使用MVVM进行UserControl绑定

Question

我目前正在进行不同的练习，以便了解UserControls的绑定。

我有以下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>

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));
        }
    }

我实际上是将标签内容绑定到字符串xValue。每次我从外部传感器接收数据时，都会触发sensor_PropertyChanged()方法，我实际上想要将xValue定义为我从传感器接收的double X。问题是propertyChangedEventHandler总是等于null。为什么会出现这个问题？因为我确信View和VievModel之间的绑定确实有效。