Start bringing attributes API to AM

This is the first PR of a WIP to bring the attributes API to
ActiveModel.  It is not yet ready for public API.

The `attributes_dirty_test.rb` file was created based on `dirty_test.rb`,
and the simplifications in the diff do much to motivate this change.

```
diff activemodel/test/cases/dirty_test.rb activemodel/test/cases/attributes_dirty_test.rb
3a4
> require "active_model/attributes"
5c6
< class DirtyTest < ActiveModel::TestCase
---
> class AttributesDirtyTest < ActiveModel::TestCase
7,41c8,12
<     include ActiveModel::Dirty
<     define_attribute_methods :name, :color, :size
<
<     def initialize
<       @name = nil
<       @color = nil
<       @size = nil
<     end
<
<     def name
<       @name
<     end
<
<     def name=(val)
<       name_will_change!
<       @name = val
<     end
<
<     def color
<       @color
<     end
<
<     def color=(val)
<       color_will_change! unless val == @color
<       @color = val
<     end
<
<     def size
<       @size
<     end
<
<     def size=(val)
<       attribute_will_change!(:size) unless val == @size
<       @size = val
<     end
---
>     include ActiveModel::Model
>     include ActiveModel::Attributes
>     attribute :name, :string
>     attribute :color, :string
>     attribute :size, :integer
```

3 files changed, 376 insertions, 0 deletions

diff --git a/activemodel/lib/active_model/attributes.rb b/activemodel/lib/active_model/attributes.rb
new file mode 100644
index 0000000000..3e34d3b83a
--- /dev/null
+++ b/activemodel/lib/active_model/attributes.rb
@@ -0,0 +1,89 @@
+# frozen_string_literal: true
+
+require "active_model/type"
+
+module ActiveModel
+  module Attributes #:nodoc:
+    extend ActiveSupport::Concern
+    include ActiveModel::AttributeMethods
+    include ActiveModel::Dirty
+
+    included do
+      attribute_method_suffix "="
+      class_attribute :attribute_types, :_default_attributes, instance_accessor: false
+      self.attribute_types = {}
+      self._default_attributes = {}
+    end
+
+    module ClassMethods
+      def attribute(name, cast_type = Type::Value.new, **options)
+        self.attribute_types = attribute_types.merge(name.to_s => cast_type)
+        self._default_attributes = _default_attributes.merge(name.to_s => options[:default])
+        define_attribute_methods(name)
+      end
+
+      private
+
+        def define_method_attribute=(name)
+          safe_name = name.unpack("h*".freeze).first
+          ActiveModel::AttributeMethods::AttrNames.set_name_cache safe_name, name
+
+          generated_attribute_methods.module_eval <<-STR, __FILE__, __LINE__ + 1
+            def __temp__#{safe_name}=(value)
+              name = ::ActiveModel::AttributeMethods::AttrNames::ATTR_#{safe_name}
+              write_attribute(name, value)
+            end
+            alias_method #{(name + '=').inspect}, :__temp__#{safe_name}=
+            undef_method :__temp__#{safe_name}=
+          STR
+        end
+    end
+
+    def initialize(*)
+      super
+      clear_changes_information
+    end
+
+    private
+
+      def write_attribute(attr_name, value)
+        name = if self.class.attribute_alias?(attr_name)
+          self.class.attribute_alias(attr_name).to_s
+        else
+          attr_name.to_s
+        end
+
+        cast_type = self.class.attribute_types[name]
+
+        deserialized_value = ActiveModel::Type.lookup(cast_type).cast(value)
+        attribute_will_change!(name) unless deserialized_value == attribute(name)
+        instance_variable_set("@#{name}", deserialized_value)
+        deserialized_value
+      end
+
+      def attribute(name)
+        if instance_variable_defined?("@#{name}")
+          instance_variable_get("@#{name}")
+        else
+          default = self.class._default_attributes[name]
+          default.respond_to?(:call) ? default.call : default
+        end
+      end
+
+      # Handle *= for method_missing.
+      def attribute=(attribute_name, value)
+        write_attribute(attribute_name, value)
+      end
+  end
+
+  module AttributeMethods #:nodoc:
+    AttrNames = Module.new {
+      def self.set_name_cache(name, value)
+        const_name = "ATTR_#{name}"
+        unless const_defined? const_name
+          const_set const_name, value.dup.freeze
+        end
+      end
+    }
+  end
+end
diff --git a/activemodel/test/cases/attributes_dirty_test.rb b/activemodel/test/cases/attributes_dirty_test.rb
new file mode 100644
index 0000000000..26b0e85db3
--- /dev/null
+++ b/activemodel/test/cases/attributes_dirty_test.rb
@@ -0,0 +1,193 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "active_model/attributes"
+
+class AttributesDirtyTest < ActiveModel::TestCase
+  class DirtyModel
+    include ActiveModel::Model
+    include ActiveModel::Attributes
+    attribute :name, :string
+    attribute :color, :string
+    attribute :size, :integer
+
+    def save
+      changes_applied
+    end
+
+    def reload
+      clear_changes_information
+    end
+  end
+
+  setup do
+    @model = DirtyModel.new
+  end
+
+  test "setting attribute will result in change" do
+    assert !@model.changed?
+    assert !@model.name_changed?
+    @model.name = "Ringo"
+    assert @model.changed?
+    assert @model.name_changed?
+  end
+
+  test "list of changed attribute keys" do
+    assert_equal [], @model.changed
+    @model.name = "Paul"
+    assert_equal ["name"], @model.changed
+  end
+
+  test "changes to attribute values" do
+    assert !@model.changes["name"]
+    @model.name = "John"
+    assert_equal [nil, "John"], @model.changes["name"]
+  end
+
+  test "checking if an attribute has changed to a particular value" do
+    @model.name = "Ringo"
+    assert @model.name_changed?(from: nil, to: "Ringo")
+    assert_not @model.name_changed?(from: "Pete", to: "Ringo")
+    assert @model.name_changed?(to: "Ringo")
+    assert_not @model.name_changed?(to: "Pete")
+    assert @model.name_changed?(from: nil)
+    assert_not @model.name_changed?(from: "Pete")
+  end
+
+  test "changes accessible through both strings and symbols" do
+    @model.name = "David"
+    assert_not_nil @model.changes[:name]
+    assert_not_nil @model.changes["name"]
+  end
+
+  test "be consistent with symbols arguments after the changes are applied" do
+    @model.name = "David"
+    assert @model.attribute_changed?(:name)
+    @model.save
+    @model.name = "Rafael"
+    assert @model.attribute_changed?(:name)
+  end
+
+  test "attribute mutation" do
+    @model.instance_variable_set("@name", "Yam".dup)
+    assert !@model.name_changed?
+    @model.name.replace("Hadad")
+    assert !@model.name_changed?
+    @model.name_will_change!
+    @model.name.replace("Baal")
+    assert @model.name_changed?
+  end
+
+  test "resetting attribute" do
+    @model.name = "Bob"
+    @model.restore_name!
+    assert_nil @model.name
+    assert !@model.name_changed?
+  end
+
+  test "setting color to same value should not result in change being recorded" do
+    @model.color = "red"
+    assert @model.color_changed?
+    @model.save
+    assert !@model.color_changed?
+    assert !@model.changed?
+    @model.color = "red"
+    assert !@model.color_changed?
+    assert !@model.changed?
+  end
+
+  test "saving should reset model's changed status" do
+    @model.name = "Alf"
+    assert @model.changed?
+    @model.save
+    assert !@model.changed?
+    assert !@model.name_changed?
+  end
+
+  test "saving should preserve previous changes" do
+    @model.name = "Jericho Cane"
+    @model.save
+    assert_equal [nil, "Jericho Cane"], @model.previous_changes["name"]
+  end
+
+  test "setting new attributes should not affect previous changes" do
+    @model.name = "Jericho Cane"
+    @model.save
+    @model.name = "DudeFella ManGuy"
+    assert_equal [nil, "Jericho Cane"], @model.name_previous_change
+  end
+
+  test "saving should preserve model's previous changed status" do
+    @model.name = "Jericho Cane"
+    @model.save
+    assert @model.name_previously_changed?
+  end
+
+  test "previous value is preserved when changed after save" do
+    assert_equal({}, @model.changed_attributes)
+    @model.name = "Paul"
+    assert_equal({ "name" => nil }, @model.changed_attributes)
+
+    @model.save
+
+    @model.name = "John"
+    assert_equal({ "name" => "Paul" }, @model.changed_attributes)
+  end
+
+  test "changing the same attribute multiple times retains the correct original value" do
+    @model.name = "Otto"
+    @model.save
+    @model.name = "DudeFella ManGuy"
+    @model.name = "Mr. Manfredgensonton"
+    assert_equal ["Otto", "Mr. Manfredgensonton"], @model.name_change
+    assert_equal @model.name_was, "Otto"
+  end
+
+  test "using attribute_will_change! with a symbol" do
+    @model.size = 1
+    assert @model.size_changed?
+  end
+
+  test "reload should reset all changes" do
+    @model.name = "Dmitry"
+    @model.name_changed?
+    @model.save
+    @model.name = "Bob"
+
+    assert_equal [nil, "Dmitry"], @model.previous_changes["name"]
+    assert_equal "Dmitry", @model.changed_attributes["name"]
+
+    @model.reload
+
+    assert_equal ActiveSupport::HashWithIndifferentAccess.new, @model.previous_changes
+    assert_equal ActiveSupport::HashWithIndifferentAccess.new, @model.changed_attributes
+  end
+
+  test "restore_attributes should restore all previous data" do
+    @model.name = "Dmitry"
+    @model.color = "Red"
+    @model.save
+    @model.name = "Bob"
+    @model.color = "White"
+
+    @model.restore_attributes
+
+    assert_not @model.changed?
+    assert_equal "Dmitry", @model.name
+    assert_equal "Red", @model.color
+  end
+
+  test "restore_attributes can restore only some attributes" do
+    @model.name = "Dmitry"
+    @model.color = "Red"
+    @model.save
+    @model.name = "Bob"
+    @model.color = "White"
+
+    @model.restore_attributes(["name"])
+
+    assert @model.changed?
+    assert_equal "Dmitry", @model.name
+    assert_equal "White", @model.color
+  end
+end
diff --git a/activemodel/test/cases/attributes_test.rb b/activemodel/test/cases/attributes_test.rb
new file mode 100644
index 0000000000..064cba40e3
--- /dev/null
+++ b/activemodel/test/cases/attributes_test.rb
@@ -0,0 +1,94 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "active_model/attributes"
+
+module ActiveModel
+  class AttributesTest < ActiveModel::TestCase
+    class ModelForAttributesTest
+      include ActiveModel::Model
+      include ActiveModel::Attributes
+
+      attribute :integer_field, :integer
+      attribute :string_field, :string
+      attribute :decimal_field, :decimal
+      attribute :string_with_default, :string, default: "default string"
+      attribute :date_field, :string, default: -> { Date.new(2016, 1, 1) }
+      attribute :boolean_field, :boolean
+    end
+
+    class ChildModelForAttributesTest < ModelForAttributesTest
+    end
+
+    class GrandchildModelForAttributesTest < ChildModelForAttributesTest
+      attribute :integer_field, :string
+    end
+
+    test "properties assignment" do
+      data = ModelForAttributesTest.new(
+        integer_field: "2.3",
+        string_field: "Rails FTW",
+        decimal_field: "12.3",
+        boolean_field: "0"
+      )
+
+      assert_equal 2, data.integer_field
+      assert_equal "Rails FTW", data.string_field
+      assert_equal BigDecimal.new("12.3"), data.decimal_field
+      assert_equal "default string", data.string_with_default
+      assert_equal Date.new(2016, 1, 1), data.date_field
+      assert_equal false, data.boolean_field
+
+      data.integer_field = 10
+      data.string_with_default = nil
+      data.boolean_field = "1"
+
+      assert_equal 10, data.integer_field
+      assert_nil data.string_with_default
+      assert_equal true, data.boolean_field
+    end
+
+    test "dirty" do
+      data = ModelForAttributesTest.new(
+        integer_field: "2.3",
+        string_field: "Rails FTW",
+        decimal_field: "12.3",
+        boolean_field: "0"
+      )
+
+      assert_equal false, data.changed?
+
+      data.integer_field = "2.1"
+
+      assert_equal false, data.changed?
+
+      data.string_with_default = "default string"
+
+      assert_equal false, data.changed?
+
+      data.integer_field = "5.1"
+
+      assert_equal true, data.changed?
+      assert_equal true, data.integer_field_changed?
+      assert_equal({ "integer_field" => [2, 5] }, data.changes)
+    end
+
+    test "nonexistent attribute" do
+      assert_raise ActiveModel::UnknownAttributeError do
+        ModelForAttributesTest.new(nonexistent: "nonexistent")
+      end
+    end
+
+    test "children inherit attributes" do
+      data = ChildModelForAttributesTest.new(integer_field: "4.4")
+
+      assert_equal 4, data.integer_field
+    end
+
+    test "children can override parents" do
+      data = GrandchildModelForAttributesTest.new(integer_field: "4.4")
+
+      assert_equal "4.4", data.integer_field
+    end
+  end
+end