TwoBinding updating issue

[felipeleon73]

First of all... Thanx for this very nice library!

I have this issue:
I've cloned SubBindings sample and modified with:
-- Program.fs

• added in the Counter Module

    let setConter value c =
        let a = { c with CounterValue = value + 1 }
        { a with StepSize = c.StepSize + 1 }

• added to type Msg
  | SetCounter of CounterId * int

• added to update function
  | SetCounter (cid,value) -> { m with AllCounters = m.AllCounters |> mapCounter cid (Counter.setCounter value) }

• added to counterBindings

      "CounterValue" |> Binding.twoWay
        (fun (m, c) -> string c.CounterValue)
        (fun v (m, c) -> SetCounter (c.Id, int v))

-- MainWindows.xml

• modified the textblock showing CounterValue in
  <TextBox Text="{Binding CounterValue}" Width="100" Margin="0,5,10,5" />

Works all but this:
if i insert a number (eg 20) in the textbox and change focus:

1. the model is update (CounterValue is now 20 and StepSize is now 2)
2. the StepSize Slider is update to 2
   3. the texbox remain 20 (ISSUE)

now if i hit the "+" button, correctly:

1. the model is update (CounterValue is now 23 and StepSize remain 2)
2. the StepSize Slider remain 2
3. the texbox is updated to 23

Why i've th bold issue?
Thanx
PS sorry for my poor english, i'm italian
(i attach the modified files)
Files.zip

[cmeeren]

As far as I know, WPF doesn't update the value of a control (or at least a TextBox) that has focus, even when PropertyChanged is triggered for the binding source. What seems to happen in this case is that the model update is triggered as (but not after) the TextBox is losing focus, and the TextBox first loses focus after the whole update loop completes. Therefore, the TextBox is not updated with the new model value.

This is supported by the logs: Even after PropertyChanged is fired for CounterValue, the TryGetMember isn't called for that property, which means that the view doesn't try to get the updated value.

In other words: It's just the way WPF works.

[felipeleon73]

thanx... Good explanation!
are there possible workarounds for this behavior?

[cmeeren]

I don't know of any simple workarounds. You could try playing around with storing the input value directly in the model and setting the XAML binding trigger to PropertyChanged, and use the LostFocus event to send a different message where you do the transformations you want (example of how to bind to events in #33 (comment)). That's what I do for most input values. For example, for a currency input, I store the raw text value in the model as a string (updating on every keypress), and on LostFocus, I send a different message that formats the string according to currency formatting rules, which updates the value in the TextBox.

[felipeleon73]

following your suggestions i've tryed to modify the ViewModel Module like this:

  let notifyPropertyChanged propName =
    log "[VM] Triggering PropertyChanged for binding %s" propName
    //propertyChanged.Trigger(this, PropertyChangedEventArgs propName)
    let triggerPropertyChanged = new Action(fun () -> propertyChanged.Trigger(this, PropertyChangedEventArgs propName))
    System.Windows.Threading.Dispatcher.CurrentDispatcher.BeginInvoke(triggerPropertyChanged) |> ignore

pratically, instead of trigger the propertyChanged directly, i've send it to dispatcher.
now everything works, but I don't know if that causes other (maybe performance) issues

[cmeeren]

Interesting! I'm also unsure what the side-effects of that change is, though it would change how people are used to WPF TextBoxes working, which might not be a good thing. Also it might not always work; there might be a race condition involved when you dispatch it like that. (I don't know the WPF internals well enough to say.)

[cmeeren]

This has been fixed in 2.0.0-beta-10.