Make Adapters registerable so they are not namespace-constrained

Changes:

- Introduce Adapter::get for use by Serializer.adapter
- Move Adapter-finding logic from Adapter::adapter_class into Adapter::get

Introduced interfaces:

- non-inherited methods
```ruby
ActiveModel::Serializer::Adapter.adapter_map     # a Hash<adapter_name, adapter_class>
ActiveModel::Serializer::Adapter.adapters        # an Array<adapter_name>
ActiveModel::Serializer::Adapter.register(name, klass) # adds an adapter to the adapter_map
ActiveModel::Serializer::Adapter.get(name_or_klass)    # raises Argument error when adapter not found
```

- Automatically register adapters when subclassing

```ruby
      def self.inherited(subclass)
        ActiveModel::Serializer::Adapter.register(subclass.to_s.demodulize, subclass)
      end
```

- Preserves subclass method `::adapter_class(adapter)`

```ruby
      def self.adapter_class(adapter)
        ActiveModel::Serializer::Adapter.get(adapter)
      end
```

- Serializer.adapter now uses `Adapter.get(config.adapter)` rather than have duplicate logic

test/adapter_test.rb

@@ -19,16 +19,6 @@ module ActiveModel
         assert_equal @serializer, @adapter.serializer
       end
 
-      def test_adapter_class_for_known_adapter
-        klass = ActiveModel::Serializer::Adapter.adapter_class(:json_api)
-        assert_equal ActiveModel::Serializer::Adapter::JsonApi, klass
-      end
-
-      def test_adapter_class_for_unknown_adapter
-        klass = ActiveModel::Serializer::Adapter.adapter_class(:json_simple)
-        assert_nil klass
-      end
-
       def test_create_adapter
         adapter = ActiveModel::Serializer::Adapter.create(@serializer)
         assert_equal ActiveModel::Serializer::Adapter::FlattenJson, adapter.class

test/serializers/adapter_for_test.rb

@@ -1,6 +1,8 @@
 module ActiveModel
   class Serializer
     class AdapterForTest < Minitest::Test
+      UnknownAdapterError = ::ActiveModel::Serializer::Adapter::UnknownAdapterError
+
       def setup
         @previous_adapter = ActiveModel::Serializer.config.adapter
       end
@@ -20,6 +22,7 @@ module ActiveModel
         adapter = ActiveModel::Serializer.adapter
         assert_equal ActiveModel::Serializer::Adapter::Null, adapter
       ensure
+        ActiveModel::Serializer.config.adapter = @previous_adapter
       end
 
       def test_overwrite_adapter_with_class
@@ -32,7 +35,7 @@ module ActiveModel
       def test_raises_exception_if_invalid_symbol_given
         ActiveModel::Serializer.config.adapter = :unknown
 
-        assert_raises ArgumentError do
+        assert_raises UnknownAdapterError do
           ActiveModel::Serializer.adapter
         end
       end
@@ -40,10 +43,123 @@ module ActiveModel
       def test_raises_exception_if_it_does_not_know_hot_to_infer_adapter
         ActiveModel::Serializer.config.adapter = 42
 
-        assert_raises ArgumentError do
+        assert_raises UnknownAdapterError do
           ActiveModel::Serializer.adapter
         end
       end
+
+      def test_adapter_class_for_known_adapter
+        klass = ActiveModel::Serializer::Adapter.adapter_class(:json_api)
+        assert_equal ActiveModel::Serializer::Adapter::JsonApi, klass
+      end
+
+      def test_adapter_class_for_unknown_adapter
+        assert_raises UnknownAdapterError do
+          ActiveModel::Serializer::Adapter.adapter_class(:json_simple)
+        end
+      end
+
+      def test_adapter_map
+        expected_adapter_map = {
+          'json'.freeze              => ActiveModel::Serializer::Adapter::Json,
+          'json_api'.freeze          => ActiveModel::Serializer::Adapter::JsonApi,
+          'flatten_json'.freeze      => ActiveModel::Serializer::Adapter::FlattenJson,
+          'null'.freeze              => ActiveModel::Serializer::Adapter::Null
+        }
+        assert_equal ActiveModel::Serializer::Adapter.adapter_map, expected_adapter_map
+      end
+
+      def test_adapters
+        assert_equal ActiveModel::Serializer::Adapter.adapters.sort, [
+          'flatten_json'.freeze,
+          'json'.freeze,
+          'json_api'.freeze,
+          'null'.freeze
+        ]
+      end
+
+      def test_get_adapter_by_string_name
+        assert_equal ActiveModel::Serializer::Adapter.get('json'.freeze), ActiveModel::Serializer::Adapter::Json
+      end
+
+      def test_get_adapter_by_symbol_name
+        assert_equal ActiveModel::Serializer::Adapter.get(:json), ActiveModel::Serializer::Adapter::Json
+      end
+
+      def test_get_adapter_by_class
+        klass = ActiveModel::Serializer::Adapter::Json
+        assert_equal ActiveModel::Serializer::Adapter.get(klass), klass
+      end
+
+      def test_get_adapter_from_environment_registers_adapter
+        ActiveModel::Serializer::Adapter.const_set(:AdapterFromEnvironment, Class.new)
+        klass = ::ActiveModel::Serializer::Adapter::AdapterFromEnvironment
+        name = 'adapter_from_environment'.freeze
+        assert_equal ActiveModel::Serializer::Adapter.get(name), klass
+        assert ActiveModel::Serializer::Adapter.adapters.include?(name)
+      ensure
+        ActiveModel::Serializer::Adapter.adapter_map.delete(name)
+        ActiveModel::Serializer::Adapter.send(:remove_const, :AdapterFromEnvironment)
+      end
+
+      def test_get_adapter_for_unknown_name
+        assert_raises UnknownAdapterError do
+          ActiveModel::Serializer::Adapter.get(:json_simple)
+        end
+      end
+
+      def test_adapter
+        assert_equal ActiveModel::Serializer.config.adapter, :flatten_json
+        assert_equal ActiveModel::Serializer.adapter, ActiveModel::Serializer::Adapter::FlattenJson
+      end
+
+      def test_register_adapter
+        new_adapter_name  = :foo
+        new_adapter_klass = Class.new
+        ActiveModel::Serializer::Adapter.register(new_adapter_name, new_adapter_klass)
+        assert ActiveModel::Serializer::Adapter.adapters.include?('foo'.freeze)
+        assert ActiveModel::Serializer::Adapter.get(:foo), new_adapter_klass
+      ensure
+        ActiveModel::Serializer::Adapter.adapter_map.delete(new_adapter_name.to_s)
+      end
+
+      def test_inherited_adapter_hooks_register_adapter
+        Object.const_set(:MyAdapter, Class.new)
+        my_adapter = MyAdapter
+        ActiveModel::Serializer::Adapter.inherited(my_adapter)
+        assert_equal ActiveModel::Serializer::Adapter.get(:my_adapter), my_adapter
+      ensure
+        ActiveModel::Serializer::Adapter.adapter_map.delete('my_adapter'.freeze)
+        Object.send(:remove_const, :MyAdapter)
+      end
+
+      def test_inherited_adapter_hooks_register_demodulized_adapter
+        Object.const_set(:MyNamespace, Module.new)
+        MyNamespace.const_set(:MyAdapter, Class.new)
+        my_adapter = MyNamespace::MyAdapter
+        ActiveModel::Serializer::Adapter.inherited(my_adapter)
+        assert_equal ActiveModel::Serializer::Adapter.get(:my_adapter), my_adapter
+      ensure
+        ActiveModel::Serializer::Adapter.adapter_map.delete('my_adapter'.freeze)
+        MyNamespace.send(:remove_const, :MyAdapter)
+        Object.send(:remove_const, :MyNamespace)
+      end
+
+      def test_inherited_adapter_hooks_register_subclass_of_registered_adapter
+        Object.const_set(:MyAdapter, Class.new)
+        my_adapter = MyAdapter
+        Object.const_set(:MySubclassedAdapter, Class.new(MyAdapter))
+        my_subclassed_adapter = MySubclassedAdapter
+        ActiveModel::Serializer::Adapter.inherited(my_adapter)
+        ActiveModel::Serializer::Adapter.inherited(my_subclassed_adapter)
+        assert_equal ActiveModel::Serializer::Adapter.get(:my_adapter), my_adapter
+        assert_equal ActiveModel::Serializer::Adapter.get(:my_subclassed_adapter), my_subclassed_adapter
+      ensure
+        ActiveModel::Serializer::Adapter.adapter_map.delete('my_adapter'.freeze)
+        ActiveModel::Serializer::Adapter.adapter_map.delete('my_subclassed_adapter'.freeze)
+        Object.send(:remove_const, :MyAdapter)
+        Object.send(:remove_const, :MySubclassedAdapter)
+      end
     end
   end
 end

test/test_helper.rb

@@ -39,6 +39,15 @@ else
 end
 
 require 'active_model_serializers'
+# eager load autoloaded adapters
+# rubocop:disable Lint/Void
+require 'active_model/serializer/adapter'
+ActiveModel::Serializer::Adapter::Null
+ActiveModel::Serializer::Adapter::Json
+ActiveModel::Serializer::Adapter::FlattenJson
+ActiveModel::Serializer::Adapter::JsonApi
+# rubocop:enable Lint/Void
+require 'active_model/serializer/adapter'
 
 require 'support/stream_capture'