How not to lose binding source updates?

Question

Suppose I have a modal dialog with a textbox and OK/Cancel buttons. And it is built on MVVM - i.e. it has a ViewModel object with a string property that the textbox is bound to.

Say, I enter some text in the textbox and then grab my mouse and click "OK". Everything works fine: at the moment of click, the textbox loses focus, which causes the binding engine to update the ViewModel's property. I get my data, everybody's happy.

Now suppose I don't use my mouse. Instead, I just hit Enter on the keyboard. This also causes the "OK" button to "click", since it is marked as IsDefault="True". But guess what? The textbox doesn not lose focus in this case, and therefore, the binding engine remains innocently ignorant, and I don't get my data. Dang!

Another variation of the same scenario: suppose I have a data entry form right in the main window, enter some data into it, and then hit Ctrl+S for "Save". Guess what? My latest entry doesn't get saved!

This may be somewhat remedied by using UpdateSourceTrigger=PropertyChanged, but that is not always possible.

One obvious case would be the use of StringFormat with binding - the text keeps jumping back into "formatted" state as I'm trying to enter it.

And another case, which I have encountered myself, is when I have some time-consuming processing in the viewmodel's property setter, and I only want to perform it when the user is "done" entering text.

This seems like an eternal problem: I remember trying to solve it systematically from ages ago, ever since I've started working with interactive interfaces, but I've never quite succeeded. In the past, I always ended up using some sort of hacks - like, say, adding an "EnsureDataSaved" method to every "presenter" (as in "MVP") and calling it at "critical" points, or something like that...

But with all the cool technologies, as well as empty hype, of WPF, I expected they'd come up with some good solution.

Answer 1

At critical points, you can force the binding to push through to your view model:

var textBox = Keyboard.FocusedElement as TextBox;
BindingOperations.GetBindingExpression(textBox, TextBox.TextProperty).UpdateSource();

Edit:

OK, since you don't want hacks we have to face the ugly truth:

• In order to implement a clean view, the properties exposed by your view model should be friendly to frequent binding updates.

An analogy we can use is a text editor. If the application was a giant text box bound to a file on disk, every keystroke would result in writing the whole file. Even the concept of saving is not needed. That's perversely correct but terribly inefficient. We all immediately see that the view model needs to expose a buffer for the view to bind to and this re-introduces the concept of save and forces state handling in our view model.

Yet, we see this is still not efficient enough. For even medium-sized files the overhead of updating the whole-file buffer on every keystroke becomes unbearable. Next we expose commands in our view model to efficiently manipulate the buffer, never actually exchanging the whole buffer with the view.

So we conclude that in order to achieve efficiency and responsiveness with pure MVVM, we need to expose an efficient view model. That means that all text boxes can be bound through to properties with no ill effects. But, it also means that you have to push state down into the view model to handle that. And that's OK because a view model is not the model; it's job is it to handle the needs of the view.

It's true that we can rapidly prototype user interfaces by taking advantage of shortcuts like binding on focus changes. But binding on focus changes can have negative consequences in real applications and if so then we should simply not use it.

What is the alternative? Expose a property friendly to frequent updates. Call it the same thing as the old inefficient property was called. Implement your fast property using the slow property with logic that depends on the state of the view model. The view model gets the save command. It knows whether the fast property has been pushed through to the slow property. It can decide if when and where the slow property will be synched to the model.

But you say, haven't we just moved the hack from the view to the view model? No, we have lost some elegance and simplicity, but go back to the text editor analogy. We have to solve the problem, and it is the view model's job to solve it.

If we want to use pure MVVM and we want efficiency and responsiveness, then lame heuristics like let's avoid updating the binding source until the element loses focus won't help. They introduce as many problems as they solve. In that case, we should let the view model do its job, even if means adding complexity.

Assuming we accept it, how can we manage the complexity? We can implement a generic wrapper utility class to buffer the slow property and allow the view model to hook its get and set methods. Our utility class can auto-register for save command events to reduce the amount of boilerplate code in our view model.

If we do it right, then all the parts of the view model that were fast enough to be used with property changed binding will all still be the same, and the others that were worthy of asking the question "Is this property too slow?" will have a small amount of code to address the issue, and the view is none the wiser.

Answer 2

This is a tricky one and I agree a non-hack and more-or-less code free solution should be found. Here are my thoughts:

1. The view is responsible because it sets IsDefault to true and allows for this 'problem'
2. The ViewModel should not be responsible in any way to fix this it might introduce dependencies from VM to V and thus breaking the pattern.
3. Without adding (C#) code to the View all you can do is either change bindings (e.g. to UpdateSourceTrigger=PropertyChanged) or add code to a base class of the Button. In the base class of the button you might be able to shift focus to the button before executing the command. Still hack-ish but cleaner than adding code to the VM.

So at the moment the only 'nice' solutions I see require the view developers to stick to a rule; set the binding in a specific way or use a special button.

Answer 3

The problem is that the TextBox's text has a default source trigger of LostFocus instead of PropertyChanged. IMHO this was a wrong default choice since it is quite unexpected and can cause all sorts of problems (such as the ones you describe).

1. The simplest solution would be to always explicitly use UpdateSourceTrigger=PropertyChanged (as the others suggested).
2. If this isn't feasible (for whatever reason), I would handle the Unloaded, Closing or Closed events and manually update the binding (as shown by Rick).

Unfortunately it seems that certain scenarios are still a bit problematic with a TextBox, so some workarounds are necessary. For example, see my question. You might want to open a Connect bug (or two) with your specific problems.

EDIT: Pressing Ctrl+S with focus on the TextBox, I would say the behavior is correct. After all, you are executing a command. This has nothing to do with the current (keyboard) focus. The command may even depend on the focused element! You are not clicking on a button or similar, which would cause the focus to change (however, depending on the button, it may fire the same command as before).

So if you want to only update the bound Text when you lose focus from the TextBox, but at the same time you want to fire a command with the newest contents of TextBox (i.e. the changes without it having lost focus), this does not match up. So either you have to change your binding to PropertyChanged, or manually update the binding.

EDIT #2: As for your two cases why you cannot always use PropertyChanged:

1. What precisely are you doing with StringFormat? In all my UI work so far I use StringFormat to reformat data I am getting from the ViewModel. However, I am not sure how using StringFormat with data which is then again edited by the user should work. I am guessing you want to format the text for display, and then "unformat" the text the user enters for further processing in your ViewModel. From your description, it seems it isn't "unformatted" correctly all the time.
   1. Open a Connect bug where it isn't working as it should.
   2. Write your own ValueConverter which you use in the binding.
   3. Have a separate property with the last "valid" value and use that value in your ViewModel; only update it once you get another "valid" value from the property you use in databinding.
2. If you have a long-running property setter (ex. a "validate" step), I would do the long-running part in a separate method (getters and setters should normally be relatively "fast"). Then run that method in a worker thread/threadpool/BackgroundWorker (make it abortable so you can restart it with a new value once the user enters more data) or similar.

Answer 4

I would add a Click event handler for the default button. The button's event handler is executed prior the command will be called, so the data bindings can be updated by changing the focus in the event handler.

private void Button_Click(object sender, RoutedEventArgs e) {
    ((Control)sender).Focus();
}

However, I don't know if similar approach can be used with other shorcut keys.

Answer 5

Yes, I have quite some experience. WPF and Silverlight still have their pain areas. MVVM doesn't solve it all; it is not a magic bullet and the support in the frameworks is getting better but still lacks. E.g., I still find editing deep child collections a problem.

At the moment I handle these situations case by case because a lot depends on the way the individual view have to work. This is what I spend most of my time on because I generate a lot of plumbing using T4 so I have time left for these quirks.

Answer 6

What you think about proxy command and KeyBinding to ENTER key?

EDITED: There we have one utility command (like converter), which requires knowledge about concrete view. This command can be reused for any dialog with same bug. And you add this functionality/hack only in view where this bug exists, and VM will be clear.

VM creates to adapt business to view and must provide some specific functionality like data conversion, UI commands, additional/helper fields, notifications and hacks/workarounds. And if we have leaks between levels in MVVM we have problems with: high connectivity, code reuse, unit testing for VM, pain code.

Usage in xaml (no IsDefault on Button):

<Window.Resources>
    <model:ButtonProxyCommand x:Key="proxyCommand"/>
</Window.Resources>

<Window.InputBindings>
    <KeyBinding Key="Enter"
          Command="{Binding Source={StaticResource proxyCommand}, Path=Instance}" 
          CommandParameter="{Binding ElementName=_okBtn}"/>
</Window.InputBindings>
<StackPanel>
    <TextBox>
        <TextBox.Text>
            <Binding Path="Text"></Binding>
        </TextBox.Text>
    </TextBox>
    <Button Name="_okBtn" Command="{Binding Command}">Ok</Button>
</StackPanel>

There used special proxy command, which receive element (CommandParameter) to move focus and execute. But this class requires ButtonBase for CommandParameter:

public class ButtonProxyCommand : ICommand
{
    public bool CanExecute(object parameter)
    {
        var btn = parameter as ButtonBase;

        if (btn == null || btn.Command == null)
            return false;

        return btn.Command.CanExecute(btn.CommandParameter);
    }

    public event EventHandler CanExecuteChanged;

    public void Execute(object parameter)
    {
        if (parameter == null)
            return;

        var btn = parameter as ButtonBase;

        if (btn == null || btn.Command == null)
            return;

        Action a = () => btn.Focus();
        var op = Dispatcher.CurrentDispatcher.BeginInvoke(a);

        op.Wait();
        btn.Command.Execute(btn.CommandParameter);
    }

    private static ButtonProxyCommand _instance = null;
    public static ButtonProxyCommand Instance
    {
        get
        {
            if (_instance == null)
                _instance = new ButtonProxyCommand();

            return _instance;
        }
    }
}

This is just idea, not complete solution.