Add page service type

the page service can be used to initialize and call a service. This type is validating that the service exists and that it inherit from BasePageService.

Author: kulturbande

app/types/alchemy/page_service_type.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module Alchemy
+  class PageServiceType < ActiveModel::Type::Value
+    def cast(value)
+      return nil if value.nil?
+
+      value.constantize
+    end
+
+    def assert_valid_value(value)
+      return if value.nil?
+
+      begin
+        klass = value.constantize
+      rescue NameError
+        raise ArgumentError, "Service class \"#{value}\" could not be found. Make sure it is defined and available."
+      end
+
+      unless klass < BasePageService
+        raise ArgumentError, "Service class \"#{value}\" must be a subclass of Alchemy::BasePageService."
+      end
+    end
+  end
+end

spec/types/alchemy/page_service_type_spec.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+module Alchemy
+  RSpec.describe PageServiceType do
+    subject(:type) { described_class.new }
+
+    # A class without a #call method
+    let(:non_service_class) do
+      Class.new
+    end
+
+    before do
+      stub_const("NotAService", non_service_class)
+    end
+
+    describe "#cast" do
+      context "with nil" do
+        it "returns nil" do
+          expect(type.cast(nil)).to be_nil
+        end
+      end
+
+      context "with a valid class name" do
+        it "returns the class constant" do
+          expect(type.cast("DummyPageService")).to eq(DummyPageService)
+        end
+      end
+    end
+
+    describe "#assert_valid_value" do
+      it "accepts nil" do
+        expect { type.assert_valid_value(nil) }.not_to raise_error
+      end
+
+      it "accepts a valid service class name" do
+        expect { type.assert_valid_value("DummyPageService") }.not_to raise_error
+      end
+
+      it "raises for a non-existent class" do
+        expect { type.assert_valid_value("DoesNotExist") }.to raise_error(
+          ArgumentError, /could not be found/
+        )
+      end
+
+      it "raises for a class that is not a subclass of BasePageService" do
+        expect { type.assert_valid_value("NotAService") }.to raise_error(
+          ArgumentError, /must be a subclass of Alchemy::BasePageService/
+        )
+      end
+    end
+  end
+end