当视图模型在MVVM中更改时，如何使视图更新？

Question

I am trying to implement MVVM, but my view is not updating when the view model changes. This is my view model:

public class ViewModelDealDetails : INotifyPropertyChanged
{
    private Deal selectedDeal;

    public Deal SelectedDeal
    {
        get { return selectedDeal; }
        set
        {
            selectedDeal = value;
            OnPropertyChanged();
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

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

In my XAML for the view I have this:

<Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
       <StackPanel>
           <TextBlock Text="{Binding Path=SelectedDeal.Title, Mode=TwoWay}"></TextBlock>
       </StackPanel>
</Grid>

The deal class:

public class Deal
{
    private string title;
    private float price;

    public Deal()
    {
        this.title = "Example";    
    }

    public Deal(string title, float price)
    {
        this.title = title;
        this.price = price;
    }

    public string Title
    {
        get { return title; }
        set { title = value; }
    }

    public float Price
    {
        get { return price; }
        set { price = value; }
    }
}

When the application starts the value is correct, but when SelectedDeal changes, the view doesn't. What am I missing?

Answer 1

The path of your binding is nested.To make it work, your Deal class should implement INotifyPropertyChanged too. Otherwise, it will not be triggered unless SelectedDeal is changed. I would suggest you make your view models all inherited from BindableBase . It would make your life much easier.

   public class ViewModelDealDetails: BindableBase
    {
        private Deal selectedDeal;

        public Deal SelectedDeal
        {
            get { return selectedDeal; }
            set { SetProperty(ref selectedDeal, value); }
        }

    }

    public class Deal: BindableBase
    {
        private string title;

        public string Title
        {
            get { return title; }
            set { SetProperty(ref title, value); }
        }
    }

The above code should work.

BTW: If you have no access to the code of Deal class, then to trigger the binding you will have to recreate an instance of SelectedDeal each time when the value of Title is changed.