ReactiveUI with WPF DatePicker

Question

I can't get the binding to a WPF DatePicker work as expected with ReactiveUI. The binding is set up in code like this:

this.Bind(this.ViewModel, x => x.MyDate, x => x.DPMyDate.SelectedDate, DPMyDate.Events().LostKeyboardFocus).DisposeWith(disposables);

In the XAML file it looks like this:

<DatePicker Name="DPMyDate"></DatePicker>

and the bound property is this:

private DateTime myDate;
public DateTime MyDate
{
    get { return myDate; }
    set
    {
        this.RaiseAndSetIfChanged(ref myDate, value);
    }
}

Changing the date via the calendar works as expected. When I change the date with the keyboard, however, the bound property gets not changed! Interestingly when I move the focus out of the DatePicker after I changed the date, then back in and back out again (without changing anything) the original change gets set. I guess it has something to do with the internal focus handling of the DatePicker with its TextBox but I don't understand whether it's and issue with WPF or ReactivUI.

Any idea?

Update 1

As a test I assigned a handler to the DatePickers LostKeyboardFocus event directly. This event seems to fire everytime the control looses focus. ReactiveUI, however, doesn't pick up the first focus loss!

NicolasR NicolasR · Accepted Answer · 2019-10-30T13:39:44

Sorry for answering my own question now 40 minutes after I posted it. But a minute ago I found out that binding to LostKeyboardFocus is the wrong way for DatePickers. One should bind to SelectedDateChanged!

In LostKeyboardFocus SelectedDate still has its original value and that is why ReactiveUI does correctly not change the property on the first focus loss. On further focus losses it is then always one change behind.

I was somehow under the impression this should work as with TextBoxes where you often want to trigger a change only when the box looses focus but DatePicker is special here.

ReactiveUI with WPF DatePicker

1 Answers