如何解决Caliburn.Micro中的循环参考

Question

I am working with Caliburn.Micro v2.0.1 on a Windows 8.1 Unversal (WinRT) project.

I followed the Caliburn.Micro Working with WinRT example.

My code looks as follows:

App.xaml.cs

protected override void OnLaunched(LaunchActivatedEventArgs args)
{
    Initialize();
    DisplayRootViewFor<LoginViewModel>();
}

protected override void PrepareViewFirst(Frame rootFrame)
{
    _container.RegisterNavigationService(rootFrame);
}

LoginViewModel.cs

public LoginViewModel(INavigationService navigationService, ...)
{
   ...
}

The issue

The OnLaunched is called first.

Initialize() Configures the WinRT container.

1. The DisplayRootViewFor<LoginViewModel> invokes an instance of the LoginViewModel and results in a Null exception because NavigationService has not yet been registered by PrepareViewFirst(Frame)
2. PrepareViewFirst(Frame) is not yet called, having a dependency on the RootFrame that should be configured by OnLaunched

Thus LoginViewModel is dependent on RegisterNavigationService and RegisterNavigationService is dependent on DisplayRootViewFor<LoginViewModel>() which is dependent on LoginViewModel

Is there any way to overcome this circular reference issue?

Answer 1

Register your services in the container before resolving the Views - this way all dependencies are available in the particular Dependency Injection container and you can use ServiceLocator to find them.

Typically I've always done this in the OnStartup() method of App.xaml.cs .

Answer 2

You should register/configure your container at the composition root , the earliest access point of your application.

This point depends on what kind of Application you have:

• ASP.NET MVC: Global.asax.cs
• Console Application: the Main Method
• WPF: Application.OnStartup
• WinPhone 7: See https://stackoverflow.com/a/7310492/455493

etc.

Check the Windows 7 lifecycle on http://msdn.microsoft.com/en-us/magazine/hh148153.aspx