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，并能够插入新的记录到数据库中。然而，当我加载这些记录并尝试更新它们时，记录的新实例正在后台创建，并使用属性更改进行更新。因此，当UI调用save命令时，原始加载的记录实例没有更改，也不会被保存。

据我所知这是事件的顺序

• 加载实例1
• 开始更新属性
• NotifyPropertyChanging被称为
• 新实例2已加载
• New Instance2已更新
• 为实例1调用UI保存更改
• 没有更改，因为实例1没有更新

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;
}

/*这是视图模型*/

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

在你的MyDataContext中，我认为你想要下面的东西，一个基本的单例概念，这样你就可以在同一个对象上工作，从而保存相同的对象。

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

edit-注意，这可能会对您的应用程序产生重大影响。可能需要重新设计一个新的创建时，如果/当你应该特别设置为null