WPF: Нестандартное окно / Хабр

Holy long question, Batman!

Q1:
The process would work, I don’t know about using the LoginModel to talk to the MainWindowViewModel however.

You could try something like LoginView -> LoginViewModel -> [SecurityContextSingleton || LoginManagerSingleton] -> MainWindowView

I know that singleton’s are considered anti-patterns by some, but I find this to be easiest for situations like these. This way, the singleton class can implement the INotifyPropertyChanged interface and raise events whenever a loginout event is detected.

Implement the LoginCommand on either the LoginViewModel or the Singleton (Personally, I would probably implement this on the ViewModel to add a degree of separation between the ViewModel’s and the “back-end” utility classes). This login command would call a method on the singleton to perform the login.

Q2:
In these cases, I typically have (yet another) singleton class to act as the PageManager or ViewModelManager. This class is responsible for creating, disposing and holding references to the Top-level pages or the CurrentPage (in a single-page only situation).

My ViewModelBase class also has a property to hold the current instance of the UserControl that is displaying my class, this is so I can hook the Loaded and Unloaded events. This provides me the ability to have virtual OnLoaded(), OnDisplayed() and OnClosed() methods that can be defined in the ViewModel so the page can perform loading and unloading actions.

As the MainWindowView is displaying the ViewModelManager.CurrentPage instance, once this instance changes, the Unloaded event fires, my page’s Dispose method is called, and eventually GC comes in and tidy’s up the rest.

Q3:
I’m not sure if I understand this one, but hopefully you just mean “Display login page when user not logged in”, if this is the case, you could instruct your ViewModelToViewConverter to ignore any instructions when the user is not logged in (by checking the SecurityContext singleton) and instead only show the LoginView template, this is also helpful in cases where you want pages that only certain users have rights to see or use where you can check the security requirements before constructing the View, and replacing it with a security prompt.

Sorry for the long answer, hope this helps 🙂

Edit:
Also, you have misspelled “Management”

Edit for questions in comments

How would the LoginManagerSingleton talk directly to the
MainWindowView. Shouldn’t everything go through the
MainWindowViewModel so that there is no code behind on the
MainWindowView

Sorry, to clarify – I don’t mean the LoginManager interacts directly with the MainWindowView (as this should be just-a-view), but rather that the LoginManager just sets a CurrentUser property in response to the call that the LoginCommand makes, which in turn raises the PropertyChanged event and the MainWindowView (which is listening for changes) reacts accordingly.

The LoginManager could then call PageManager.Open(new OverviewScreen()) (or PageManager.Open("overview.screen") when you have IOC implemented) for example to redirect the user to the default screen users see once logged in.

The LoginManager is essentially the last step of the actual login process and the View just reflects this as appropriate.

Also, in typing this, it has occurred to me that rather than having a LoginManager singleton, all this could be housed in the PageManager class. Just have a Login(string, string) method, which sets the CurrentUser on successful log in.

I understand the idea of a PageManagerView, basically through a PageManagerViewModel

I wouldn’t design PageManager to be of View-ViewModel design, just an ordinary house-hold singleton that implements INotifyPropertyChanged should do the trick, this way the MainWindowView can react to the changing of the CurrentPage property.

Is ViewModelBase an abstract class you created?

Yes. I use this class as the base class of all my ViewModel’s.

This class contains

When a logged in detected, CurrentControl is set to a new View

Personally, I would only hold the instance of the ViewModelBase that is currently being displayed. This is then referenced by the MainWindowView in a ContentControl like so: Content="{Binding Source={x:Static vm:PageManager.Current}, Path=CurrentPage}".

I also then use a converter to transform the ViewModelBase instance in to a UserControl, but this is purely optional; You could just rely on ResourceDictionary entries, but this method also allows the developer to intercept the call and display a SecurityPage or ErrorPage if required.

Then when the application starts it detects no one is logged in, and
thus creates a LoginView and sets that to be the CurrentControl.
Rather than harding it that the LoginView is displayed by default

You could design the application so that the first page that is displayed to the user is an instance of the OverviewScreen. Which, since the PageManager currently has a null CurrentUser property, the ViewModelToViewConverter would intercept this and the rather than display the OverviewScreenView UserControl, it would instead show the LoginView UserControl.

If and when the user successfully logs in, the LoginViewModel would instruct the PageManager to redirect to the original OverviewScreen instance, this time displaying correctly as the CurrentUser property is non-null.

How do people get around this limitation as you mention as do others, singletons are bad

I’m with you on this one, I like me a good singleton. However, the use of these should be limited to be used only where necessary. But they do have perfectly valid uses in my opinion, not sure if any one else wants to chime in on this matter though?

Edit 2:

Do you use a publicly available framework/set of classes for MVVM

No, I’m using a framework that I have created and refined over the last twelve months or so. The framework still follows most the MVVM guidelines, but includes some personal touches that reduces the amount of overall code required to be written.

For example, some MVVM examples out there set up their views much the same as you have; Whereas the View creates a new instance of the ViewModel inside of its ViewObject.DataContext property. This may work well for some, but doesn’t allow the developer to hook certain Windows events from the ViewModel such as OnPageLoad().

OnPageLoad() in my case is called after all controls on the page have been created and have come in to view on screen, which may be instantly, within a few minutes after the constructor is called, or never at all. This is where I do most of my data loading to speed up the page loading process if that page has multiple child pages inside tabs that are not currently selected, for example.

But not only that, by creating the ViewModel in this manner increases the amount of code in each View by a minimum of three lines. This may not sound like much, but not only are these lines of code essentially the same for all views creating duplicate code, but the extra line count can add up quite quickly if you have an application that requires many Views. That, and I’m really lazy.. I didn’t become a developer to type code.

What I will do in a future revision through your idea of a page
manager would be to have several views open at once like a tabcontrol,
where a page manager controls pagetabs instead of just a single
userControl. Then tabs can be selected by a separate view binded to
the page manager

In this case, the PageManager won’t need to hold a direct reference to each of the open ViewModelBase classes, only those at the top-level. All other pages will be children of their parent to give you more control over the hierarchy and to allow you to trickle down Save and Close events.

If you put these in an ObservableCollection<ViewModelBase> property in the PageManager, you will only then need to create the MainWindow’s TabControl so that it’s ItemsSource property points to the Children property on the PageManager and have the WPF engine do the rest.

Can you expand a bit more on the ViewModelConverter

Sure, to give you an outline it would be easier to show some code.

    public override object Convert(object value, SimpleConverterArguments args)
    {
        if (value == null)
            return null;

        ViewModelBase vm = value as ViewModelBase;

        if (vm != null && vm.PageTemplate != null)
            return vm.PageTemplate;

        System.Windows.Controls.UserControl template = GetTemplateFromObject(value);

        if (vm != null)
            vm.PageTemplate = template;

        if (template != null)
            template.DataContext = value;

        return template;
    }

Reading through this code in sections it reads:

This is the code in the converter that does most of the grunt work, reading through the sections you can see:

• Main try..catch block used to catch any class construction errors including,
  • Page does not exist,
  • Run-time error in constructor code,
  • And fatal errors in XAML.
• convertViewModelTypeToViewType() just tries to find the View that corresponds to the ViewModel and returns the type code that it thinks it should be (this may be null).
• If this is not null, create a new instance of the type.
• If we fail to find a View to use, try to create the default page for that ViewModel type. I have a few additional ViewModel base classes that inherit from ViewModelBase that provide a separation of duties between the types of page’s they are.
  • For example, a SearchablePage class will simply display a list of all objects in the system of a certain type and provide the Add, Edit, Refresh and Filter commands.
  • A MaintenancePage will retrieve the full object from the database, dynamically generate and position the controls for the fields that the object exposes, creates children pages based on any collection the object has and provides the Save and Delete commands to use.
• If we still don’t have a template to use, throw an error so the developer knows something went wrong.
• In the catch block, any run-time error that occurs are displayed to the user in a friendly ErrorPage.

This all allows me to focus on only creating ViewModel classes as the application will simple display the default pages unless the View pages have been explicitly overridden by the developer for that ViewModel.