Question

I’m writing a WPF calculator app. In this application I have TextBox that get and set the input according to the OnWindowKeyDown event (input insert/validated and returned to the TextBox as the user type).

for example if the user type:

3-> validate == true--> print to TextBox '3'
y-> validate == false -> ignore
3-> validate == true --> print to TeextBox "33"

I want to get the input char by char (I have a State Pattern class that react to each char added), but I need to return the result as a string.

How can I transfer my data from and to the textBox, with the MVVM design?

Answer 1

It should be enough to bind a Text property of TextBox to ViewModel‘s property.

For example:

//ViewModel 
public class MyViewModel : INotifyPropertyChanged
{
     private string textBoxText = string.Empty;
     public string TextBoxText 
     {
        get {return textBoxText;}
        set {
           textBoxText = value;
           OnPropertyChanged("TextBoxText "..);
        }
     }
}

Bind MyViewModel to a DataContext of your Form, TextBox or whatever… in short make it available to your TextBoxin some way.

Define a Converter whom methods will be invoked every time Text property of TextBox set.

public class TextContenyConverter: IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {

        // validate input and return appropriate value
    }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}

After binding in XAML considering that DataContext is an object of type MyViewModel

where TextContenyConverterObject is an object of type TextContenyConverter defined like a static resource.

This just an example.

This is another good explantion on ValueConverters.