插件分层实战：Service层到底该放业务逻辑还是数据组装？

最近重构老插件时发现Service层越来越臃肿，仔细分析发现很多同事把两类代码混在一起：

典型问题代码：

class UserService {
    public function getUserWithOrders($uid) {
        // 数据获取
        $user = UserModel::find($uid);
        $orders = OrderModel::where('uid', $uid)->get();
        
        // 业务逻辑
        if($user->vip_level > 3) {
            $orders = $this->applyVipDiscount($orders);
        }
        
        // 数据组装
        return [
            'user' => $user->toArray(),
            'orders' => array_map(function($order){
                return [
                    'id' => $order->id,
                    'price' => $order->price * 0.8 // 折扣计算
                ];
            }, $orders)
        ];
    }
}

推荐拆分方案：

class UserModel {
    public static function getWithOrders($uid) {
        return self::with('orders')->find($uid);
    }
}

class DiscountService {
    public function applyVipPolicy($orders, $vipLevel) {
        return $vipLevel > 3 
            ? array_map(fn($o) => $o->setAttribute('discount', 0.8), $orders)
            : $orders;
    }
}

class UserController {
    public function profile() {
        $data = UserModel::getWithOrders($uid);
        $data['orders'] = (new DiscountService())
            ->applyVipPolicy($data['orders'], $data['user']->vip_level);
        
        return response()->json([
            'user' => UserTransformer::format($data['user']),
            'orders' => OrderTransformer::collection($data['orders'])
        ]);
    }
}

关键原则：Service层应该像调味料，Model层提供原料，Controller负责摆盘。当需要新增折扣类型时，只需修改DiscountService，保持Model稳定。