在ViewModel中实体模型更改时更新UI

Question

I am trying to come up with a good way of implementing the MVVM pattern using Entity-Framework where my entities are my models. My DataContext is my viewmodel. This is a small reproduction of the problem.

View

<TextBox Text="{Binding MyText}" />

ViewModel :

I have the requirement of needing to navigate record by record from my DB. When a button is clicked in the View a command is sent to the Viewmodel that executes nextRecord() . EF does its magic and _myObject is the next row/record from the database

public class myViewModel: INotifyPropertyChanged
{
    private MyEntityObject _myObject;

    public string MyText
    {
        get { return _myObject.MyText; }
        set
        {
            if (_myObject.MyText != value)
            {
                _myObject.MyText = value;
                OnPropertyChanged("MyText");
            }
        }
    }

    private void _nextRecord()
    {
      _myObject = myEntitiesContext.NextRecord() //pseudocode
    }
}

Autogenerated Entity Model

public partial class MyEntityObject
{
     public string MyText { get; set; }
}

Since the View has no knowledge of _myObject changing, it doesn't update when _myObject changes. A few approaches I have thought of.

1. I haven't tested wrapping my entities in a INotifyPropertyChanged wrapper class but am wary to do this as I have a lot of entity objects.

2. I could call OnPropertyChanged("...") for all properties, but some of my entities have a lot of properties to them, which would be ugly. Possible to use reflection to make it cleaner, but I may have properties that aren't databound.

3. I might be able to defer this to the UI, somehow refreshing the bindings when I click "Next Record", but this breaks MVVM and looks dirty

How can I get the UI to recognize changes from _myObject ?

Answer 1

As I've mentioned in the comments, calling OnPropertyChanged("") or OnPropertyChanged(null) invalidates all properties and is equivalent to calling OnPropertyChanged for each and every property. This behavior is also documented here :

The PropertyChanged event can indicate all properties on the object have changed by using either null or String.Empty as the property name in the PropertyChangedEventArgs.

This means that you can simply add a call to OnPropertyChanged("") when you update your object to force WPF to reevaluate all bindings to your view model:

private void _nextRecord()
{
    _myObject = myEntitiesContext.NextRecord();
    OnPropertyChanged("");
}

That being said, I'd still go with @Anand's solution (+1). There's an ongoing debate on whether it's OK or not for the viewmodel to expose the model as a property, and I tend to go with exposing it until you need to introduce some view model specific logic. Most of the time you won't have to and it's not worth the trouble of wrapping model properties.

Answer 2

The problem with your code is that when _myObject changes the MyText property changed event is not fired. A work around would be to create a new property to hold you entity and make this property as your Grid s DataContext in your view as shown below. Now when this line is executed MyObject = myEntitiesObject.NextRecord() your view will be notified about the change.

public class myViewModel : INotifyPropertyChanged
{

    private MyEntityObject _myObject;

    public MyEntityObject MyObject
    {
        get { return _myObject; }
        set {
            if (_myObject != value)
            {
                _myObject = value;
                OnPropertyChanged("MyObject");
            }
        }
    }


    private void _nextRecord()
    {
      MyObject = myEntitiesObject.NextRecord() //pseudocode
    }
}

View:

    <Grid DataContext="{Binding MyObject}"> 
        <TextBlock Text="{Binding MyText}"/>
    </Grid>

Answer 3

An extremely simple but not very elegant solution that I believe would meet needs: upon switching records, set the DataContext to null, then back to the ViewModel.

However, there are arguably more elegant alternatives that require more work to meet all requirements. See Anand's answer for an improvement upon this.

Answer 4

View中的标记应该具有设置值的mode和UpdateSourceTrigger属性。