How can I get the number of products "on-sale" for a specific category?

I'm woocommerce 2.8, How can I count the products existing in the shop "on sale" for a specific category (ex: computers) ?

I found a way to count all products :

$count_posts = wp_count_posts( 'product' );
return $count_posts->publish;

But how can I get the number of products "on-sale" for a specific category ?

Thanks

$args = array(
    'post_type'      => 'product',
    'posts_per_page' => -1,
    'tax_query'     => array(
        array(
            'taxonomy'  => 'product_cat',
            'field'     => 'id', 
            'terms'     => here goes your category id, you can also go for slug
        )
    ),
    'meta_query'     => array(
        'relation' => 'OR',
        array(
            'key'           => '_sale_price',
            'value'         => 0,
            'compare'       => '>',
            'type'          => 'numeric'
        ),
        array(
            'key'           => '_min_variation_sale_price',
            'value'         => 0,
            'compare'       => '>',
            'type'          => 'numeric'
        )
    )
);
$query = new WP_Query($args);
$onSaleCount = $query->found_posts;

The above looks for simple and variable products on sale belonging to a category mentioned under tax_query.

I'm not sure if there's a better method, but this should work.

LE - Changed args to exclude products out of stock as per comments.

$args = array(
    'post_type' => 'product',
    'posts_per_page' => -1,
    'tax_query' => array(
        array(
            'taxonomy' => 'product_cat',
            'field' => 'id',
            'terms' => here goes your category id, you can also go for slug
        )
    ),
    'meta_query' => array(
        'relation' => 'AND',
        array(
            'relation' => 'OR',
            array(
                'key' => '_sale_price',
                'value' => 0,
                'compare' => '>',
                'type' => 'numeric'
            ),
            array(
                'key' => '_min_variation_sale_price',
                'value' => 0,
                'compare' => '>',
                'type' => 'numeric'
            )
        ),
        array(
            'key' => '_stock_status',
            'value' => 'instock'
        ),
    )
);