Minor refactor - discounts services (#470)

This PR encompasses the refactoring of the Discount Service, introducing significant enhancements to the discount calculation and retrieval processes. A notable change is the implementation of the GetDiscountAmountProvider through the use of the Mediator pattern, enabling the calculation of discounts via a plugin mechanism. Additionally, the Mediator pattern has been applied to facilitate the retrieval of a list of applied discounts, ensuring a more streamlined and efficient process. The Discount Service has been further modularized by separating it into IDiscountValidationService and IDiscountProviderLoader. This division not only clarifies the service's responsibilities but also enhances its maintainability and scalability by allowing for more focused implementations of its constituent parts.

src/Business/Grand.Business.Catalog/Queries/Handlers/GetDiscountAmountProviderHandler.cs

@@ -0,0 +1,39 @@
+using Grand.Business.Core.Interfaces.Catalog.Discounts;
+using Grand.Business.Core.Queries.Catalog;
+using Grand.Domain.Catalog;
+using Grand.Domain.Customers;
+using Grand.Domain.Discounts;
+using MediatR;
+
+namespace Grand.Business.Catalog.Queries.Handlers;
+
+public class GetDiscountAmountProviderHandler : IRequestHandler<GetDiscountAmountProvider, double>
+{
+    private readonly IDiscountProviderLoader _discountProviderLoader;
+
+    public GetDiscountAmountProviderHandler(IDiscountProviderLoader discountProviderLoader)
+    {
+        _discountProviderLoader = discountProviderLoader;
+    }
+
+    public Task<double> Handle(GetDiscountAmountProvider request, CancellationToken cancellationToken)
+    {
+        return GetDiscountAmountProvider(request.Discount, request.Customer, request.Product, request.Amount);
+    }
+
+    /// <summary>
+    /// Get amount from discount amount provider 
+    /// </summary>
+    /// <param name="discount"></param>
+    /// <param name="product"></param>
+    /// <param name="amount"></param>
+    /// <param name="customer"></param>
+    /// <returns></returns>
+    private async Task<double> GetDiscountAmountProvider(Discount discount, Customer customer, Product product, double amount)
+    {
+        var discountAmountProvider = _discountProviderLoader.LoadDiscountAmountProviderBySystemName(discount.DiscountPluginName);
+        if (discountAmountProvider == null)
+            return 0;
+        return await discountAmountProvider.DiscountAmount(discount, customer, product, amount);
+    }
+}

src/Business/Grand.Business.Catalog/Queries/Handlers/GetDiscountUsageHistoryQueryHandler.cs

@@ -0,0 +1,35 @@
+using Grand.Business.Core.Queries.Catalog;
+using Grand.Data;
+using Grand.Domain;
+using Grand.Domain.Discounts;
+using MediatR;
+
+namespace Grand.Business.Catalog.Queries.Handlers;
+
+public class GetDiscountUsageHistoryQueryHandler : IRequestHandler<GetDiscountUsageHistoryQuery, IPagedList<DiscountUsageHistory>>
+{
+    private readonly IRepository<DiscountUsageHistory> _discountUsageHistoryRepository;
+
+    public GetDiscountUsageHistoryQueryHandler(IRepository<DiscountUsageHistory> discountUsageHistoryRepository)
+    {
+        _discountUsageHistoryRepository = discountUsageHistoryRepository;
+    }
+
+    public async Task<IPagedList<DiscountUsageHistory>> Handle(GetDiscountUsageHistoryQuery request, CancellationToken cancellationToken)
+    {
+        var query = from d in _discountUsageHistoryRepository.Table
+            select d;
+
+        if (!string.IsNullOrEmpty(request.DiscountId))
+            query = query.Where(duh => duh.DiscountId == request.DiscountId);
+        if (!string.IsNullOrEmpty(request.CustomerId))
+            query = query.Where(duh => duh.CustomerId == request.CustomerId);
+        if (!string.IsNullOrEmpty(request.OrderId))
+            query = query.Where(duh => duh.OrderId == request.OrderId);
+        if (request.Canceled.HasValue)
+            query = query.Where(duh => duh.Canceled == request.Canceled.Value);
+        query = query.OrderByDescending(c => c.CreatedOnUtc);
+
+        return await PagedList<DiscountUsageHistory>.Create(query, request.PageIndex, request.PageSize);
+    }
+}

src/Business/Grand.Business.Catalog/Services/Discounts/DiscountProviderLoader.cs

@@ -0,0 +1,64 @@
+using Grand.Business.Core.Interfaces.Catalog.Discounts;
+
+namespace Grand.Business.Catalog.Services.Discounts;
+
+public class DiscountProviderLoader : IDiscountProviderLoader
+{
+    private readonly IEnumerable<IDiscountProvider> _discountProviders;
+    private readonly IEnumerable<IDiscountAmountProvider> _discountAmountProviders;
+
+    public DiscountProviderLoader(IEnumerable<IDiscountProvider> discountProviders, IEnumerable<IDiscountAmountProvider> discountAmountProviders)
+    {
+        _discountProviders = discountProviders;
+        _discountAmountProviders = discountAmountProviders;
+    }
+
+    /// <summary>
+    /// Load discount provider by rule system name
+    /// </summary>
+    /// <param name="ruleSystemName">Rule system name</param>
+    /// <returns>Found discount</returns>
+    public virtual IDiscountProvider LoadDiscountProviderByRuleSystemName(string ruleSystemName)
+    {
+        var discountPlugins = LoadAllDiscountProviders();
+        foreach (var discountPlugin in discountPlugins)
+        {
+            var rules = discountPlugin.GetRequirementRules();
+
+            if (!rules.Any(x => x.SystemName.Equals(ruleSystemName, StringComparison.OrdinalIgnoreCase)))
+                continue;
+            return discountPlugin;
+        }
+        return null;
+    }
+
+    /// <summary>
+    /// Load all discount providers
+    /// </summary>
+    /// <returns>Discount providers</returns>
+    public virtual IList<IDiscountProvider> LoadAllDiscountProviders()
+    {
+        return _discountProviders.ToList();
+    }
+
+    /// <summary>
+    /// Get all discount amount providers
+    /// </summary>
+    /// <returns></returns>
+    public virtual IList<IDiscountAmountProvider> LoadDiscountAmountProviders()
+    {
+        return _discountAmountProviders.ToList();
+    }
+
+    /// <summary>
+    /// Load discount amountProviderBySystemName
+    /// </summary>
+    /// <param name="systemName"></param>
+    /// <returns></returns>
+    public virtual IDiscountAmountProvider LoadDiscountAmountProviderBySystemName(string systemName)
+    {
+        return _discountAmountProviders.FirstOrDefault(x => x.SystemName.Equals(systemName, StringComparison.OrdinalIgnoreCase));
+    }
+
+
+}