PHP电子商务系统循环依赖问题

我刚刚开始使用依赖注入，我立即遇到了一个问题:我有两个相互依赖的类。

类是Basket和Shipping。在我的Basket类中，我有以下相关方法:

public function totalShipping()
{
    return $this->_shipping->rate();
}
public function grandTotal()
{
    return $this->totalProductsPrice() + $this->totalShipping();
}
public function totalWeight()
{
    $weight = 0;
    $products = $this->listProducts();
    foreach ($products as $product) {
        $weight += $product['product_weight'];
    }
    return ($weight == '') ? 0 : $weight;
}

$this->_shipping是Shipping类

在Shipping类中，我有以下相关方法:

public function rate()
{   
    if (isset($_SESSION['shipping']['method_id'])) {
        $methodId = $_SESSION['shipping']['method_id'];
        return $this->_rates[$methodId]['Shipping Price'];
    }
    // Method not set
    return NULL;
}
// Available Methods depend on country and the total weight of products added to the customer's basket. E.g. USA and over 10kg
public function listAvailableMethods()
{   
    $rates = array();
    if (isset($_SESSION['customer']['shipping_address']['country_code'])) {
        foreach ($this->_rates as $method_id => $rate) {
            if (($_SESSION['customer']['shipping_address']['country_code'] == $rate['Country']) && ($this->_basket->totalWeight() > $rate['Weight From']) && ($this->_basket->totalWeight() < $rate['Weight To'])) {
                $rates[$method_id] = $rate;
            }
        }
    }
    return $rates;
}

$this->_basket是Basket类的一个实例。

我完全不知道如何解决这个循环依赖。提前感谢您的帮助。

在Shipping类中我也有这个方法:

public function setMethod($method_id)
{
    // A check to make sure that the method_id is one of the provided methods
    if ( !array_key_exists($method_id, $this->listAvailableMethods()) ) return false;
    $_SESSION['shipping'] = array(
        'method_id' => $method_id
    );
}