Changing a Label when a Bool variable turns True

Question

i'm not really sure how to explain this... i'm going to put the code in psuedo code for easy reading

pretty much I want a label to change it's text when a bool variable of a class changes... i'm not sure what I need to use since i'm using WPF and the class can't just change the label i don't think?

do i need some sort of event? or a WPF event? thanks for any help

public MainWindow()
{
   SomeClass temp = new SomeClass();

   void ButtonClick(sender, EventArgs e)
   {  
      if (temp.Authenticate)
        label.Content = "AUTHENTICATED";
   }
}

public SomeClass
{
  bool _authenticated;

  public bool Authenticate()
  {
    //send UDP msg
    //wait for UDP msg for authentication
    //return true or false accordingly
  }
}

I would do this with data triggers in Xaml, avoiding any code-behind at all... — Gayot Fow, May 13 '14 at 21:51

score 8 · Accepted Answer · edited May 23 '17 at 12:23

Another WPF approach other than BradledDotNet answer is to use triggers. This will be purely XAML based without converter code.

XAML:

<Label>
  <Label.Style>
    <Style TargetType="Label">
      <Style.Triggers>
        <DataTrigger Binding="{Binding Path=IsAuthenticated}" Value="True">
          <Setter Property="Content" Value="Authenticated" />
        </DataTrigger>
      </Style.Triggers>
    </Style>
  </Label.Style>
</Label>

Same rule applies as mentioned by BradledDotNet, ViewModel should be attached and "IsAuthenticated" property should raise the PropertyChanged event.

-- Add some boilerplate code as suggested by Gayot Fow --

View Code Behind:

For simplicity I will just set the DataContext of the view in code behind. For a better design you can use ViewModelLocator as explained here.

public partial class SampleWindow : Window
{
  SampleModel _model = new SampleModel();

  public SampleWindow() {
    InitializeComponent();
    DataContext = _model; // attach the model/viewmodel to DataContext for binding in XAML
  }
}

Example Model (it should really be ViewModel):

The main point here is the attached model/viewmodel has to implement INotifyPropertyChanged.

public class SampleModel : INotifyPropertyChanged
{
  bool _isAuthenticated = false;

  public bool IsAuthenticated {
    get { return _isAuthenticated; }
    set {
      if (_isAuthenticated != value) {
        _isAuthenticated = value;
        OnPropertyChanged("IsAuthenticated"); // raising this event is key to have binding working properly
      }
    }
  }

  public event PropertyChangedEventHandler PropertyChanged;

  void OnPropertyChanged(string propName) {
    if (PropertyChanged != null) {
      PropertyChanged(this, new PropertyChangedEventArgs(propName));
    }
  }
}

While other answers will work, this approach is a lot more scalable and less code-intensive, hence the "preferred" approach. A single line of Xaml can simultaneously change the background colour to green on the same binding source for example. @techhero, I would suggest amending your answer to include the requisite boiler plate for the INPC because the OP is new to WPF, as well as future consumers of this question. — Gayot Fow, May 13 '14 at 22:45
something at work came up but promise to review everyone's answer here when my time frees up later today or tomorrow...as my main goal is to learn.. so i really thank everyone for their time — user1189352, May 13 '14 at 23:10

score 4 · Answer 2 · answered May 13 '14 at 21:43

Since we are using WPF, I would use a binding and converter to accomplish this:

<Page.Resources>
   <local:BoolToAuthenticationStringConverter x:Key="BoolToAuthenticationStringConverter"/>
</Page>

<Label Content="{Binding Path=IsAuthenticated, Converter={StaticResource BoolToAuthenticationStringConverter}"/>

Then a converter that looks like:

public class BoolToAuthenticationStringConverter : IValueConverter
{
   public object Convert (...)
   {
      if ((bool)value)
         return "Authenticated!";
      else
         return "Not Authenticated!";
   }

   public object ConvertBack (...)
   {
      //We don't care about this one for this converter
      throw new NotImplementedException();
   }
}

In our XAML, we declare the converter in the resources section, then use it as the "Converter" option of the binding for "Content". In the code, we cast value to a bool (IsAuthenticated is assumed to be a bool in your ViewModel), and return an appropriate string. Make sure your ViewModel implements INotifyPropertyChanged and that IsAuthenticated raises the PropertyChanged event for this to work!

Note: Since you won't be changing the Label via the UI, you don't need to worry about ConvertBack. You could set the mode to OneWay to make sure that it never gets called though.

Granted, this is very specific to this scenario. We could make a generic one:

<Label Content="{Binding Path=IsAuthenticated, Converter={StaticResource BoolDecisionToStringConverter}, ConverterParameter='Authenticated;Not Authenticated'}"/>

Then a converter that looks like:

public class BoolDecisionToStringConverter : IValueConverter
{
   public object Convert (...)
   {
      string[] args = String.Split((String)parameter, ';');
      if ((bool)value)
         return args[0];
      else
         return args[1];
   }

   public object ConvertBack (...)
   {
      //We don't care about this one for this converter
      throw new NotImplementedException();
   }
}

something came up but promise i'll still look at yours and Jeroen's answer! — user1189352, May 13 '14 at 21:49

score 2 · Answer 3 · answered May 13 '14 at 21:21

2

Yes, you need an event.

Consider a sample like this:

public SomeClass {
    public event EventHandler<EventArgs> AuthenticateEvent;  
    boolean isAuthenticated;

    public bool Authenticate()
    {
        // Do things
        isAuthenticated = true;
        AuthenticateEvent(this, new EventArgs());
    }
}

public MainWindow()
{
   SomeClass temp = new SomeClass();

   public MainWindow(){
      temp.AuthenticateEvent+= OnAuthentication;
      temp.Authenticate();
   }

   private void OnAuthentication(object sender, EventArgs e){
       Dispatcher.Invoke(() => {
           label.Content = "AUTHENTICATED";
       }); 
   }
}

Now whenever temp fires the Authenticate event, it will change your label.

answered May 13 '14 at 21:21

Jeroen Vannevel

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kk will read right now and get back to you, thank you very much – user1189352 May 13 '14 at 21:21
even better would be binding with a converter, but this will technically work. – BradleyDotNET May 13 '14 at 21:23
`Yes, you need an event.1` I wouldn't say that so sure. I could implement it with async/await without any event. – L.B May 13 '14 at 21:23
i'd be interested in seeing it for knowledge purposes @L.B. if you were interested in showing. – user1189352 May 13 '14 at 21:24
@BradleyDotNET: if there is a more common way in WPF to do this, feel free to add it as an answer. I have limited experience with WPF so I'm approaching this from a general C# perspective. – Jeroen Vannevel May 13 '14 at 21:24
@JeroenVannevel, Ok, thanks for the response! I posted a "WPF" way of doing it. – BradleyDotNET May 13 '14 at 21:43

Changing a Label when a Bool variable turns True

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