SwiftGen Stencil template to create UIColor and UIImage extensions for all asset catalogs.

asset_extensions.stencil

/// Attention: Changes made to this file will not have any effect and will be reverted 
/// when building the project. Please adjust the Stencil template `asset_extensions.stencil` instead.
/// See https://github.com/SwiftGen/SwiftGen#bundled-templates-vs-custom-ones for more information.

import UIKit

// MARK: - Private Helper -

private final class BundleToken {}
private let bundle = Bundle(for: BundleToken.self)

{# Macro to create a static property for a given asset and class type #}
{% macro staticAsset type asset %}
static let {{asset.name|swiftIdentifier:"pretty"|lowerFirstWord|escapeReservedKeywords}} = {{type}}(named: "{{asset.value}}", in: bundle, compatibleWith: nil)!
{% endmacro %}
{# Create an UIColor extension for all color assets in asset catalogs #}
// MARK: - Colors -

extension UIColor {
    {% for catalog in catalogs %}
        {% for asset in catalog.assets %}
            {% if asset.type == "color" %}
                {% filter indent:4 %}{% call staticAsset "UIColor" asset %}{% endfilter %}
            {% endif %}
        {% endfor %}
    {% endfor %}
}

{# Create an UIImage extension for all image assets in asset catalogs #}
// MARK: - Images -

extension UIImage {
    {% for catalog in catalogs %}
        {% for asset in catalog.assets %}
            {% if asset.type == "image" %}
                {% filter indent:4 %}{% call staticAsset "UIImage" asset %}{% endfilter %}
            {% endif %}
        {% endfor %}
    {% endfor %}
}

Hey, not sure to be honest, probably best to look at the stencil documentation or look at the templates included with SwiftGen (in case they already handle this).