Styjun

I'm using x:Bind and INotifyPropertyChanged to update UI in UWP application. But it behaves like OneTime binding even though I set it to OneWay.

Bindings.Update() works, but I want to know why INotifyPropertyChanged fails.

XAML

<TextBlock Text="x:Bind staffVM.Name, Mode=OneWay"/>

Code-behind:

private StaffViewModel staffVM;
private void ListView_SelectionChanged(object sender, SelectionChangedEventArgs e)

 // I want to change staffVM according to ListView's selection.
 staffVM = staffListView.SelectedItem as StaffViewModel;
 staffVM.Update(); // If change this to Bindings.Update(), It works.

ViewModel:

public class StaffViewModel: INotifyPropertyChanged

 private Character character;
 public string Name => character.name == string.Empty ? null : character.name;
 public void Update()
 
 RaisePropertyChanged(string.Empty);
 

 public event PropertyChangedEventHandler PropertyChanged;

 public void RaisePropertyChanged([CallerMemberName]string propName = null)
 
 PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
 

Firstly, you need to specify the name of the variable that you want to update:

public void Update()

 RaisePropertyChanged(nameof(Name));

Documentation and sample: https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.data.inotifypropertychanged.propertychanged

Secondly, by default x:Bind is OneTime
To fix it, add Mode="OneWay"

Mode Specifies the binding mode, as one of these strings: "OneTime", "OneWay", or "TwoWay". The default is "OneTime". Note that this differs from the default for Binding, which is "OneWay" in most cases.

Please read documentation
https://docs.microsoft.com/en-us/windows/uwp/xaml-platform/x-bind-markup-extension

The problem here is not on the level of the StaffViewModel class, but on the level of the page. When you do:

staffVM = staffListView.SelectedItem as StaffViewModel;

The UI has no notification about the fact that the staffVM field has changed. So the binding is still pointing to the old instance of StaffViewModel. Hence when you do staffVM.Update(), it does notify about changes, but the UI is not listening to that instance - it is still listening to notifications on the first selected item. Bindings.Update() fixes this because it completely re-evaluates all bindings so it will "get" the new value of staffVM field.

Solution would be to implement INotifyPropertyChanged on the Page and encapsulate the staffVM in a property which raises PropertyChanged event.

Ideally I would however suggest creating a "root" view model, which you will set only once and will not change and which will contain the selected item as its property. This way you don't have to implement INotifyPropertyChanged in the Page and its code-behind will be simpler. As a result you will have something like the following in the code-behind:

public RootViewModel VM get; = new RootViewModel();

And in XAML:

<TextBlock Text="x:Bind VM.SelectedStaff.Name, Mode=OneWay"/>