Merge pull request #353 from SAFE-Stack/dependency-injection

Add DI recipe

Author: isaacabraham

docs/recipes/patterns/add-dependency-injection.md

@@ -0,0 +1,54 @@
+> This recipe is not a detailed discussion of the pros and cons of Dependency Injection (DI) compared to other patterns. It simply illustrates how to use it within a SAFE Stack application!
+
+1. Create a class that you wish to inject with a dependency (in this example, we use the built-in `IConfiguration` type that is included in ASP .NET):
+
+    ```fsharp
+    open Microsoft.Extensions.Configuration
+
+    type DatabaseRepository(config:IConfiguration) =
+        member _.SaveDataToDb (text:string) =
+            let connectionString = config["SqlConnectionString"]
+            // Connect to SQL using the above connection string etc.
+            Ok 1
+    ```
+
+    > Instead of functions or modules, DI in .NET and F# only works with classes.
+
+2. Register your type with ASP .NET during startup within the `application { }` block.
+    ```diff
+    ++ open Microsoft.Extensions.DependencyInjection
+
+       application {
+           //...
+    ++     service_config (fun services -> services.AddSingleton<DatabaseRepository>())
+    ```
+
+    > [This section](https://learn.microsoft.com/en-us/aspnet/core/fundamentals/dependency-injection?view=aspnetcore-8.0#lifetime-and-registration-options) of the official ASP .NET Core article explain the distinction between different lifetime registrations, such as Singleton and Transient.
+
+3. Ensure that your Fable Remoting API can access the `HttpContext` type by using the `fromContext` builder function.
+    ```diff
+    --  |> Remoting.fromValue createFableRemotingApi
+    ++  |> Remoting.fromContext createFableRemotingApi
+    ```
+
+4. Within your Fable Remoting API, use the supplied `context` to retrieve your dependency:
+
+    ```diff
+    ++ open Microsoft.AspNetCore.Http
+
+       let createFableRemotingApi
+    ++     (context:HttpContext) =
+    ++     let dbRepository = context.GetService<DatabaseRepository>()
+           // ...
+           // Return the constructed API record value...
+    ```
+
+    > Giraffe provides the `GetService<'T>` extension to allow you to quickly retrieve a dependency from the `HttpContext`.
+
+    This will instruct ASP .NET to get a handle to the `DatabaseRepository` object; ASP .NET will automatically supply the `IConfiguration` object to the constructor. Whether a new `DatabaseRepository` object is constructed on each call depends on the lifetime you have registered it with.
+
+> You can have your types depend on other types that you create, as long as they are registering into ASP .NET Core's DI container using methods such as `AddSingleton` etc.
+
+## Further Reading
+* [Official documentation on DI in ASP .NET Core](https://learn.microsoft.com/en-us/aspnet/core/fundamentals/dependency-injection?view=aspnetcore-8.0)
+* [Archived example PR to update the SAFE Template Todo App to use DI](https://github.com/SAFE-Stack/SAFE-template/pull/466/files)

mkdocs.yml

@@ -42,7 +42,7 @@ extra:
   analytics:
     provider: google
     property: G-WED2S9FLSL
-    
+
 nav:
   - Home: "index.md"
   - Introduction: "intro.md"
@@ -107,6 +107,8 @@ nav:
           - Migrate to Paket from NuGet: "recipes/package-management/migrate-to-paket.md"
           - Migrate to NuGet from Paket: "recipes/package-management/migrate-to-nuget.md"
           - Sync NuGet and NPM Packages: "recipes/package-management/sync-nuget-and-npm-packages.md"
+      - Patterns:
+          - Use Dependency Injection: "recipes/patterns/add-dependency-injection.md"
       - Client / Server:
           - Use Giraffe instead of Saturn: "recipes/client-server/saturn-to-giraffe.md"
           - Handle server errors on the client: "recipes/client-server/server-errors-on-client.md"