Documented plug-in modules.

docs/en/PlugIn-Modules.md

@@ -1,3 +1,233 @@
 # Plug-In Modules
 
-TODO
+It is possible to load [modules](Module-Development-Basics.md) as plug-ins. That means you may not reference to a module's assembly in your solution, but you can load that module in the application startup just like any other module.
+
+## Basic Usage
+
+`IServiceCollection.AddApplication<T>()` extension method can get options to configure the plug-in sources.
+
+**Example: Load plugins from a folder**
+
+````csharp
+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.DependencyInjection;
+using Volo.Abp.Modularity.PlugIns;
+
+namespace MyPlugInDemo.Web
+{
+    public class Startup
+    {
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services.AddApplication<MyPlugInDemoWebModule>(options =>
+            {
+                options.PlugInSources.AddFolder(@"D:\Temp\MyPlugIns");
+            });
+        }
+
+        public void Configure(IApplicationBuilder app)
+        {
+            app.InitializeApplication();
+        }
+    }
+}
+````
+
+* This is the `Startup` class of a typical ASP.NET Core application.
+* `PlugInSources.AddFolder` gets a folder path and to load assemblies (typically `dll`s) in that folder.
+
+That's all. ABP will discover the modules in the given folder, configure and initialize them just like regular modules.
+
+### Plug-In Sources
+
+`options.PlugInSources` is actually a list of `IPlugInSource` implementations and `AddFolder` is just a shortcut for the following expression:
+
+````csharp
+options.PlugInSources.Add(new FolderPlugInSource(@"D:\Temp\MyPlugIns"));
+````
+
+> `AddFolder()` only looks for the assembly file in the given folder, but not looks for the sub-folders. You can pass `SearchOption.AllDirectories` as a second parameter to explore plug-ins also from the sub-folders, recursively.
+
+There are two more built-in Plug-In Source implementations:
+
+* `PlugInSources.AddFiles()` gets a list of assembly (typically `dll`) files. This is a shortcut of using `FilePlugInSource` class.
+* `PlugInSources.AddTypes()` gets a list of module class types. If you use this, you need to load the assemblies of the modules yourself, but it provides flexibility when needed. This is a shortcut of using `TypePlugInSource` class.
+
+If you need, you can create your own `IPlugInSource` implementation and add to the `options.PlugInSources` just like the others.
+
+## Example: Creating a Simple Plug-In
+
+Create a simple **Class Library Project** in a solution:
+
+![simple-plugin-library](images/simple-plugin-library.png)
+
+You can add ABP Framework packages you need to use in the module. At least, you should add the `Volo.Abp.Core` package to the project:
+
+````
+Install-Package Volo.Abp.Core
+````
+
+Every [module](Module-Development-Basics.md) must declare a class derived from the `AbpModule`. Here, a simple module class that resolves a service and initializes it on the application startup:
+
+````csharp
+using Microsoft.Extensions.DependencyInjection;
+using Volo.Abp;
+using Volo.Abp.Modularity;
+
+namespace MyPlugIn
+{
+    public class MyPlungInModule : AbpModule
+    {
+        public override void OnApplicationInitialization(ApplicationInitializationContext context)
+        {
+            var myService = context.ServiceProvider
+                .GetRequiredService<MyService>();
+            
+            myService.Initialize();
+        }
+    }
+}
+````
+
+`MyService` can be any class registered to [Dependency Injection](Dependency-Injection.md) system, as show below:
+
+````csharp
+using Microsoft.Extensions.Logging;
+using Volo.Abp.DependencyInjection;
+
+namespace MyPlugIn
+{
+    public class MyService : ITransientDependency
+    {
+        private readonly ILogger<MyService> _logger;
+
+        public MyService(ILogger<MyService> logger)
+        {
+            _logger = logger;
+        }
+
+        public void Initialize()
+        {
+            _logger.LogInformation("MyService has been initialized");
+        }
+    }
+}
+````
+
+Build the project, open the build folder, find the `MyPlugIn.dll`:
+
+![simple-plug-in-dll-file](images/simple-plug-in-dll-file.png)
+
+Copy `MyPlugIn.dll` into the plug-in folder (`D:\Temp\MyPlugIns` for this example).
+
+If you have configured the main application like described above (see Basic Usage section), you should see the `MyService has been initialized` log in the application startup.
+
+## Example: Creating a Plug-In With Razor Pages
+
+Creating plug-ins with views inside requires a bit more attention.
+
+> This example assumes you've [created a new web application](https://abp.io/get-started) using the application startup template and MVC / Razor Pages UI.
+
+Create a new **Class Library** project in a solution:
+
+![simple-razor-plugin](images/simple-razor-plugin.png)
+
+Edit the `.csproj` file content:
+
+````xml
+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+    <PropertyGroup>
+        <TargetFramework>net5.0</TargetFramework>
+        <OutputType>Library</OutputType>
+        <IsPackable>true</IsPackable>
+    </PropertyGroup>
+
+    <ItemGroup>
+      <PackageReference Include="Volo.Abp.AspNetCore.Mvc.UI.Theme.Shared" Version="4.0.1" />
+    </ItemGroup>
+
+</Project>
+````
+
+* Changed `Sdk` to `Microsoft.NET.Sdk.Web`.
+* Added `OutputType` and `IsPackable` properties.
+* Added `Volo.Abp.AspNetCore.Mvc.UI.Theme.Shared` NuGet package.
+
+> Depending on [Volo.Abp.AspNetCore.Mvc.UI.Theme.Shared](https://www.nuget.org/packages/Volo.Abp.AspNetCore.Mvc.UI.Theme.Shared) package is not required. You can reference to a more base package like [Volo.Abp.AspNetCore.Mvc](https://www.nuget.org/packages/Volo.Abp.AspNetCore.Mvc/). However, if you will build a UI page/component, it is suggested to reference to the [Volo.Abp.AspNetCore.Mvc.UI.Theme.Shared](https://www.nuget.org/packages/Volo.Abp.AspNetCore.Mvc.UI.Theme.Shared) package since it is the most high-level package without depending on a particular [theme](UI/AspNetCore/Theming.md). If there is no problem to depend on a particular theme, you can directly reference to the theme's package to be able to use the theme-specific features in your plug-in.
+
+Then create your module class in the plug-in:
+
+````csharp
+using System.IO;
+using System.Reflection;
+using Microsoft.AspNetCore.Mvc.ApplicationParts;
+using Microsoft.Extensions.DependencyInjection;
+using Volo.Abp.AspNetCore.Mvc.UI.Theme.Shared;
+using Volo.Abp.Modularity;
+
+namespace MyMvcUIPlugIn
+{
+    [DependsOn(typeof(AbpAspNetCoreMvcUiThemeSharedModule))]
+    public class MyMvcUIPlugInModule : AbpModule
+    {
+        public override void PreConfigureServices(ServiceConfigurationContext context)
+        {
+            PreConfigure<IMvcBuilder>(mvcBuilder =>
+            {
+                //Add plugin assembly
+                mvcBuilder.PartManager.ApplicationParts.Add(new AssemblyPart(typeof(MyMvcUIPlugInModule).Assembly));
+
+                //Add views assembly
+                var viewDllPath = Path.Combine(Path.GetDirectoryName(typeof(MyMvcUIPlugInModule).Assembly.Location), "MyMvcUIPlugIn.Views.dll");
+                var viewAssembly = new CompiledRazorAssemblyPart(Assembly.LoadFrom(viewDllPath));
+                mvcBuilder.PartManager.ApplicationParts.Add(viewAssembly);
+            });
+        }
+    }
+}
+````
+
+* Depending on the `AbpAspNetCoreMvcUiThemeSharedModule` since we added the related NuGet package.
+* Adding the plug-in's assembly to the `PartManager` of ASP.NET Core MVC. This is required by ASP.NET Core. Otherwise, your controllers inside the plug-in doesn't work.
+* Adding the plug-in's views assembly to the `PartManager` of ASP.NET Core MVC. This is required by ASP.NET Core. Otherwise, your views inside the plug-in doesn't work.
+
+You can now add a razor page, like `MyPlugInPage.cshtml` inside the `Pages` folder:
+
+````html
+@page
+@model MyMvcUIPlugIn.Pages.MyPlugInPage
+<h1>Welcome to my plug-in page</h1>
+<p>This page is located inside a plug-in module! :)</p>
+````
+
+Now, you can build the plug-in project. It will produce the following output:
+
+![simple-razor-plug-in-dll-file](images/simple-razor-plug-in-dll-file.png)
+
+Copy the `MyMvcUIPlugIn.dll` and `MyMvcUIPlugIn.Views.dll` into the plug-in folder (`D:\Temp\MyPlugIns` for this example).
+
+If you have configured the main application like described above (see Basic Usage section), you should be able to visit the `/MyPlugInPage` URL when your application:
+
+![simple-plugin-output](images/simple-plugin-output.png)
+
+## Discussions
+
+In real world, your plug-in may have some external dependencies. Also, your application might be designed to support plug-ins. All these are your own system requirements. What ABP does is just loading modules on the application startup. What you do inside that modules is up to you.
+
+However, we can provide a few suggestions for some common cases.
+
+### Library Dependencies
+
+For package/dll dependencies, you can copy the related dlls to the plug-in folder. ABP automatically loads all assemblies in the folder and your plug-in will work as expected.
+
+> See [Microsoft's documentation](https://docs.microsoft.com/en-us/dotnet/core/tutorials/creating-app-with-plugin-support#plugin-with-library-dependencies) for some additional explanations for that case.
+
+### Database Schema
+
+If your module uses a relational database and [Entity Framework Core](Entity-Framework-Core.md), it will need to have its tables available in the database. There are different ways to ensure the tables have been created when an application uses the plug-in. Some examples;
+
+1. The Plugin may check if the database tables does exists and create the tables on the application startup or migrate them if the plug-in has been updated and requires some schema changes. You can use EF Core's migration API to do that.
+2. You can improve the `DbMigrator` application to find migrations of the plug-ins and execute them.
+
+There may be other solutions. For example, if your DB admin doesn't allow you to change the database schema in the application code, you may need to manually send a SQL file to the database admin to apply it to the database.

docs/en/docs-nav.json

@@ -349,7 +349,8 @@
               "path": "Module-Development-Basics.md"
             },
             {
-              "text": "Plug-In Modules"
+              "text": "Plug-In Modules",
+              "path": "PlugIn-Modules.md"
             },
             {
               "text": "Customizing the Application Modules",