Talk to the View

View/ViewModel data binding isn't always the best way to trigger a behavior in the View. When the binding logic becomes too convoluted and difficult to follow, it's better to switch to a different strategy and let the ViewModel talk directly to the view. 

We'll revive the View code-behind and teach the ViewModel to call into the View via an interface. 

The 06-HappyHour tutorial folder holds the state of this solution at the end of the lesson.

Reset the view after each new drink order

Notice that, as you added new drink orders, the old value remains in the TextBox. Keep pressing the button and you place a new order for the same drink. That works for me; once I’ve found my drink, I tend to stay with it all night.

It may not work for the business. The specification says the View should prepare for a new choice after each order. We should clear the TextBox and set the focus so it's easy to make a new choice.

Should the ViewModel clear the TextBox and set focus? We don’t think so. The ViewModel shouldn’t know about TextBoxes and input focus. These are the details of interaction design decisions that are the proper concern of the View, not the ViewModel. The ViewModel should signal its intention to the View – it wants the View to “reset itself”, whatever that may mean.  The View should respond to that signal in the visual and functional way that the View designer deemed appropriate. This is a concrete example of the principle of “separation of concerns.”

Supervising Controller

There is no easy way in XAML to set the TextBox focus at the ViewModel’s request.

An obvious approach would be to put the TextBox focus logic in a reset method in the View’s code-behind and let the ViewModel call that method as appropriate.

This approach violates a fundamental tenet of MVVM: the ViewModel should never have a reference to the View. We’re going to do it anyway.

We’ll do it in a way that preserves separation of concerns. We won’t call the method “ClearTextBoxAndGiveItFocus”; the ViewModel doesn’t need to know those details. We’ll call it “ReadyForNewDrink” because that’s the intent.

We’ll define the ReadyForNewDrink method in an interface that insulates the ViewModel from any particular concrete View.
 
Finally, we’ll write the ViewModel code such that it only calls ReadyForNewDrink if the View supports the interface; if the View doesn’t, no big deal.

When our ViewModel maintains a reference to the View and tells the View what to do through an interface, we’re using the Supervising Controller pattern. This is another in the family of UI Architectures  that, like MVVM, help us maintain separation between the easily tested, non-visual presentation logic and the hard-to-test UI widgets on screen.

Define the interface

1. Select the ViewModels folder.

2. Add | Class | "IMainPage.cs". 

3. Replace entire contents with this:

4. Open MainPageViewModel.

MainPageViewModel can’t call ReadyForNewDrink until it "becomes aware" of a View that implements it.  Fortunately, MainPageViewModel inherits from Screen …

… and Screen implements the IViewAware interface as described below. What you need to know is that Screen calls its OnViewAttached method when the View becomes available. MainPageViewModel can override that method to gain access to the View … as shown here.

Casting the View as IMainPage makes its ReadyForNewDrink method accessible.

5. Revise the AddDrinkOrder method to ready the view after adding a new drink order to the collection. 

Notice that our ViewModel remains testable despite its dependence on a View. That dependence is limited to the IMainPage interface which is trivial to fake. 

Of course we’ll have to remember to pass a fake view into the MainPageViewModel when we test it. We can do that by casting the tested MainPageViewModel to IViewAware and calling AttachView with the fake view during setup of AddDrinkOrder tests.

Teach MainPage to get ready … in code-behind

Now that the MainPageViewModel is calling IMainPage.ReadyForNewDrink, we have to make MainPage implement that method … and it can only do so in code-behind. 

There is no code-behind for the MainPage at the moment. We deleted the MainPage.xaml.cs file a few lessons ago because we didn’t need it. It is our strong preference to eliminate code and files that aren’t contributing. No problem; we can restore it now that we need it.

1. Project | Add | Class | "MainPage.xaml.cs"

2. Replace entire contents with this:

Our implementation resets the view for the next new drink order by clearing the TextBox and giving it focus. You could restore the constructor that calls InitializeComponent if you wish. We decline to do so; we'll let Cocktail call InitializeComponent for us.

3. Rebuild and run [F5]

Enter a drink name and click the button. Immediately after, you should see that the TextBox is clear, the button disabled, and focus is in the TextBox where it is ready for your next drink.

Last call

In this lesson, we found ourselves wanting a user experience that is difficult to accomplish by MVVM-style data binding. The easier approach is to write the desired behavior in the View code-behind and invoke it from the ViewModel. 

We wrote a View interface (IMainPage) to abstract the View from the ViewModel. We made sure that interface member names expressed the intention ("ReadyForNewDrink") rather than dictating the user interaction ("ClearTextBoxAndGiveItFocus").

We saw that a ViewModel derived from the Screen class gains access to the View through the OnViewAttached method and thereby becomes capable of calling View members through the View interface.

This business of a ViewModel calling a View through an interface is an example of the Supervising Controller pattern which is an alternative to the more common ViewModel pattern.

We recommend that you stick with the ViewModel pattern when you can; it generally yields code that is cleaner and easier to maintain (no need for view interfaces).

But don’t be dogmatic about it. Use Supervising Controller when the ViewModel initiates a change in the View that isn’t easy to communicate through data binding.

Ask the Mixologist

This lesson is finished.  Feel free to move on directly to the next one. This "Ask the Mixologist" section is an optional digression from the lesson’s main course to related points of interest.

Is View code-behind evil?

We are not opposed to code in the code-behind. We’re wary of it because code-behind is difficult to test and developers have a bad habit of hiding business logic there. But we don’t mind if the code is simple (no conditional logic) and confined to purely local matters of design and usability. Try not to make a habit of it ... please.

Shouldn’t the MainPage code-behind call InitializeComponent?

Certainly something has to call InitializeComponent before the view can appear on screen. That something is the Caliburn Micro view composition process which is why we were able to delete the code-behind in the first place.

If you feel more comfortable with a more traditional looking code-behind that has a constructor that calls InitializeComponent, go ahead and add it. There is no harm in calling it twice.

Why IMainPage belongs to the ViewModels

You might think IMainPage belongs in the Views folder with a HappyHour.Views namespace because it prescribes functionality to be implemented by a View.  In fact it belongs with the ViewModels. 

The unshakeable principle is this: Views may depend upon ViewModels but ViewModels may not depend upon Views. To understand why, let’s indulge a couple of thought experiments.

Suppose that IMainPage were defined in the Views namespace and folder. Then suppose we later discover a good reason to breakout the ViewModels into their own assembly. 

Because MainPageViewModel depends upon IMainPage, the ViewModels assembly would have to depend upon the Views assembly. But we know that MainPage must implement IMainPage which means the Views assembly would have to depend upon the ViewModels assembly. We’d be trapped in a circular dependency – Views depends upon ViewModels which depends upon Views – and circular dependencies are disallowed in .NET.

Suppose we want to reuse our ViewModels to support multiple clients written in different XAML technologies such as Silverlight, WPF, Windows Phone and Windows 8. We couldn’t do that; the MainPageViewModel dependence on IMainPage in the Silverlight HappyHour Views assembly would pin the entire ViewModels assembly to the Silverlight client.

These problems disappear when IMainPage is defined in ViewModels. The Views assembly must depend upon the ViewModels assembly. But there is no reciprocal dependency from ViewModels to Views. The Views in Silverlight, WPF, Windows Phone and Windows 8 could each depend on the same ViewModels assembly (assuming other aspects of the technologies permitted such dependence).

Define View interfaces of the Supervising Controller pattern in the ViewModels namespace, not the Views namespace.

What is IViewAware?

The Screen class implements IViewAware, an interface that makes a ViewModel "aware" of the View to which it is attached. That "awareness" is well short of deep familiarity; such familiarity would break the separation we seek between the visual manifestation of the View and the non-visual view support that is the proper role of the ViewModel.

But there is no harm (and plenty of benefit) in knowing a few things about a View when it is abstracted behind this interface.

1. GetView is called first, giving the ViewModel an opportunity to provide its companion View instance directly rather than rely on the framework to create it. A ViewModel could return a concrete cached View. If it returns null, that means the framework should create the View. In HappyHour, where the MainPageViewModel is only displayed once, the base class implementation returns null and a new View is created.

2. AttachView is called next with the concrete View to which this ViewModel is bound (either the one returned by GetView or the one created by the framework). Screen implements this method privately. MainPageViewModel is not involved but can learn about the view by overriding OnViewAttached ... as we did in this lesson.

3. ViewAttached is the event raised after AttachView completes. It informs other interested components that this ViewModel has been bound to its View and is ready for business.  
MainPageViewModel doesn’t bother listening for this event because it gets the same information from OnViewAttached. OnViewAttached is called before handlers of the ViewAttached event.