Application boot process

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Last updated 3 years ago

Application boot process

What happens under the hood when we write this really trivial piece of code?

public partial class App : Application
{
    public App()
    {
        this.AddRadicalApplication<MainView>();
    }
}

As we have already seen in the we are doing 2 main choices:

We boot using the default IoC container provided by Microsoft.Extensions.DeendencyInjection;
We declare that the MainView window is the main/shell window of our application;

Internally the application boot process is not so trivial as it appears from the outside, when the Startup event is raised by the WPF application the bootstrapper:

Identifies assemblies to scan

In order to configure the IoC containers assemblies needs to scanned to load types that need to be registered for DI. This is accomplished by the assembly scanner. It's possible to customize some of the assembly scanner behaviors by using the AssemblyScanner property of the BootstrapConfiguration instance, like in the following snippet:

public partial class App : Application
{
    public App()
    {
        this.AddRadicalApplication<MainView>(configuration =>
        {
            var scanner = configuration.AssemblyScanner;
            scanner.DirectorySearchOptions = SearchOption.TopDirectoryOnly; //default value
        });
    }
}

Register additional types in the IoC container

To register custom types, other than the ones already automatically registered via bootstrap conventions, a dependencies installer is required. Create a class that implements the IDependenciesInstaller interface. A class instance will be automatically created at runtime and the Install method will be invoked:

public class MyCustomInstaller : IDependenciesInstaller
{
   public void Install(BootstrapConventions conventions, IServiceCollection services, IEnumerable<Type> assemblyScanningResults)
   {
      services.AddSingleton<MyCustomSingleton>();
   }
}

Creates the service provider

Once assemblies and types are scanned and identified through bootstrap conventions the default IoC container provided by Microsoft.Extensions.DeendencyInjection is created. In case an instance of the created IServiceProvider is required outside the scope of the Radical application, it can be retrieved using the following snippet:

public partial class App : Application
{
    public App()
    {
        IServiceProvider container = null;
        this.AddRadicalApplication<MainView>(configuration =>
        {
            configuration.OnBootCompleted(serviceProvider => container = serviceProvider);
        });
    }
}

ShutdownMode

WPF applications have the concept of ShutdownMode. Application bootstrapper does not change in any way the default value of the Application.Current.ShutdownMode unless explicitly requested by user:

public partial class App : Application
{
    public App()
    {
        this.AddRadicalApplication<MainView>(configuration =>
        {
            configuration.OverrideShutdownMode(ShutdownMode.OnLastWindowClose);
        });
    }
}

Principal initialization

Once the application services are setup the bootstrapper takes care of setting up the Thread.CurrentPrincipal, the default behavior is to use the current user Windows identity. This behavior can be changed by setting a different principal right after the boot process is completed, using the OnBootCompleted handler;

Culture & UICulture

After setting up the principal and finally returning control to the application the boot process has the option to setup the Culture and the UICulture of the current Thread. The default behavior is to use values of the hosting OS. The default behavior can be overwritten in the following way:

public partial class App : Application
{
    public App()
    {
        this.AddRadicalApplication<MainView>(configuration =>
        {
           configuration.UseCulture(container=>new CultureInfo("it-IT"));
           configuration.UseUICulture(container=>new CultureInfo("it-IT"));
        });
    }
}

Boot

Once everything is setup the bootstrapper gives us the ability to take part into the boot process before the main window is shown:

public partial class App : Application
{
    public App()
    {
        this.AddRadicalApplication<MainView>(configuration =>
        {
           configuration.OnBooting(container=>
          {
             //boot is in progress, UI is not visible yet.
          });
        });
    }
}

BootCompleted

The last event in the process is the one used to show the main window, we have the opportunity to be notified using the exposed handler:

public partial class App : Application
{
    public App()
    {
        this.AddRadicalApplication<MainView>(configuration =>
    {
       configuration.OnBootCompleted(container=>
       {
          //UI is fully setup
       });
    });
    }
}

Intercepting unhandled exceptions

if we need to be notified whenever an unhandled exception occurs in our application we can use the provided hook:

public partial class App : Application
{
    public App()
    {
        this.AddRadicalApplication<MainView>(configuration =>
        {
        configuration.OnUnhandledException(ex=> { /* deal with exception */ });
        });
    }
}

Handling the application Shutdown

As for the startup we can also handle the shutdown process of the application:

public partial class App : Application
{
    public App()
    {
        this.AddRadicalApplication<MainView>(configuration =>
        {
            configuration.OnShuttingDown(args=> { });
        });
    }
}

When the application shuts down the provided delegate is invoked passing in wehter the boot was completed or not, and the reason why the application is shutting down:

public enum ApplicationShutdownReason
{
    /// <summary>
    /// The application has been shutdown using the Radical canonical behaviors.
    /// In this case the shutdown process can be canceled.
    /// </summary>
    UserRequest = 0,

    /// <summary>
    /// The application is shutting down because another
    /// instance is already running and the application
    /// is marked as singleton.
    /// </summary>
    MultipleInstanceNotAllowed = 1,

    /// <summary>
    /// The application is shutting down because the operating system session is ending.
    /// </summary>
    SessionEnding,

    /// <summary>
    /// The application has been shut down using the App.Current.Shutdown() method.
    /// </summary>
    ApplicationRequest,
}

As we can see we can easily determine why the application is shutting down. Currently there is no way from the application bootstrapper to cancel the shutdown process, in order to achieve that we need to subscribe to the ApplicationShutdownRequested message via the message broker.