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.

7 files changed, 212 insertions, 2 deletions

diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
index 88df1a9..944b8dc 100644
--- a/.rubocop_todo.yml
+++ b/.rubocop_todo.yml
@@ -13,7 +13,7 @@ Metrics/AbcSize:
 # Offense count: 2
 # Configuration parameters: CountComments.
 Metrics/ClassLength:
-  Max: 209
+  Max: 212
 
 # Offense count: 7
 Metrics/CyclomaticComplexity:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index b25e660..2d123ff 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,7 +12,7 @@ scheme are considered to be bugs.
 
 ### Added
 
-* Your contribution here.
+* [#323](https://github.com/intridea/hashie/pull/323): Added `Hashie::Extensions::Mash::DefineAccessors` - [@marshall-lee](https://github.com/marshall-lee).
 
 ### Changed
 
diff --git a/README.md b/README.md
index 6a88955..77456ac 100644
--- a/README.md
+++ b/README.md
@@ -644,6 +644,31 @@ end
 
 However, on Rubies less than 2.0, this means that every key you send to the Mash will generate a symbol. Since symbols are not garbage-collected on older versions of Ruby, this can cause a slow memory leak when using a symbolized Mash with data generated from user input.
 
+### Mash Extension:: DefineAccessors
+
+This extension can be mixed into a Mash so it makes it behave like `OpenStruct`. It reduces the overhead of `method_missing?` magic by lazily defining field accessors when they're requested.
+
+```ruby
+class MyHash < ::Hashie::Mash
+  include Hashie::Extensions::Mash::DefineAccessors
+end
+
+mash = MyHash.new
+MyHash.method_defined?(:foo=) #=> false
+mash.foo = 123
+MyHash.method_defined?(:foo=) #=> true
+
+MyHash.method_defined?(:foo) #=> false
+mash.foo #=> 123
+MyHash.method_defined?(:foo) #=> true
+```
+
+You can also extend the existing mash without defining a class:
+
+```ruby
+mash = ::Hashie::Mash.new.with_accessors!
+```
+
 ## Dash
 
 Dash is an extended Hash that has a discrete set of defined properties and only those properties may be set on the hash. Additionally, you can set defaults for each property. You can also flag a property as required. Required properties will raise an exception if unset. Another option is message for required properties, which allow you to add custom messages for required property.
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
diff --git a/spec/hashie/extensions/mash/define_accessors_spec.rb b/spec/hashie/extensions/mash/define_accessors_spec.rb
new file mode 100644
index 0000000..de7abe7
--- /dev/null
+++ b/spec/hashie/extensions/mash/define_accessors_spec.rb
@@ -0,0 +1,90 @@
+require 'spec_helper'
+
+describe Hashie::Extensions::Mash::DefineAccessors do
+  let(:args) { [] }
+
+  shared_examples 'class with dynamically defined accessors' do
+    it 'defines reader on demand' do
+      expect(subject.method_defined?(:foo)).to be_falsey
+      instance.foo
+      expect(subject.method_defined?(:foo)).to be_truthy
+    end
+
+    it 'defines writer on demand' do
+      expect(subject.method_defined?(:foo=)).to be_falsey
+      instance.foo = :bar
+      expect(subject.method_defined?(:foo=)).to be_truthy
+    end
+
+    it 'defines predicate on demand' do
+      expect(subject.method_defined?(:foo?)).to be_falsey
+      instance.foo?
+      expect(subject.method_defined?(:foo?)).to be_truthy
+    end
+
+    it 'defines initializing reader on demand' do
+      expect(subject.method_defined?(:foo!)).to be_falsey
+      instance.foo!
+      expect(subject.method_defined?(:foo!)).to be_truthy
+    end
+
+    it 'defines underbang reader on demand' do
+      expect(subject.method_defined?(:foo_)).to be_falsey
+      instance.foo_
+      expect(subject.method_defined?(:foo_)).to be_truthy
+    end
+
+    context 'when initializing from another hash' do
+      let(:args) { [{ foo: :bar }] }
+
+      it 'does not define any accessors' do
+        expect(subject.method_defined?(:foo)).to be_falsey
+        expect(subject.method_defined?(:foo=)).to be_falsey
+        expect(subject.method_defined?(:foo?)).to be_falsey
+        expect(subject.method_defined?(:foo!)).to be_falsey
+        expect(subject.method_defined?(:foo_)).to be_falsey
+        expect(instance.foo).to eq :bar
+      end
+    end
+  end
+
+  context 'when included in Mash subclass' do
+    subject { Class.new(Hashie::Mash) { include Hashie::Extensions::Mash::DefineAccessors } }
+    let(:instance) { subject.new(*args) }
+
+    describe 'this subclass' do
+      it_behaves_like 'class with dynamically defined accessors'
+
+      describe 'when accessors are overrided in class' do
+        before do
+          subject.class_eval do
+            def foo
+              if self[:foo] != 1
+                :bar
+              else
+                super
+              end
+            end
+          end
+        end
+
+        it 'allows to call super' do
+          expect(instance.foo).to eq :bar
+          instance.foo = 2
+          expect(instance.foo).to eq :bar
+          instance.foo = 1
+          expect(instance.foo).to eq 1
+        end
+      end
+    end
+  end
+
+  context 'when Mash instance is extended' do
+    let(:instance) { Hashie::Mash.new(*args).with_accessors! }
+    subject { instance.singleton_class }
+
+    describe 'its singleton class' do
+      it_behaves_like 'class with dynamically defined accessors'
+    end
+  end
+end