Prevents checkout if the WooCommerce cart only contains items from a specific category

wc-prevent-checkout-for-cart-with-specific-category.php

<?php // only copy this line if needed

/**
 * Renders a notice and prevents checkout if the cart
 * only contains products in a specific category
 */
function sv_wc_prevent_checkout_for_category() {
	
	// set the slug of the category for which we disallow checkout
	$category = 'clothing';
	
	// get the product category
	$product_cat = get_term_by( 'slug', $category, 'product_cat' );
	
	// sanity check to prevent fatals if the term doesn't exist
	if ( is_wp_error( $product_cat ) ) {
		return;
	}
	
	$category_name = '<a href="' . get_term_link( $category, 'product_cat' ) . '">' . $product_cat->name . '</a>';
	
	// check if this category is the only thing in the cart
	if ( sv_wc_is_category_alone_in_cart( $category ) ) {
		
		// render a notice to explain why checkout is blocked
		wc_add_notice( sprintf( 'Hi there! Looks like your cart only contains products from the %1$s category &ndash; you must purchase a product from another category to check out.', $category_name ), 'error' );
	}
}
add_action( 'woocommerce_check_cart_items', 'sv_wc_prevent_checkout_for_category' );


/**
 * Checks if a cart contains exclusively products in a given category
 * 
 * @param string $category the slug of the product category
 * @return bool - true if the cart only contains the given category
 */
function sv_wc_is_category_alone_in_cart( $category ) {
        
	// check each cart item for our category
	foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
		
		// if a product is not in our category, bail out since we know the category is not alone
		if ( ! has_term( $category, 'product_cat', $cart_item['data']->id ) ) {
			return false;
		}
	}
        
	// if we're here, all items in the cart are in our category
	return true;
}