Qfyilyi

My MainWindow has 4 radio buttons. The user has to choose one, then press on a button that opens up a new window. Depending on the radio button selected, I want to change the background that appears in the new window. This is my code:

public partial class Practice : Window

{

    public Practice()

    {

        InitializeComponent();



        if (((MainWindow)Application.Current.MainWindow).BinomialRadio.IsChecked == true)

        {



        }

        else if (((MainWindow)Application.Current.MainWindow).HypergeometricRadio.IsChecked == true)

        {



        }

        else if (((MainWindow)Application.Current.MainWindow).PoissonRadio.IsChecked == true)

        {

            Background = new ImageBrush(new BitmapImage(new Uri(BaseUriHelper.GetBaseUri(this), "images/poisson_practice_screen.jpg")));

        }

        else

        {



        }

    }

}

The new window already has a default background that I set in the properties of the XAML code. This code above runs and executes well, but the picture does not change. I found a quick fix, which is basically to remove the background (so that the new window always has a blank background), and then set it every time it opens. Is there any better way to do this?

Thank you to everyone for their help

The reason that the background of the Practice window does not update is because you set its background in the constructor of the window, which only runs when the window is created. In order for it to update, you have to add event handlers on each of the checkboxes for the Checked event and update the background in the handler.

However, the easiest and most recommended way to do this is using data binding. Data binding is a construct in WPF and other frameworks where you declaratively indicate which properties are linked together, so that you don't have to update them manually. No writing tedious event event handlers or keeping track of complicated changes.

Practice.xaml.cs:

public partial class Practice : Window

{

    // INotifyPropertyChanged implementation is important!

    // Without it, WPF has no way of knowing that you changed your property...

    public class PracticeModel : INotifyPropertyChanged

    {

        private BitmapImage _background;



        public BitmapImage Background 

        { 

            get => _background;

            set { _background = value; PropertyChanged?.Invoke(nameof(Background)); }

        }



        public event PropertyChangedEventHandler PropertyChanged;

    }



    public Practice()

    {

        InitializeComponent();



        // DataContext specifies which object the bindings are bound to

        this.DataContext = new PracticeModel();

    }

}

Practice.xaml:

<Window x:Class="MyApp.Practice" Background="{Binding Background}">

    <!-- your content here; all other attributes of Window omitted for brevity -->

</Window>

MainWindow.xaml.cs:

public partial class MainWindow : Window

{

    public Practice.PracticeModel PracticeModel { get; set; } = new Practice.PracticeModel();



    // ...



    public OnButtonClicked(RoutedEventArgs e) 

    {

        var window = new Practice();

        // DataContext specifies which object the bindings are bound to

        window.DataContext = this.PracticeModel;

        window.Show();

    }



    public OnPoissonRadioChecked(RoutedEventArgs e) 

    {

        PracticeModel.Background = new BitmapImage(new Uri(BaseUriHelper.GetBaseUri(this), "images/poisson_practice_screen.jpg"));

    }



    // likewise for other radio buttons ...

}

MainWindow.xaml:

<Window x:Class="MyApp.MainWindow">

    <RadioButton Group="TheButtons" x:Name="BinomialRadio" IsChecked="True" Checked="OnBinomialRadioChecked" />

    <RadioButton Group="TheButtons" x:Name="HypergeometricRadio" Checked="OnHypergeometricRadioChecked" />

    <RadioButton Group="TheButtons" x:Name="PoissonRadio" Checked="OnPoissonRadioChecked" />

    <RadioButton Group="TheButtons" x:Name="TheOtherRadio" Checked="OnTheOtherRadioChecked" />

</Window>

When you change the property on PracticeModel, the PropertyChanged event is fired. This tells WPF that the property has changed, and it automatically updates all of the relevant bindings. This will quickly become very useful when you want to have more than one dynamically updating property. Additionally, data binding can automatically convert from a string or a Uri to an ImageSource, so you might not even need to create the BitmapImage yourself (and if you don't have to, then don't.)

As you might have noticed, there are still event handlers in this code. That's because I didn't want to introduce too much complexity at the same time, and data-binding radio buttons properly can be kind of confusing for someone who is not accustomed to this. I hope this helps!