Modular Web Application with ASP.NET Core

直接撸别人的文章，还未来得及翻译，见谅。
Background
There are few things we need to address to make our application modularized:

How can MVC know about our controllers when they are in other class libraries, in other folder and not being referenced by the host
How can the ViewEngine pick up the right location for the Views in modules
How to register services used in modules
How to serve static file: js, css, image for modules
How to register domain entities in modules to DbContext

Using the code
Below is general folder structure I have come up with

The Modular.WebHost is the ASP.NET Core project and it will act as the host. It will bootstrap the app and load all the modules it found in the Modules folder.

Each module contains all the stuff for itself to run including Controllers, Services, Views and event static files.

For easy development, in the visual studio solution I create a "Modules" solution items and add module projects in Modular.WebHost/Modules physical folder.

In order to prevent Modular.WebHost to compile stuff in Modules folder, we need to exclude them in the project.json.

1. First we will scan all the assemblies in each module and load them up

var moduleRootFolder = new DirectoryInfo(Path.Combine(_hostingEnvironment.ContentRootPath, "Modules"));
    var moduleFolders = moduleRootFolder.GetDirectories();

    foreach (var moduleFolder in moduleFolders)
    {
        var binFolder = new DirectoryInfo(Path.Combine(moduleFolder.FullName, "bin"));
        if (!binFolder.Exists)
        {
            continue;
        }

        foreach (var file in binFolder.GetFileSystemInfos("*.dll", SearchOption.AllDirectories))
        {
            Assembly assembly = null;
            try
            {
                 assembly = AssemblyLoadContext.Default.LoadFromAssemblyPath(file.FullName);
            }
            catch (FileLoadException ex)
            {
                if (ex.Message == "Assembly with same name is already loaded")
                {
                    // Get loaded assembly
                    assembly = Assembly.Load(new AssemblyName(Path.GetFileNameWithoutExtension(file.Name)));
                }
                else
                {
                    throw;
                }
            }

            if (assembly.FullName.Contains(moduleFolder.Name))
            {
                modules.Add(new ModuleInfo { Name = moduleFolder.Name, Assembly = assembly, Path = moduleFolder.FullName });
            }
        }
    }

Then module assemblies will be added to MVC by ApplicationPart

 var mvcBuilder = services.AddMvc();
    foreach (var module in modules)
    {
        // Register controller from modules
        mvcBuilder.AddApplicationPart(module.Assembly);
    }

1. For the view, a custom ModuleViewLocationExpander is used to help the view engine lookup up the right module folder the views

services.Configure<RazorViewEngineOptions>(options =>
    {
        options.ViewLocationExpanders.Add(new ModuleViewLocationExpander());
    });

public class ModuleViewLocationExpander : IViewLocationExpander
    {
        private const string _moduleKey = "module";

        public IEnumerable<string> ExpandViewLocations(ViewLocationExpanderContext context, IEnumerable<string> viewLocations)
        {
            if (context.Values.ContainsKey(_moduleKey))
            {
                var module = context.Values[_moduleKey];
                if (!string.IsNullOrWhiteSpace(module))
                {
                    var moduleViewLocations = new string[]
                    {
                       "/Modules/Modular.Modules." + module + "/Views/{1}/{0}.cshtml",
                       "/Modules/Modular.Modules." + module + "/Views/Shared/{0}.cshtml"
                    };

                    viewLocations = moduleViewLocations.Concat(viewLocations);
                }
            }
            return viewLocations;
        }

        public void PopulateValues(ViewLocationExpanderContext context)
        {
            var controller = context.ActionContext.ActionDescriptor.DisplayName;
            var moduleName = controller.Split('.')[2];
            if(moduleName != "WebHost")
            {
                context.Values[_moduleKey] = moduleName;
            }
        }
    }

1. Each module contains a ModuleInitializer.cs where services for that module is registered

 // Register dependency in modules
    var moduleInitializerInterface = typeof(IModuleInitializer);
    foreach(var module in modules)
    {
        // Register dependency in modules
        var moduleInitializerType = module.Assembly.GetTypes().Where(x => typeof(IModuleInitializer).IsAssignableFrom(x)).FirstOrDefault();
        if(moduleInitializerType != null && moduleInitializerType != typeof(IModuleInitializer))
        {
            var moduleInitializer = (IModuleInitializer)Activator.CreateInstance(moduleInitializerType);
            moduleInitializer.Init(services);
        }
    }

Please note that services within modules already registered by Autofac

1. And this is how I serve static files for modules

// Serving static file for modules
foreach(var module in modules)
{
    var wwwrootDir = new DirectoryInfo(Path.Combine(module.Path, "wwwroot"));
    if (!wwwrootDir.Exists)
    {
        continue;
    }

    app.UseStaticFiles(new StaticFileOptions()
    {
        FileProvider = new PhysicalFileProvider(wwwrootDir.FullName),
        RequestPath = new PathString("/"+ module.SortName)
    });
}

1. For Entity Framework
   Every domain entities need to inherit from Entity, then on the "OnModelCreating" method, we find them and register them to DbContext

 private static void RegisterEntities(ModelBuilder modelBuilder, IEnumerable<Type> typeToRegisters)
    {
        var entityTypes = typeToRegisters.Where(x => x.GetTypeInfo().IsSubclassOf(typeof(Entity)) && !x.GetTypeInfo().IsAbstract);
        foreach (var type in entityTypes)
        {
            modelBuilder.Entity(type);
        }
    }

Sometimes, we might also need to do some custom mappings for our model, let take look at the sample below

public class ModuleACustomModelBuilder : ICustomModelBuilder
    {
        public void Build(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Sample>()
                .Property(x => x.Name).HasColumnName("TestName");
        }
    }

All the classes that implement the ICustomModelBuilder will be hooked and called in the "OnModelCreating" of the ModularDbContext

 private static void RegisterCustomMappings(ModelBuilder modelBuilder, IEnumerable<Type> typeToRegisters)
    {
        var customModelBuilderTypes = typeToRegisters.Where(x => typeof(ICustomModelBuilder).IsAssignableFrom(x));
        foreach(var builderType in customModelBuilderTypes)
        {
            if (builderType != null && builderType != typeof(ICustomModelBuilder))
            {
                var builder = (ICustomModelBuilder)Activator.CreateInstance(builderType);
                builder.Build(modelBuilder);
            }
        }
    }

1. Strong typed view
   There is a known issue with 1.0.0 on how MVC finds compilation assemblies for class libraries. We can workaround by adding the modules assemblies to the list of compilation assemblies directly.

var mvcBuilder = services.AddMvc()
    .AddRazorOptions(o =>
    {
        foreach (var module in modules)
        {
            o.AdditionalCompilationReferences.Add(MetadataReference.CreateFromFile(module.Assembly.Location));
        }
    });

Yeah, and now we are done. Please checkout the source code for more details.