C＃SQLite INotifyPropertyChanged实现

Question

I'm using SQLite in my application to store some Message objects. I display a list of Message items in "View 1". When I change a property in the Edit View "View 2", I want the property to also change in the list.

Classes

Message.cs

class Message : INotifyPropertyChanged
{
    private uint _id;
    public uint Id 
    { 
        get
        {
            return _id;
        }
        set
        {
            // Trigger INotifyPropertyChanged
            Set("Id", ref _id, value);
        }
    }

    private string _content;
    public string Content 
    {
        get
        {
            return _content;
        }
        set
        {
            // Trigger INotifyPropertyChanged
            Set("Content", ref _content, value);
        }
    }

    ...
}

MessageViewModel.cs

class MessageViewModel : INotifyPropertyChanged
{
    private Message message;

    ...

    private string _content;
    public string Content 
    {
        get
        {
            return message.Content;
        }
        set
        {
            // Set value
            message.Content = value;

            // Trigger INotifyPropertyChanged
            RaisePropertyChanged();
        }
    }

    ...
}

View 1

View1.xaml

The datacontext is View1ViewModel

View1ViewModel.cs

private List<MessageViewModel> _messages;
public List<MessageViewModel> Messages
{
    get
    {
        return _messages;
    }
    set
    {
        // Trigger INotifyPropertyChanged
        Set("Messages", ref _messages, value);
    }
}

...

private async void loadMessages()
{
    // Get the messages from SQLite database
    var messages = await newMessages();
    Messages = new MessageViewModelCollection(messages);
}

View 2

View2ViewModel.cs

private MessageViewModel _message;
public MessageViewModel Message
{
    get
    {
        return _message;
    }
    set
    {
        // Trigger INotifyPropertyChanged
        Set("Message", ref _message, value);
    }
}

...

private async void loadMessage()
{
    // Get the message from SQLite database by Id
    var message = await newMessage(messageId);
    Message = new MessageViewModel(message);
}

The functions newMessages and newMessage(uint messageId) return new Message objects from the database.

I normally use the INotifyPropertyChanged implementation, but this doesn't work. I query the database 2 times, once for the list (View 1), and once for the edit page (View 2). The SQLite returns two different copies of the Message object, therefore the INotifyPropertyChanged meganism will not work (Only in the current page, not the pages in the backstack).

I could fix the problem by re-using the Message item from the list, but I cannot always do this in all views.

Is there a way to make the INotifyPropertyChanged work in this scenario? Or do I need a different approach to update the values?

Answer 1

You need a different approach. IMHO, the best thing would be to separate the data model from the view model. So have a MessageVm that binds to the WPF view, and put the logic into it to update itself appropriately (along with firing any necessary INotifyPropertyChanged 's) from the Message objects that get passed into it.

There are some automatic mapping tools like Automapper which might, to some degree, alleviate the pain that comes along with this approach.

But you really should separate the view model from the data model, the decoupling of these layers is one of the principle tenets of WPF programming.