Detect PropertyChanged in custom TextBox

Question

I'm have implemented a custom TextBox:

public class MyTextBox : TextBox
{
    // ...
}

that I'm using from XAML:

<MyTextBox Text="{Binding MyProperty}" />

and it's bound to a property in my ViewModel.

public class MyDataContext : INotifyPropertyChanged
{
    public string MyProperty
    {
        get { return _myPropertyBackingField; }
        set
        {
            _myPropertyBackingField = value;
            PropertyChanged(this, new PropertyChangedEventArgs("MyProperty"));
        }
    }

    // ...
}

Question: How can I, in MyTextBox, detect that MyProperty is changed?

MyProperty = "NewValue";

Preferably, I would like to distinguish a programmatical change from when the change was triggered by the user editing the value. That is, I don't think overriding OnPropertyChanged works for me.

Answer 1

OP here.

I realised after a while that it's the binding that keeps track of updating the TextBox from the source (DataContext). So a possible path to take would be to call GetBindingExpression(TextProperty) and work something out from that.

However, I solved it by overriding TextBoxBase.OnTextChanged:

protected override void OnTextChanged(TextChangedEventArgs e)
{
    base.OnTextChanged(e);
    if (!IsFocused)
    {
        // Do stuff here
    }
}

Since the control is not focused, the change must have been done programatically. This is not perfect since a programatical change might come when the TextBox has focus, but it is good enough for me.

Answer 2

You can register to the PropertyChanged event of the TextBox's DataContext.

var dataContext = DataContext as MyDataContext;
dataContext.PropertyChanged += dataContext_PropertyChanged;

// check for the propertyname and react
void dataContext_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName == "MyProperty")
    {
        // Do things
    }
}

So if your viewmodel raises PropertyChanged you textbox also gets notified. But I think that's bad practice. What do you want to achieve?