Auto-inject dependencies into components

By using dry-system's auto_inject feature, we can get rid of the
boilerplate code that injects dependencies into components.

We create a `SolidusAdmin::Import` constant that can be used to dynamically
create a module that gets included into a component. This module will
provide instance methods to fetch the declared dependencies from the
component.

We also register a top level `main_nav_items` dependency to avoid
calling a private in nature `.within_namespace` method.

[1] - https://dry-rb.org/gems/dry-system/1.0/dependency-auto-injection/

admin/app/components/solidus_admin/base_component.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "solidus_admin/system/import"
+
 module SolidusAdmin
   # BaseComponent is the base class for all components in Solidus Admin.
   class BaseComponent < ViewComponent::Base

admin/app/components/solidus_admin/main_nav_component.rb

@@ -3,16 +3,15 @@
 module SolidusAdmin
   # Renders the main navigation of Solidus Admin.
   class MainNavComponent < BaseComponent
-    def initialize(main_nav_item_component: component("main_nav_item"), items: container.within_namespace("main_nav"))
-      @main_nav_item_component = main_nav_item_component
-      @items = items
-      super
-    end
+    include Import[
+      "main_nav_item_component",
+      items: "main_nav_items"
+    ]
 
     erb_template <<~ERB
       <nav>
         <%=
-          render @main_nav_item_component.with_collection(
+          render main_nav_item_component.with_collection(
             sorted_items
           )
         %>
@@ -22,7 +21,7 @@ def initialize(main_nav_item_component: component("main_nav_item"), items: conta
     private
 
     def sorted_items
-      @items.sort_by(&:position)
+      items.sort_by(&:position)
     end
   end
 end

admin/lib/solidus_admin/container.rb

@@ -23,6 +23,8 @@ class Container < Dry::System::Container
     end
 
     # Returns all the registered components for a given namespace.
+    #
+    # @api private
     def self.within_namespace(namespace)
       keys.filter_map do
         _1.start_with?("#{namespace}#{config.namespace_separator}") && resolve(_1)

admin/lib/solidus_admin/system/import.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+require "solidus_admin/container"
+
+module SolidusAdmin
+  Import = Container.injector
+end

admin/lib/solidus_admin/system/providers/main_nav.rb

@@ -12,6 +12,10 @@ module Providers
           register("second_item", MainNavItem.new(title: "Second item", position: "20"))
           register("third_item", MainNavItem.new(title: "Third item", position: "30"))
         end
+
+        container.register("main_nav_items") do
+          Container.within_namespace("main_nav")
+        end
       end
     end
   end