Conditional payment method in SilverShop

CheckoutStep_CustomPaymentMethod.php

<?php

use SilverStripe\Omnipay\GatewayInfo;

/**
 */
class CheckoutStep_CustomPaymentMethod extends CheckoutStep_PaymentMethod
{
    public static function get_supported_gateways(Order $order = null)
    {
        $gateways = GatewayInfo::getSupportedGateways();
        $member = Member::currentUser();

        // Remove the invoice option if the member can't pay by invoice.
        // How you check that and what gateways you remove is entirely up to you.
        if (!$member || !$member->AllowInvoice()) {
            unset($gateways['Manual']);
            return $gateways;
        }

        return $gateways;
    }

    private static $allowed_actions = array(
        'paymentmethod',
        'PaymentMethodForm'
    );

    protected function checkoutconfig()
    {
        $config = new CheckoutComponentConfig(ShoppingCart::curr(), false);
        // The important bit here is to use the custom checkout component
        $config->addComponent(new CustomPaymentCheckoutComponent());

        return $config;
    }

    public function paymentmethod()
    {
        return array(
            'OrderForm' => $this->PaymentMethodForm()
        );
    }
}

CustomPaymentCheckoutComponent.php

<?php

class CustomPaymentCheckoutComponent extends PaymentCheckoutComponent
{

    public function getFormFields(Order $order) {
        $fields = new FieldList();
        $gateways = CheckoutStep_CustomPaymentMethod::get_supported_gateways();

        if(count($gateways) > 1){
            $fields->push(
                new OptionsetField(
                    'PaymentMethod',
                    _t("CheckoutField.PaymentType", "Payment Type"),
                    $gateways,
                    array_keys($gateways)
                )
            );
        }
        if(count($gateways) == 1){
            $fields->push(
                HiddenField::create('PaymentMethod')->setValue(key($gateways))
            );
        }

        return $fields;
    }

    public function getRequiredFields(Order $order) {
        return array('PaymentMethod');
    }

    public function validateData(Order $order, array $data) {
        $result = new ValidationResult();
        if(!isset($data['PaymentMethod'])){
            $result->error("Payment method not provided", "PaymentMethod");
            throw new ValidationException($result);
        }
        $methods = CheckoutStep_CustomPaymentMethod::get_supported_gateways();
        if(!isset($methods[$data['PaymentMethod']])){
            $result->error("Gateway not supported", "PaymentMethod");
            throw new ValidationException($result);
        }
    }

    public function getData(Order $order) {
        $gateways = CheckoutStep_CustomPaymentMethod::get_supported_gateways();
        $selected = Checkout::get($order)->getSelectedPaymentMethod();
        if(isset($gateways[$selected])){
            return array(
                'PaymentMethod' => Checkout::get($order)->getSelectedPaymentMethod()
            );
        } else {
            return array();
        }
    }

    public function setData(Order $order, array $data) {
        if(isset($data['PaymentMethod'])){
            Checkout::get($order)->setPaymentMethod($data['PaymentMethod']);
        }
    }
}