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+This file includes details for developers contributing to this code base.
+
+## Code Architecture
+
+This package follows a simple dependency-injection pattern. There are two types of components within this pattern. Both are registered in [the Plugin class](/includes/classes/Plugin.php).
+
+1. Modules: Modules are loaded on every execution. They implement the `Module` interface and include a `register` method that performs setup actions. The `register` method is called automatically when the module is instantiated.
+2. Services: Services are loaded when they are declared as dependencies for Modules and other Services. They implement the `Service` interface, but if a service should only be instantiated once and shared throughout the system, it should instead implement the `SharedService` interface.
+
+### Declaring Dependencies
+
+Once a Service has been declared in [the Plugin class](/includes/classes/Plugin.php) and implements either `Service` or `SharedService`, it will automatically be passed to any module or service in the system that declares it via a type hint in its own constructor. For example:
+
+
+```PHP
+// Service class added to the services array in Plugin.php
+class MyService implements Service {
+    public function say_hello() {
+        echo 'hello';
+    }
+}
+
+class MyModule implements Module {
+    private $my_service;
+
+    public function construct( Service $my_service ) { // Automatically passed in.
+        $this->my_service = $my_service;
+    }
+
+    public function register() {
+        // setup actions
+    }
+
+    public function greet() {
+        $this->my_service->say_hello();
+    }
+}
+```
+
+A dependency can also be an interface. The system only allows one implementation of an interface to be registered at a time and will automatically detect the registered service class that implements that interface. For example:
+
+```PHP
+interface MyInterface {
+    public function say_hello();
+}
+
+class MyService implements Service, MyInterface {
+    public function say_hello() {
+        echo 'hello';
+    }
+}
+
+class MyModule implements Module {
+    private $my_service;
+
+    public function construct( MyInterface $my_service ) { // Automatically passed in. The dependency injector will detect the concrete class that implements the interface.
+        $this->my_service = $my_service;
+    }
+
+    public function register() {
+        // setup actions
+    }
+
+    public function greet() {
+        $this->my_service->say_hello();
+    }
+}
+```
+
+## Unit Testing
+
+Unit tests run inside a GitHub Action on Pull Requests. Running tests locally requires Docker. Once Docker is running, run tests with the following commands:
+
+```bash
+npm install # Installs the @wordpress/env package.
+npm run env-start # Starts the WordPress environment.
+npm run test:php # Runs the PHP unit tests.
+```