Migrate from Basic to MVVM and MEF Composable Patterns for a Silverlight Application - Part 3

using System;
using System.ComponentModel;
using System.ComponentModel.Composition;
using System.Linq;
using System.ServiceModel.DomainServices.Client;
using ProductApp.Common;
using ProductRiaLib.Web.Models;
using ProductRiaLib.Web.Services;

namespace ProductApp.Models
{
    [Export(ModuleID.ProductListModel, typeof(IProductListModel))]
    [PartCreationPolicy(CreationPolicy.Shared)]
    public class ProductListModel : IProductListModel
    {
        private ProductDomainContext _ctx;
        
        // Define event handlers for data operations
        public event EventHandler<QueryResultsArgs<Category>> GetCategoryLookupComplete;
        public event EventHandler<QueryResultsArgs<Product>> GetCategorizedProductsComplete;
        public event EventHandler<SubmitOperationArgs> SaveChangesComplete;

        public ProductListModel()
        {
            if (_ctx == null)
            {
                _ctx = new ProductDomainContext();
                _ctx.PropertyChanged += _ctx_PropertyChanged;
            }
        }             

        private void _ctx_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            switch (e.PropertyName)
            {
                case "HasChanges":
                    HasChanges = _ctx.HasChanges;
                    break;
                case "IsLoading":
                    IsBusy = _ctx.IsLoading;
                    break;
                case "IsSubmitting":
                    IsBusy = _ctx.IsSubmitting;
                    break;
            }
        }

        public void GetCategoryLookup()
        {
            DataAsyncHandlers.LoadQueryAsync(_ctx, _ctx.GetCategoriesQuery(), this, GetCategoryLookupComplete);
        }

        public void GetCategorizedProducts(int categoryId)
        {
            DataAsyncHandlers.LoadQueryAsync(_ctx, _ctx.GetCategorizedProductQuery(categoryId), this, GetCategorizedProductsComplete);
        }

        public void SaveChanges(string operationType)
        {
            CurrentOperation = operationType;
            DataAsyncHandlers.SaveChangesAsync(_ctx, this, SaveChangesComplete);
        }

        public void AddNewProduct(Product addedProduct)
        {
            _ctx.Products.Add(addedProduct);
            SaveChanges(OperationTypes.AddNew);
        }

        public void DeleteProduct(Product deletingProduct)
        {
            _ctx.Products.Remove(deletingProduct);
            SaveChanges(OperationTypes.Delete);
        }

        public string CurrentOperation { get; set; }

        private Boolean _hasChanges;
        //
        public Boolean HasChanges
        {
            get
            {
                return _hasChanges;
            }

            private set
            {
                if (_hasChanges != value)
                {
                    _hasChanges = value;
                    OnPropertyChanged("HasChanges");
                }
            }
        }

        private Boolean _isBusy;
        //
        public Boolean IsBusy
        {
            get
            {
                return _isBusy;
            }

            private set
            {
                if (_isBusy != value)
                {
                    _isBusy = value;
                    OnPropertyChanged("IsBusy");
                }
            }
        }

        
        // Implement INotifyPropertyChanged
        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }  
    }
}