Add Hashie::Extensions::Mash::DefineAccessors.

This patch adds an extension for Mash that makes it behave like
`OpenStruct`. It reduces overhead of `method_missing?` magic which is a
good thing! It's inspired by the recent @sferik's work on `OpenStruct` —
https://github.com/ruby/ruby/pull/1033.

When using it in `Mash` subclasses it makes them *remember* methods so
then it's more like `ActiveModel` than `OpenStruct` in this case.

To use it like `OpenStruct` one could use this shortcut:

```ruby
{ foo: 1, bar: 2 }.to_mash.with_accessors!
```

Implementation details:

It injects to class an anonymous module that stores accessor method
definitions. This is inspired by `ActiveModel` / `ActiveRecord`. It
allows to override accessors in subclass and call them via `super` if
this is intended.

3 files changed, 95 insertions, 0 deletions

diff --git a/lib/hashie.rb b/lib/hashie.rb
index b6e1d13..fd93e05 100644
--- a/lib/hashie.rb
+++ b/lib/hashie.rb
@@ -47,6 +47,7 @@ module Hashie
       autoload :KeepOriginalKeys, 'hashie/extensions/mash/keep_original_keys'
       autoload :SafeAssignment, 'hashie/extensions/mash/safe_assignment'
       autoload :SymbolizeKeys, 'hashie/extensions/mash/symbolize_keys'
+      autoload :DefineAccessors, 'hashie/extensions/mash/define_accessors'
     end
 
     module Array
diff --git a/lib/hashie/extensions/mash/define_accessors.rb b/lib/hashie/extensions/mash/define_accessors.rb
new file mode 100644
index 0000000..f13ea2e
--- /dev/null
+++ b/lib/hashie/extensions/mash/define_accessors.rb
@@ -0,0 +1,90 @@
+module Hashie
+  module Extensions
+    module Mash
+      module DefineAccessors
+        def self.included(klass)
+          klass.class_eval do
+            mod = Ext.new
+            include mod
+          end
+        end
+
+        def self.extended(obj)
+          included(obj.singleton_class)
+        end
+
+        class Ext < Module
+          def initialize
+            mod = self
+            define_method(:method_missing) do |method_name, *args, &block|
+              key, suffix = method_name_and_suffix(method_name)
+              case suffix
+              when '='.freeze
+                mod.define_writer(key, method_name)
+              when '?'.freeze
+                mod.define_predicate(key, method_name)
+              when '!'.freeze
+                mod.define_initializing_reader(key, method_name)
+              when '_'.freeze
+                mod.define_underbang_reader(key, method_name)
+              else
+                mod.define_reader(key, method_name)
+              end
+              send(method_name, *args, &block)
+            end
+          end
+
+          def define_reader(key, method_name)
+            define_method(method_name) do |&block|
+              if key? method_name
+                self.[](method_name, &block)
+              else
+                self.[](key, &block)
+              end
+            end
+          end
+
+          def define_writer(key, method_name)
+            define_method(method_name) do |value = nil|
+              if key? method_name
+                self.[](method_name, &proc)
+              else
+                assign_property(key, value)
+              end
+            end
+          end
+
+          def define_predicate(key, method_name)
+            define_method(method_name) do
+              if key? method_name
+                self.[](method_name, &proc)
+              else
+                !!self[key]
+              end
+            end
+          end
+
+          def define_initializing_reader(key, method_name)
+            define_method(method_name) do
+              if key? method_name
+                self.[](method_name, &proc)
+              else
+                initializing_reader(key)
+              end
+            end
+          end
+
+          def define_underbang_reader(key, method_name)
+            define_method(method_name) do
+              if key? method_name
+                self.[](key, &proc)
+              else
+                underbang_reader(key)
+              end
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/hashie/mash.rb b/lib/hashie/mash.rb
index 6e45c09..f41d745 100644
--- a/lib/hashie/mash.rb
+++ b/lib/hashie/mash.rb
@@ -119,6 +119,10 @@ module Hashie
       end
     end
 
+    def with_accessors!
+      extend Hashie::Extensions::Mash::DefineAccessors
+    end
+
     alias to_s inspect
 
     # If you pass in an existing hash, it will