Add :condition option for .memoize method (#7)

Now you can specify a block for instance execute
whether to use memoization or not:

```ruby
def file_mtime
  File.mtime asset_file
end

memoize :file_mtime, condition: -> { environment == 'production' }
```

lib/memery.rb

@@ -20,9 +20,9 @@ def self.method_visibility(klass, method_name)
   end
 
   module ClassMethods
-    def memoize(method_name)
+    def memoize(method_name, condition: nil)
       prepend_memery_module!
-      define_memoized_method!(method_name)
+      define_memoized_method!(method_name, condition: condition)
     end
 
     private
@@ -33,14 +33,16 @@ def prepend_memery_module!
       prepend @_memery_module
     end
 
-    def define_memoized_method!(method_name)
+    def define_memoized_method!(method_name, condition: nil)
       mod_id = @_memery_module.object_id
       visibility = Memery.method_visibility(self, method_name)
       raise ArgumentError, "Method #{method_name} is not defined on #{self}" unless visibility
 
       @_memery_module.module_eval do
         define_method(method_name) do |*args, &block|
-          return super(*args, &block) if block
+          if block || (condition && !instance_exec(&condition))
+            return super(*args, &block)
+          end
 
           @_memery_memoized_values ||= {}
 

spec/memery_spec.rb

@@ -8,6 +8,8 @@
 class A
   include Memery
 
+  attr_accessor :environment
+
   memoize def m
     m_private
   end
@@ -21,6 +23,13 @@ class A
     [x, y]
   end
 
+  def m_condition
+    CALLS << __method__
+    __method__
+  end
+
+  memoize :m_condition, condition: -> { environment == "production" }
+
   protected
 
   memoize def m_protected
@@ -166,4 +175,30 @@ class << self
       expect { klass }.to raise_error(ArgumentError, /Method foo is not defined/)
     end
   end
+
+  describe ":condition option" do
+    before do
+      a.environment = environment
+    end
+
+    context "returns true" do
+      let(:environment) { "production" }
+
+      specify do
+        values = [ a.m_condition, a.m_nil, a.m_condition, a.m_nil ]
+        expect(values).to eq([:m_condition, nil, :m_condition, nil])
+        expect(CALLS).to eq([:m_condition, nil])
+      end
+    end
+
+    context "returns false" do
+      let(:environment) { "development" }
+
+      specify do
+        values = [ a.m_condition, a.m_nil, a.m_condition, a.m_nil ]
+        expect(values).to eq([:m_condition, nil, :m_condition, nil])
+        expect(CALLS).to eq([:m_condition, nil, :m_condition])
+      end
+    end
+  end
 end