Question

I’m currently using the following code to be notified when a DependencyProperty‘s Value has changed:

DependencyPropertyDescriptor propDescriptor = DependencyPropertyDescriptor.FromProperty(property, control.GetType());
propDescriptor.AddValueChanged(control, controlChangedHandler);

This works great and is quite simple, but what I really need now is to be notified when a DependencyProperty‘s Value is about to change. I thought there would be a DependencyPropertyDescriptor.AddValueChanging() method, but it doesn’t seem to exist. Any ideas how I can create this functionality?

I need to be able to cancel the change, fire off some asynchronous backend logic, and only have the control’s property really change if the backend logic succeeds.

Answer 1

I solved the problem at hand by implementing wrapping my IODevice in an INotifyPropertyChanged implementation and binding it to the DependencyProperty.

The magic is in the fact that IODeviceWrapper.Value‘s setter doesn’t actually set the value, but rather does the IO. It turns out that when the setter is called by the DependencyProperty it’s bound to, the change hasn’t yet been committed to the DependencyProperty‘s value. Hence, IODeviceWrapper.Value‘s setter gets called in by the DependencyProperty‘s sudo-, non-existent ValueChanging event.

At this time, if the DependencyProperty reads from the Value‘s getter it will get the old value until the IO is complete. When the IO is complete IODeviceWrapper.Value‘s PropertyChanged event gets fired, and the DependencyProperty then reads the new value.

My flawed design is now working flawlessly. Here’s the code in case anyone else is interested. Ignore the naysayers.

public class IODeviceWrapper : INotifyPropertyChanged
{
    public IODeviceWrapper(IODevice ioDevice)
    {
        _ioDevice = ioDevice;

        _ioDevice.ValueChanged += ValueChanged;
    }

    private IODevice _ioDevice;

    public event PropertyChangedEventHandler PropertyChanged;

    private void ValueChanged()
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs("Value"));
        }
    }

    public int Value
    {
        get { return _ioDevice.Value; }
        set
        {
            //Do ansynchronous IO
            Task task = new Task(() => _ioDevice.DoIO(value));
            task.Start();
        }
    }
}