How to control the order of module initialization in Prism

Question

I\'m using Prism V2 with a DirectoryModuleCatalog and I need the modules to be initialized in a certain order. The desired order is specified with an attribute on each IModu

Answer 1

I didn't like the idea of using ModuleDependency because this would mean that module a would not load when module b was not present, when in fact there was no dependency. Instead I created a priority attribute to decorate the module:

/// <summary>
/// Allows the order of module loading to be controlled.  Where dependencies
/// allow, module loading order will be controlled by relative values of priority
/// </summary>
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
public sealed class PriorityAttribute : Attribute
{
    /// <summary>
    /// Constructor
    /// </summary>
    /// <param name="priority">the priority to assign</param>
    public PriorityAttribute(int priority)
    {
        this.Priority = priority;
    }

    /// <summary>
    /// Gets or sets the priority of the module.
    /// </summary>
    /// <value>The priority of the module.</value>
    public int Priority { get; private set; }
}

I then decorated the modules like this:

[Priority(200)]
[Module(ModuleName = "MyModule")]
public class MyModule : IModule

I created a new descendent of DirectoryModuleCatalog:

/// <summary>
/// ModuleCatalog that respects PriorityAttribute for sorting modules
/// </summary>
[SecurityPermission(SecurityAction.InheritanceDemand), SecurityPermission(SecurityAction.LinkDemand)]
public class PrioritizedDirectoryModuleCatalog : DirectoryModuleCatalog
{
    /// <summary>
    /// local class to load assemblies into different appdomain which is then discarded
    /// </summary>
    private class ModulePriorityLoader : MarshalByRefObject
    {
        /// <summary>
        /// Get the priorities
        /// </summary>
        /// <param name="modules"></param>
        /// <returns></returns>
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance", "CA1822:MarkMembersAsStatic"), System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Reliability", "CA2001:AvoidCallingProblematicMethods", MessageId = "System.Reflection.Assembly.LoadFrom")]
        public Dictionary<string, int> GetPriorities(IEnumerable<ModuleInfo> modules)
        {
            //retrieve the priorities of each module, so that we can use them to override the 
            //sorting - but only so far as we don't mess up the dependencies
            var priorities = new Dictionary<string, int>();
            var assemblies = new Dictionary<string, Assembly>();

            foreach (ModuleInfo module in modules)
            {
                if (!assemblies.ContainsKey(module.Ref))
                {
                    //LoadFrom should generally be avoided appently due to unexpected side effects,
                    //but since we are doing all this in a separate AppDomain which is discarded
                    //this needn't worry us
                    assemblies.Add(module.Ref, Assembly.LoadFrom(module.Ref));
                }

                Type type = assemblies[module.Ref].GetExportedTypes()
                    .Where(t => t.AssemblyQualifiedName.Equals(module.ModuleType, StringComparison.Ordinal))
                    .First();

                var priorityAttribute =
                    CustomAttributeData.GetCustomAttributes(type).FirstOrDefault(
                        cad => cad.Constructor.DeclaringType.FullName == typeof(PriorityAttribute).FullName);

                int priority;
                if (priorityAttribute != null)
                {
                    priority = (int)priorityAttribute.ConstructorArguments[0].Value;
                }
                else
                {
                    priority = 0;
                }

                priorities.Add(module.ModuleName, priority);
            }

            return priorities;
        }
    }

    /// <summary>
    /// Get the priorities that have been assigned to each module.  If a module does not have a priority 
    /// assigned (via the Priority attribute) then it is assigned a priority of 0
    /// </summary>
    /// <param name="modules">modules to retrieve priorities for</param>
    /// <returns></returns>
    private Dictionary<string, int> GetModulePriorities(IEnumerable<ModuleInfo> modules)
    {
        AppDomain childDomain = BuildChildDomain(AppDomain.CurrentDomain);
        try
        {
            Type loaderType = typeof(ModulePriorityLoader);
            var loader =
                (ModulePriorityLoader)
                childDomain.CreateInstanceFrom(loaderType.Assembly.Location, loaderType.FullName).Unwrap();

            return loader.GetPriorities(modules);
        }
        finally
        {
            AppDomain.Unload(childDomain);
        }
    }

    /// <summary>
    /// Sort modules according to dependencies and Priority
    /// </summary>
    /// <param name="modules">modules to sort</param>
    /// <returns>sorted modules</returns>
    protected override IEnumerable<ModuleInfo> Sort(IEnumerable<ModuleInfo> modules)
    {
        Dictionary<string, int> priorities = GetModulePriorities(modules);
        //call the base sort since it resolves dependencies, then re-sort 
        var result = new List<ModuleInfo>(base.Sort(modules));
        result.Sort((x, y) =>
            {
                string xModuleName = x.ModuleName;
                string yModuleName = y.ModuleName;
                //if one depends on other then non-dependent must come first
                //otherwise base on priority
                if (x.DependsOn.Contains(yModuleName))
                    return 1; //x after y
                else if (y.DependsOn.Contains(xModuleName))
                    return -1; //y after x
                else 
                    return priorities[xModuleName].CompareTo(priorities[yModuleName]);
            });

        return result;
    }
}

Finally, I changed the bootstrapper to use this new catalog:

    /// <summary>Where are the modules located</summary>
    /// <returns></returns>
    protected override IModuleCatalog GetModuleCatalog()
    {
        return new PrioritizedDirectoryModuleCatalog() { ModulePath = @".\Modules" };
    }

I'm not sure if the stuff with assembly loading is the best way to do things, but it seems to work...

Answer 2

You can use the ModuleDependency attribute on your module class to tell the loader that your module depends on other modules:

[ModuleDependency("SomeModule")]
[ModuleDependency("SomeOtherModule")]
public class MyModule : IModule
{
}

Answer 3

In the AddModule() call in the Bootstrapper, you can specify a dependency. So, you can say A depends on B depends on C, and that will determine load order.

http://msdn.microsoft.com/en-us/magazine/cc785479.aspx

Answer 4

Had a similar problem an combined Fergus Bowns answer with the SmartDirectoryCatalog suggested by Haukinger: Multiple DirectoryModuleCatalog in a Prism application . I use this for "optional dependencies". Hope this will help someone.

PS: with the actual Prism Unity 7.2 you need to replace ModuleInfo with IModuleInfo

Answer 5

Bringing this back from the dead as I seem to have found a different solution that some might find useful. I tried it out and it works but I have yet to feel out all the pros and cons.

I was using DirectoryModuleCatalog to get a list of all my modules which were all placed into a single folder. But I noticed that for the most part all my "View" modules depended on my "Service" modules, and that was a pretty common pattern. No service should depend on a view. So that got me thinking, what if we just put all the service modules into a folder and all the view modules into another and created two different catalogs in the correct order. Some digging around and I found this article that mentions something called an AggregateModuleCatalog, and it's used to concatenate together a bunch of catalogs. I found the source code for this class here. And here's how I used it:

class Bootstrapper : UnityBootstrapper
{
    protected override System.Windows.DependencyObject CreateShell() {...}
    protected override void InitializeShell() {...}

    protected override IModuleCatalog CreateModuleCatalog()
    {
        return new AggregateModuleCatalog();
    }

    protected override void ConfigureModuleCatalog()
    {
        ((AggregateModuleCatalog)ModuleCatalog).AddCatalog(new DirectoryModuleCatalog { ModulePath = "Modules.Services" });
        ((AggregateModuleCatalog)ModuleCatalog).AddCatalog(new DirectoryModuleCatalog { ModulePath = "Modules.Views" });
    }
}

And the AggregateModuleCatalog:

public class AggregateModuleCatalog : IModuleCatalog
{
    private List<IModuleCatalog> catalogs = new List<IModuleCatalog>();

    /// <summary>
    /// Initializes a new instance of the <see cref="AggregateModuleCatalog"/> class.
    /// </summary>
    public AggregateModuleCatalog()
    {
        this.catalogs.Add(new ModuleCatalog());
    }

    /// <summary>
    /// Gets the collection of catalogs.
    /// </summary>
    /// <value>A read-only collection of catalogs.</value>
    public ReadOnlyCollection<IModuleCatalog> Catalogs
    {
        get
        {
            return this.catalogs.AsReadOnly();
        }
    }

    /// <summary>
    /// Adds the catalog to the list of catalogs
    /// </summary>
    /// <param name="catalog">The catalog to add.</param>
    public void AddCatalog(IModuleCatalog catalog)
    {
        if (catalog == null)
        {
            throw new ArgumentNullException("catalog");
        }

        this.catalogs.Add(catalog);
    }

    /// <summary>
    /// Gets all the <see cref="ModuleInfo"/> classes that are in the <see cref="ModuleCatalog"/>.
    /// </summary>
    /// <value></value>
    public IEnumerable<ModuleInfo> Modules
    {
        get
        {
            return this.Catalogs.SelectMany(x => x.Modules);
        }
    }

    /// <summary>
    /// Return the list of <see cref="ModuleInfo"/>s that <paramref name="moduleInfo"/> depends on.
    /// </summary>
    /// <param name="moduleInfo">The <see cref="ModuleInfo"/> to get the</param>
    /// <returns>
    /// An enumeration of <see cref="ModuleInfo"/> that <paramref name="moduleInfo"/> depends on.
    /// </returns>
    public IEnumerable<ModuleInfo> GetDependentModules(ModuleInfo moduleInfo)
    {
        var catalog = this.catalogs.Single(x => x.Modules.Contains(moduleInfo));
        return catalog.GetDependentModules(moduleInfo);
    }

    /// <summary>
    /// Returns the collection of <see cref="ModuleInfo"/>s that contain both the <see cref="ModuleInfo"/>s in
    /// <paramref name="modules"/>, but also all the modules they depend on.
    /// </summary>
    /// <param name="modules">The modules to get the dependencies for.</param>
    /// <returns>
    /// A collection of <see cref="ModuleInfo"/> that contains both all <see cref="ModuleInfo"/>s in <paramref name="modules"/>
    /// and also all the <see cref="ModuleInfo"/> they depend on.
    /// </returns>
    public IEnumerable<ModuleInfo> CompleteListWithDependencies(IEnumerable<ModuleInfo> modules)
    {
        var modulesGroupedByCatalog = modules.GroupBy<ModuleInfo, IModuleCatalog>(module => this.catalogs.Single(catalog => catalog.Modules.Contains(module)));
        return modulesGroupedByCatalog.SelectMany(x => x.Key.CompleteListWithDependencies(x));
    }

    /// <summary>
    /// Initializes the catalog, which may load and validate the modules.
    /// </summary>
    public void Initialize()
    {
        foreach (var catalog in this.Catalogs)
        {
            catalog.Initialize();
        }
    }

    /// <summary>
    /// Adds a <see cref="ModuleInfo"/> to the <see cref="ModuleCatalog"/>.
    /// </summary>
    /// <param name="moduleInfo">The <see cref="ModuleInfo"/> to add.</param>
    public void AddModule(ModuleInfo moduleInfo)
    {
        this.catalogs[0].AddModule(moduleInfo);
    }
}

I should also mention that the article states the following:

To demonstrate multiple ways of using the ModuleCatalog, the QuickStart using Unity implements an AggregateModuleCatalog that derives from IModuleCatalog. This class is not intended to be used in a shipping application.

Why that is I'm not sure. Would love to hear any explanations as to why that might be.

Answer 6

You can replace the default IModuleInitializer for an instance of a custom class that instead of initializing the modules right after they are loaded, stores them in a modules list. When all modules have been loaded, you initialize them in whatever order you want.

How to achieve this:

1) In the bootstrapper, override the ConfigureContainer method to replace the default IModuleInitializer for a instance of the MyModuleInitializer class, yet maintaining the default initializer with a name (for example, defaultModuleInitializer):

protected override void ConfigureContainer()
{
    base.ConfigureContainer();
    var defaultContainer = Container.Resolve<IModuleInitializer>();
    Container.RegisterInstance<IModuleInitializer>("defaultModuleInitializer", defaultContainer);
    Container.RegisterType<IModuleInitializer, MyModuleInitializer>(new ContainerControlledLifetimeManager());
}

2) Create the MyModuleInitializer class that performs the desired storea-all-then-sort-and-initialize procedure:

public class MyModuleInitializer : IModuleInitializer
{
    bool initialModuleLoadCompleted = false;
    IModuleInitializer defaultInitializer = null;
    List<ModuleInfo> modules = new List<ModuleInfo>();

    public MyModuleInitializer(IUnityContainer container)
    {
        defaultInitializer = container.Resolve<IModuleInitializer>("defaultModuleInitializer");
    }

    public void Initialize(ModuleInfo moduleInfo)
    {
        if(initialModuleLoadCompleted) {
            //Module loaded on demand after application startup - use the default initializer
            defaultInitializer.Initialize(moduleInfo);
            return;
        }

        modules.Add(moduleInfo);

        if(AllModulesLoaded()) {
            SortModules();
            foreach(var module in modules) {
                defaultInitializer.Initialize(module);
            }
            modules = null;
            initialModuleLoadCompleted = true;
        }
    }

    private bool AllModulesLoaded()
    {
        //Here you check whether all the startup modules have been loaded
        //(perhaps by looking at the module catalog) and return true if so
    }

    private void SortModules()
    {
        //Here you sort the "modules" list however you want
    }
}

Note that after all the startup modules have been loaded, this class reverts to simply invoking the default initializer. Adapt the class appropriately if this is not what you need.