvider\SvgIconProvider;

class CustomIconSetProvider extends AbstractIconSetProvider
{
    public function provideIcons(): \Generator
    {
        if ($this->styles->isEnabled()) {
            $this->styles->write('/* Base styles for the icon set. */');
        }

        foreach (['dummy', 'test'] as $iconName) {
            if ($this->styles->isEnabled()) {
                $this->styles->write('/* Styles for icon "' . $iconName . '". */');
            }
            yield $this->createIcon('tx-myext-' . $iconName, $iconName, [
                'source' => 'EXT:my_ext/Resources/Public/Icons/' . $iconName . '.svg'
            ]);
        }
    }

    protected function getProviderName(): string
    {
        return SvgIconProvider::class;
    }
}
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