Add finalizer to weak instance cache

lib/roo/helpers/weak_instance_cache.rb

@@ -22,11 +22,20 @@ def instance_cache(key)
 
         unless object
           object = yield
+          ObjectSpace.define_finalizer(object, instance_cache_finalizer(key))
           instance_variable_set(key, WeakRef.new(object))
         end
 
         object
       end
+
+      def instance_cache_finalizer(key)
+        proc do
+          if instance_variable_defined?(key) && (ref = instance_variable_get(key)) && !ref.weakref_alive?
+            remove_instance_variable(key)
+          end
+        end
+      end
     end
   end
 end

spec/lib/roo/weak_instance_cache_spec.rb

@@ -0,0 +1,69 @@
+require 'spec_helper'
+
+describe Roo::Helpers::WeakInstanceCache do
+  let(:klass) do
+    Class.new do
+      include Roo::Helpers::WeakInstanceCache
+
+      def memoized_data
+        instance_cache(:@memoized_data) do
+          "Some Costly Operation" * 1_000
+        end
+      end
+    end
+  end
+
+  subject do
+    klass.new
+  end
+
+  it 'should be lazy' do
+    expect(subject.instance_variables).to_not include(:@memoized_data)
+    data = subject.memoized_data
+    expect(subject.instance_variables).to include(:@memoized_data)
+  end
+
+
+  it 'should be memoized' do
+    data = subject.memoized_data
+    expect(subject.memoized_data).to equal(data)
+  end
+
+  it 'should recalculate after GC' do
+    GC.disable
+    original_id = subject.memoized_data.object_id
+    expect(subject.memoized_data.object_id).to eq(original_id)
+
+    GC.start
+    expect(subject.memoized_data.object_id).not_to eq(original_id)
+  end
+
+  it 'must remove instance variable' do
+    expect(subject.instance_variables).to_not include(:@memoized_data)
+    GC.disable
+    subject.memoized_data
+    expect(subject.instance_variables).to include(:@memoized_data)
+
+    GC.start
+    expect(subject.instance_variables).to_not include(:@memoized_data)
+  end
+
+  context '#inspect must not raise' do
+    it 'before calculation' do
+      expect{subject.inspect}.to_not raise_error
+    end
+    it 'after calculation' do
+      GC.disable
+      subject.memoized_data
+      expect{subject.inspect}.to_not raise_error
+      expect(subject.inspect).to include("Some Costly Operation")
+      GC.start
+    end
+    it 'after GC' do
+      subject.memoized_data
+      GC.start
+      expect{subject.inspect}.to_not raise_error
+      expect(subject.inspect).to_not include("Some Costly Operation")
+    end
+  end
+end