Configure services that reference other services

* Move service configuration from the Engine to Service
* Delegate configuration mechanics to internal Service::Configurator
* Delegate service building to the concrete Service classes, allowing
  them to configure composed services.
* Implement for the Mirror service.

2 files changed, 42 insertions, 0 deletions

diff --git a/lib/active_storage/service/configurator.rb b/lib/active_storage/service/configurator.rb
new file mode 100644
index 0000000000..5054e07ec7
--- /dev/null
+++ b/lib/active_storage/service/configurator.rb
@@ -0,0 +1,31 @@
+class ActiveStorage::Service::Configurator #:nodoc:
+  def initialize(service_name, configurations)
+    @service_name, @configurations = service_name.to_sym, configurations.symbolize_keys
+  end
+
+  def build
+    service_class.build(service_config.except(:service), @configurations)
+  end
+
+  private
+    def service_class
+      resolve service_class_name
+    end
+
+    def service_class_name
+      service_config.fetch :service do
+        raise "Missing Active Storage `service: …` configuration for #{service_config.inspect}"
+      end
+    end
+
+    def service_config
+      @configurations.fetch @service_name do
+        raise "Missing configuration for the #{@service_name.inspect} Active Storage service. Configurations available for #{@configurations.keys.inspect}"
+      end
+    end
+
+    def resolve(service_class_name)
+      require "active_storage/service/#{service_class_name.to_s.downcase}_service"
+      ActiveStorage::Service.const_get(:"#{service_class_name}Service")
+    end
+end
diff --git a/lib/active_storage/service/mirror_service.rb b/lib/active_storage/service/mirror_service.rb
index 7ec166aace..eec1f2af65 100644
--- a/lib/active_storage/service/mirror_service.rb
+++ b/lib/active_storage/service/mirror_service.rb
@@ -5,6 +5,17 @@ class ActiveStorage::Service::MirrorService < ActiveStorage::Service
 
   delegate :download, :exist?, :url, to: :primary
 
+  # Stitch together from named configuration.
+  def self.build(mirror_config, all_configurations) #:nodoc:
+    primary = ActiveStorage::Service.configure(mirror_config.fetch(:primary), all_configurations)
+
+    mirrors = mirror_config.fetch(:mirrors).collect do |service_name|
+      ActiveStorage::Service.configure(service_name.to_sym, all_configurations)
+    end
+
+    new primary: primary, mirrors: mirrors
+  end
+
   def initialize(primary:, mirrors:)
     @primary, @mirrors = primary, mirrors
   end