Add default: support for ActiveSupport::CurrentAttributes.attribute

Extend the `.attribute` class method to accept a `:default` option for
its list of attributes:

```ruby
class Current < ActiveSupport::CurrentAttributes
  attribute :counter, default: 0
end
```

Internally, `ActiveSupport::CurrentAttributes` will maintain a
`.defaults` class attribute to determine default values during instance
initialization.

Author: seanpdoyle

activesupport/CHANGELOG.md

@@ -1,3 +1,13 @@
+*   Add `default:` support for `ActiveSupport::CurrentAttributes.attribute`
+
+    ```ruby
+    class Current < ActiveSupport::CurrentAttributes
+      attribute :counter, default: 0
+    end
+    ```
+
+    *Sean Doyle*
+
 *   Yield instance to `Object#with` block
 
     ```ruby

activesupport/lib/active_support/current_attributes.rb

@@ -102,7 +102,14 @@ def instance
       end
 
       # Declares one or more attributes that will be given both class and instance accessor methods.
-      def attribute(*names)
+      #
+      # ==== Options
+      #
+      # * <tt>:default</tt> - The default value for the attributes. If the value
+      # is a proc or lambda, it will be called whenever an instance is
+      # constructed. Otherwise, the value will be duplicated with +#dup+.
+      # Default values are re-assigned when the attributes are reset.
+      def attribute(*names, default: nil)
         invalid_attribute_names = names.map(&:to_sym) & INVALID_ATTRIBUTE_NAMES
         if invalid_attribute_names.any?
           raise ArgumentError, "Restricted attribute names: #{invalid_attribute_names.join(", ")}"
@@ -126,6 +133,8 @@ def attribute(*names)
         end
 
         singleton_class.delegate(*names.flat_map { |name| [name, "#{name}="] }, to: :instance, as: self)
+
+        defaults.merge! names.index_with { default }
       end
 
       # Calls this callback before #reset is called on the instance. Used for resetting external collaborators that depend on current values.
@@ -177,10 +186,12 @@ def respond_to_missing?(name, _)
         end
     end
 
+    class_attribute :defaults, instance_writer: false, default: {}
+
     attr_accessor :attributes
 
     def initialize
-      @attributes = {}
+      @attributes = merge_defaults!({})
     end
 
     # Expose one or more attributes within a block. Old values are returned after the block concludes.
@@ -200,8 +211,21 @@ def set(attributes, &block)
     # Reset all attributes. Should be called before and after actions, when used as a per-request singleton.
     def reset
       run_callbacks :reset do
-        self.attributes = {}
+        self.attributes = merge_defaults!({})
       end
     end
+
+    private
+      def merge_defaults!(attributes)
+        defaults.each_with_object(attributes) do |(name, default), values|
+          value =
+            case default
+            when Proc then default.call
+            else default.dup
+            end
+
+          values[name] = value
+        end
+      end
   end
 end

activesupport/test/current_attributes_test.rb

@@ -11,6 +11,8 @@ class CurrentAttributesTest < ActiveSupport::TestCase
   Person = Struct.new(:id, :name, :time_zone)
 
   class Current < ActiveSupport::CurrentAttributes
+    attribute :counter_integer, default: 0
+    attribute :counter_callable, default: -> { 0 }
     attribute :world, :account, :person, :request
     delegate :time_zone, to: :person
 
@@ -86,6 +88,30 @@ def after_teardown
     assert_equal "world/1", Current.world
   end
 
+  test "read and write attribute with default value" do
+    assert_equal 0, Current.counter_integer
+
+    Current.counter_integer += 1
+
+    assert_equal 1, Current.counter_integer
+
+    Current.reset
+
+    assert_equal 0, Current.counter_integer
+  end
+
+  test "read attribute with default callable" do
+    assert_equal 0, Current.counter_callable
+
+    Current.counter_callable += 1
+
+    assert_equal 1, Current.counter_callable
+
+    Current.reset
+
+    assert_equal 0, Current.counter_callable
+  end
+
   test "read overwritten attribute method" do
     Current.request = "request/1"
     assert_equal "request/1 something", Current.request