Add Calculator Contract,

Add some checking for calculators,

src/Calculation/DefaultCalculator.php

@@ -3,10 +3,11 @@
 namespace Gloudemans\Shoppingcart\Calculation;
 
 use Gloudemans\Shoppingcart\CartItem;
+use Gloudemans\Shoppingcart\Contracts\Calculator;
 
-class DefaultCalculator
+class DefaultCalculator implements Calculator
 {
-    public static function getAttribute(string $attribute, CartItem $cartItem)
+    static function getAttribute(string $attribute, CartItem $cartItem)
     {
         $decimals = config('cart.format.decimals', 2);
 

src/Calculation/GrossPrice.php

@@ -3,10 +3,11 @@
 namespace Gloudemans\Shoppingcart\Calculation;
 
 use Gloudemans\Shoppingcart\CartItem;
+use Gloudemans\Shoppingcart\Contracts\Calculator;
 
-class GrossPrice
+class GrossPrice implements Calculator
 {
-    public static function getAttribute(string $attribute, CartItem $cartItem)
+    static function getAttribute(string $attribute, CartItem $cartItem)
     {
         $decimals = config('cart.format.decimals', 2);
 

src/CartItem.php

@@ -4,9 +4,12 @@ namespace Gloudemans\Shoppingcart;
 
 use Gloudemans\Shoppingcart\Calculation\DefaultCalculator;
 use Gloudemans\Shoppingcart\Contracts\Buyable;
+use Gloudemans\Shoppingcart\Contracts\Calculator;
+use Gloudemans\Shoppingcart\Exceptions\InvalidCalculatorException;
 use Illuminate\Contracts\Support\Arrayable;
 use Illuminate\Contracts\Support\Jsonable;
 use Illuminate\Support\Arr;
+use ReflectionClass;
 
 /**
  * @property-read mixed discount
@@ -398,7 +401,14 @@ class CartItem implements Arrayable, Jsonable
                 return round($this->weight * $this->qty, $decimals);
         }
 
-        return call_user_func(config('cart.calculator', DefaultCalculator::class).'::getAttribute', $attribute, $this);
+        $class = new ReflectionClass(config('cart.calculator', DefaultCalculator::class));
+        if (!$class->implementsInterface(Calculator::class))
+        {
+            throw new InvalidCalculatorException('The configured Calculator seems to be invalid. Calculators have to implement the Calculator Contract.');
+        }
+
+        return call_user_func($class->getName().'::getAttribute', $attribute, $this);
+        
     }
 
     /**

src/Contracts/Calculator.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace Gloudemans\Shoppingcart\Contracts;
+
+use Gloudemans\Shoppingcart\CartItem;
+
+interface Calculator
+{
+    static function getAttribute(string $attribute, CartItem $cartItem);
+}

src/Exceptions/InvalidCalculatorException.php

@@ -0,0 +1,9 @@
+<?php
+
+namespace Gloudemans\Shoppingcart\Exceptions;
+
+use RuntimeException;
+
+class InvalidCalculatorException extends RuntimeException
+{
+}