Silverlight dependency property snafu, what am I doing wrong?

Question

I'm enhancing an open source control to add some functionality that I need, and I'm getting hopelessly tangled up in the following problem:

The control is a rich textbox that supports HTML but not via a property; you have to do something like this:

var sHtml = "..."
ctrl.LoadHtml(sHtml)

and

var sHtml = ctrl.SaveHtml()

So far so good. But I want to set the HTML via data binding, so I made a dependency property called Html:

    public static readonly DependencyProperty HtmlProperty =
        DependencyProperty.Register(
            "Html",
            typeof(string),
            typeof(RichTextEditor),
            new PropertyMetadata(string.Empty, new PropertyChangedCallback(HtmlChangedCallback))
            );

    public string Html
    {
        get {return (string)GetValue(HtmlProperty);}

        set {SetValue(HtmlProperty, value);}
    }

    private static void HtmlChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        //get the control
        var rte = (RichTextEditor)d;

        //got here, so load the html
        rte.TextBox.LoadHtml((string)e.NewValue);
    }

This all works fine. The problem I'm having is that I can't figure out how to notify the property system when the contents of the control have changed. The control has a ContentChanged event, so I tried this:

    private void rtb_ContentChanged(object sender, RichTextBoxEventArgs e)
    {
        //tell the html prop that it changed
        SetValue(HtmlProperty, rtb.SaveHtml());
    }

But this then triggers the HtmlChangedCallback and the re-entrance caused problems. So then I tried using a re-entrance flag, but that got messy because the sequence of events is more complex than I would have expected, and around this point I figured I must be missing something, so I'm asking here. Please help! Thanks in advance.

BTW, the control doesn't support INotifyPropertyChanged, and implementing it is out of scope, because the control is big and I don't want to do that much work.

Answer 1

Why do you need to notify property system? Are you trying to update the source of the data binding? The control does not have to implement INotifyPropertyChanges; this interface has to be implemented on the class which provides the data for data binding.

What if you ignore Save/LoadHtml for now and just store HTML as a string? Does it work without problems?

Here is the example which works fine for me

class MyControl
{
    public int Value
    {
        get
        {
            return (int)GetValue(ValueProperty);
        }

        set
        {
            SetValue(ValueProperty, value);
            Count.Text = value.ToString();
        }
    }

    public static readonly DependencyProperty ValueProperty =
        DependencyProperty.Register("Value",
            typeof(int),
            typeof(Vote),
            new PropertyMetadata(0, OnValueChanged));

    private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ((MyControl)d).Value = (int)e.NewValue;
    }

Count is a text block control.

Answer 2

This is a classic little problem that is usually solved with a simple boolean flag.

private bool suppressHtmlChanged = false;

private static void HtmlChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
{

    var rte = (RichTextEditor)d;

    if (!rte.suppressHtmlChanged)
        rte.TextBox.LoadHtml((string)e.NewValue);
}


private void rtb_ContentChanged(object sender, RichTextBoxEventArgs e)
{
    suppressHtmlChanged = true;
    SetValue(HtmlProperty, rtb.SaveHtml());
    suppressHtmlChanged = false;
}

These days such solutions seem so old fashioned don't they, often we can convince ourselves that such a "low-tech" solutions can't be right because they're not "elegant".