Notifying change of a calculated databound property in WPF

Question

As an exercise I decided to create a bicycle gear calculator in WPF. I created two private fields with setters that trigger OnPropertyChanged(), but I have one databound property, ratio, which behaves as "readonly", because it is dynamically calculated. When I run the program, the textboxes show, the initial values are correctly displayed, the "working" word in the property changed handler is displayed, but the ratio TextBlock doesn't update.

I suspect this is due to the way the property is "getted", I wonder if it would be absolutely necessary to add a private field for every, I wonder if there should be any DependencyProperty in this... But actually I have reached my knowledge limit on this and cannot get this trivial program to work.

---

This is my model:

class SingleGearsetModel : INotifyPropertyChanged
{
    public SingleGearsetModel()
    {
        crank = 44;
        cog = 16;
    }

    private int _crank;
    private int _cog;

    public int crank { 
        get{return _crank;}
        set{
            _crank = value;
            OnPropertyChanged("crank");
        }
    }

    public int cog {
        get{return _cog;}
        set{
            _cog = value;
            OnPropertyChanged("cog");
        } 
    }

    public double ratio
    {
        get {
            return (double)crank / (double)cog;
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string arg)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(arg));
            Console.Writeline("working");
        }
    }

} // end class

This is my XAML:

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="CalculadorFixaWPF.MainWindow"
    x:Name="Window"
    Title="MainWindow"
    Width="640" Height="480">

    <DockPanel x:Name="LayoutRoot">
            <TextBox Text="{Binding crank, Mode=TwoWay}"/>
            <TextBox Text="{Binding cog, Mode=TwoWay}"/>
            <TextBlock Text="{Binding ratio, StringFormat={}{0:0.00}}"/>
    </DockPanel>
</Window>

And this is in my code-behind (MainWindow.xaml.cs):

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

Thanks for reading!

Answer 1

Since Ratio is a calculated field, you want to add a notification every time the value might change. This will happen if crank or cog are changed.

So, after you notify on them being changed, also notify thatratio has changed:

public int crank 
{ 
    get{return _crank;}
    set{
        _crank = value;
        OnPropertyChanged("crank");

        // *** Notify that ratio has also been changed ***
        OnPropertyChanged("ratio");
    }
}

Same goes for cog.

Edit: Based on your comments, here is how you can register to the PropertyChanged event from inside your class and raise a PropertyChanged for ratio:

// Constructor
public SingleGearsetModel()
{
    ....
   PropertyChanged += SingleGearsetModel_PropertyChanged;
}

// PropertyChanged event handler
void SingleGearsetModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
        if (e.PropertyName == "cog" || e.PropertyName == "crank")
        {
            OnPropertyChanged("ratio");
        }
}

Using this approach, you don't need to add the OnPropertyChanged("ratio") inside the setters of the properties - whenever they are raised, you get the event(in SingleGearsetModel_PropertyChanged) and raise the OnPropertyChanged("ratio")