CODEWITHSUNDEEP
c#wpfinotifypropertychanged
user2011909
user2011909

Reputation: 139

How to start an event changed from a INotifyPropertyChanged

I have two windows. In the first window I would like to start the second window with some preference values (e. g. "MaxWords"). The second window holds a class with an interface for INotifyPropertyChanged. This works as expected...

public partial class PreviewPreferences : Window
{
    public PreviewPreferences()
    {
        InitializeComponent();
        this.DataContext = new ViewModel();
    }
}

public class ViewModel
{
    public Preferences preferences { get; private set; }

    public ViewModel()
    {
        preferences = new Preferences();
    }
}

public class Preferences : INotifyPropertyChanged
{
    private int _maxWords = 10;

    /// <summary>
    /// Default constructor
    /// </summary>
    public Preferences() { }

    /// <summary>
    /// Max words
    /// </summary>
    public int MaxWords
    {
        get { return this._maxWords; }
        set { this._maxWords = value; this.OnPropertyChanged("MaxWords"); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged([CallerMemberName] string name = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
    }
}

The second window should updating the first window and adds some user controls to the grid. The question is not how to add the controls... it is more how to raise the event from the preference class that the value MaxWords is changed?

private void button_preview_preferences_Click(object sender, RoutedEventArgs e)
    {
        PreviewPreferences previewPreferences = new PreviewPreferences();
        previewPreferences.Show();
        Preferences preferences = new Preferences();
        preferences.PropertyChanged += HandleChangedPreferences;
    }

    private void HandleChangedPreferences(object sender, PropertyChangedEventArgs e)
    {
        // this will never be raised
        for (int i = 0; i < MaxWords; i++)
        {
            ...
        }
    }

Upvotes: 0

Views: 59

Answers (1)

ASh
ASh

Reputation: 35680

you have two instance of Preferences in button_preview_preferences_Click method. The first and important one (the one that changes) is hidden in PreviewPreferences DataContext:

private void button_preview_preferences_Click(object sender, RoutedEventArgs e)
{
    var previewPreferences = new PreviewPreferences();

    var preferences = (previewPreferences.DataContext as ViewModel).preferences;
    preferences.PropertyChanged += HandleChangedPreferences;

    previewPreferences.Show();
}

I suggest to invert the logic - create preferences outside ViewModel, and create ViewModel outside PreviewPreferences view:

public partial class PreviewPreferences : Window
{
    public PreviewPreferences()
    {
        InitializeComponent();
    }
}

public class ViewModel
{
    public Preferences preferences { get; private set; }

    public ViewModel(Preferences p)
    {
        preferences = p;
    }
}

private void button_preview_preferences_Click(object sender, RoutedEventArgs e)
{
    var preferences = new Preferences();
    preferences.PropertyChanged += HandleChangedPreferences;

    var previewPreferences = new PreviewPreferences();
    previewPreferences.DataContext = new ViewModel(preferences);
    previewPreferences.Show();
}

Upvotes: 1

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