c# - Windows Phone 8 MVVM Linq 表在 NotifyPropertyChanging 上创建新实例

Question

我有一个 Linq DataContext 作为应用程序的数据库。我已经设置了 MVVM 模式并且能够将新记录插入到数据库中。但是，当我加载这些记录并尝试更新它们时，会在后台创建一个新的记录实例，并随着属性更改进行更新。因此，当他们的 UI 调用保存命令时，最初加载的记录实例没有更改，也没有保存。

据我所知这是事件的顺序

• 加载实例 1
• 开始更新属性
• 调用 NotifyPropertyChanging
• 新实例 2 已加载
• 新实例 2 已更新
• 从实例 1 的 UI 调用保存更改
• 未进行任何更改，因为实例 1 尚未更新

下面是我的代码:

/* 这是实体 */

[Table]
public class User : IDisposable, INotifyPropertyChanged, INotifyPropertyChanging
{
    private MyDataContext context;

    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(String propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (null != handler)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public event PropertyChangingEventHandler PropertyChanging;
    private void NotifyPropertyChanging(String propertyName)
    {
        PropertyChangingEventHandler handler = PropertyChanging;
        if (null != handler)
        {
            handler(this, new PropertyChangingEventArgs(propertyName));
        }
    }

    public void Dispose()
    {
        context.Dispose();
    }

    private Guid _id;
    [Column(IsPrimaryKey = true, IsDbGenerated = false, DbType = "UNIQUEIDENTIFIER NOT NULL", CanBeNull = false, AutoSync = AutoSync.OnInsert)]
    public Guid Id
    {
        get { return _id; }
        set
        {
            if (_id != value)
            {
                NotifyPropertyChanging("Id");
                _id = value;
                NotifyPropertyChanged("Id");
            }
        }
    }

    private string _name;
    [Column(CanBeNull = false)]
    public string Name
    {
        get { return _name; }
        set
        {
            if (_name != value)
            {
                NotifyPropertyChanging("Name"); // This line creates the new entity
                _name = value;
                NotifyPropertyChanged("Name");
            }
        }
    }

    public User()
    {
        this.context = MyDataContext.GetContext();
    }

    public override void SaveChanges()
    {
        if (_id == Guid.Empty)
        {
            this.Id = Guid.NewGuid();
            context.Users.InsertOnSubmit(this);
            context.SubmitChanges();
        }
        else
        {
            context.SubmitChanges();
        }
    }

    public static User NewInstance()
    {
        return new User
        {
            Name = String.Empty
        };
    }
}

/* 这是数据上下文 */

public class MyDataContext : DataContext
{
    // Specify the connection string as a static, used in main page and app.xaml.
    public static string ConnectionString = "Data Source=isostore:/MyApp.sdf;Password=pwd";

    public MyDataContext(string connectionString) : base(connectionString) { }

    public static MyDataContext GetContext()
    {
        var context = new MyDataContext(ConnectionString);
        return context;
    }

    public Table<User> Users;
}

/* 这是 View 模型 */

public sealed class UserViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    protected void NotifyPropertyChanged(String propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    private User _user;

    public UserViewModel(Guid id)
    {
        using (MyDataContext context = new MyDataContext(MyDataContext.ConnectionString))
        {
            _User = context.User.First(u => u.Id == id);
        }
    }

    public UserViewModel(User user)
    {
        _user = user;
    }

    public UserViewModel()
    {
        _user = User.NewInstance();
    }

    public string Name
    {
        get { return _user.Name; }
        set
        {
            _user.Name = value;
            NotifyPropertyChanged("Name");
        }
    }

    private ICommand _saveCommand;
    public ICommand SaveCommand
    {
        get
        {
            return _saveCommand ?? (_saveCommand = new GenericCommand(() =>
            {
                _user.SaveChanges();
            }, true));
        }
    }
}

Answer 1

在您的 MyDataContext 中，我认为您需要下面的内容，这是一个基本的单例概念，这样您就可以处理同一个对象，从而保存同一个对象并进行更改。

private static DataContext context = null;

public static MyDataContext GetContext()
{
    if(context == null)
        context = new MyDataContext(ConnectionString);
    return context;
}

edit- 请注意，这会对您的应用程序产生重大影响。可能需要在创建新的时重新设计，以及是否/何时应该专门将其设置为 null。