UserControl binding using MVVM

Question

I'm currently working on different exercises in order to understand bindings for UserControls.

I have following UserControl:

<UserControl x:Class="Overvaagning.View.AccGyrValues" 
         xmlns:vm="clr-namespace:Overvaagning.Model"
         mc:Ignorable="d" Width="404.478" Height="467.164">

<UserControl.DataContext>
    <vm:Accelerometer/>
</UserControl.DataContext>


<Grid Margin="10,0,0,0">    
    <Label x:Name="labelValX" Content="{Binding Path=xValue}" HorizontalAlignment="Left" Margin="282,51,0,0" VerticalAlignment="Top" FontSize="30" RenderTransformOrigin="0.519,0.24" Width="88" Height="44"/>
</Grid>

And a ViewModel:

public class Accelerometer : Values, INotifyPropertyChanged, Overvaagning.Model.Values.ISensorOperations
{
    public SensorAccelerometer BTAccSensor { get; internal set; }

    // Property changed event handleren initialiseres 
    public event PropertyChangedEventHandler PropertyChanged;

    // Her initiliserer jeg Universalt Entydigt Identification (UUID) for at få adgang til Accelerometer tjenesten i kittet
    static Guid SensorDeviceUuid = Guid.Parse("F000AA10-0451-4000-B000-000000000000");
    public Accelerometer()
    {

        // Definer sensoren til en accelerometer sensor. 
        BTAccSensor = new SensorAccelerometer();
        // Sensoren service UUID defineres til Accelerometer UUID.
        BTAccSensor.SensorDeviceUuid = SensorDeviceUuid;

        // Denne UUID er en karakteristisk UUID i accelerometer tjenesten der har opgaven til at aktivere accelerometeret
        BTAccSensor.DataUuid = Guid.Parse("F000AA11-0451-4000-B000-000000000000");
        // Denne UUID bruges til at konfigurere acceleromteret
        BTAccSensor.ConfigurationUuid = Guid.Parse("F000AA12-0451-4000-B000-000000000000");
        // Denne UUID bruges til at modificere acceleromter følsomheden, alt afhængig af, hvor mange notifikationer
        // accelerometer skal sende pr. sekund 
        BTAccSensor.SpeedUuid = Guid.Parse("F000AA13-0451-4000-B000-000000000000");
        // PropertyChanged defineres til den metode der skal kaldes, så snart der modtages data fra accelerometet
        BTAccSensor.PropertyChanged += sensor_PropertyChanged;
    }

    public sbyte XRaw { get; internal set; }
    public sbyte YRaw { get; internal set; }
    public sbyte ZRaw { get; internal set; }
    public byte ValueToSend;
    public double X { get; internal set; }
    public double Y { get; internal set; }
    public double Z { get; internal set; }
    public double G { get; internal set; }

    public static Task<IList<Values.SensorInitData>> CreateAllAsync(Readings readings, FrameworkElement el)
    {
        return SensorCreateAll<Accelerometer>.CreateAllAsync(SensorDeviceUuid, readings, el);
    }

    public Task<int> InitializeAsync(GattDeviceService device, Readings readings, FrameworkElement el, byte ValueToSend)
    {
        return BTAccSensor.InitializeAsync(device, readings, el, ValueToSend);
    }

    private string _xval;

    public string xValue
    {
        get { return _xval; }
        set
        {
            if (value != _xval)
            {
                _xval = value;
                OnPropertyChanged("xValue");
            }
        }
    }
    public void sensor_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        sbyte value;
        switch (e.PropertyName)
        {
            case "B0Raw":
                value = (sbyte)BTAccSensor.B0Raw;
                if (value != XRaw)
                {
                    XRaw = value;
                    X = value / 16.0;
                    X = Math.Round(X, 2);
                    xValue = X.ToString();                
                }
                break;
            case "B1Raw":
                value = (sbyte)BTAccSensor.B1Raw;
                if (value != YRaw)
                {
                    YRaw = value;
                    Y = ((double)(sbyte)value) / 16.0;
                    Y = Math.Round(Y, 2);

                    G = Math.Sqrt((X * X) + (Y * Y) + (Z * Z));
                    G = Math.Round(G, 2);
                }
                break;
            case "B2Raw":
                value = (sbyte)BTAccSensor.B2Raw;
                if (value != ZRaw)
                {
                    ZRaw = value;
                    Z = ((double)(sbyte)value) / 16.0;
                    Z = Math.Round(Z, 2);

                }
                break;
        }
    }

    // Metoden for propertychanged eventhandler
    protected virtual void OnPropertyChanged(string propName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propName));
        }
    }

I'm actually binding the label content to string xValue. Everytime I receive a data from an external sensor, the sensor_PropertyChanged() method is triggered, and I actually want to define the xValue as the double X I'm receiving from the sensor. The problem is that the propertyChangedEventHandler becomes always equals to null. Why does this problem occur ?, since I'm sure of the binding between the View and VievModel actually works.

possible duplicate of [PropertyChanged event always null](http://stackoverflow.com/questions/1512627/propertychanged-event-always-null) — Sinatr, Nov 20 '14 at 13:32
Please explain your problem better. You said that *the sensor_PropertyChanged() method is triggered*, but that *the propertyChangedEventHandler becomes always equals to null*... that can't be true... the `sensor_PropertyChanged` method can only be called if the `PropertyChangedEventHandler event` is *not* `null`. Follow the example from the [`INotifyPropertyChanged` Interface](http://msdn.microsoft.com/en-us/library/system.componentmodel.inotifypropertychanged(v=vs.110).aspx) page on MSDN for help with this interface. — Sheridan, Nov 20 '14 at 14:10
in your switch your are setting xValue only in one case. Try to add Xvalue to all cases. May be you just do not get B0Raw — Sasha, Nov 24 '14 at 10:04

UserControl binding using MVVM

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