Allow helper methods defined in the block's context to call DSL methods

Previously, it turns out that this wasn't possible, which made
refactoring code that used Docile to extract common helper methods,
and re-use them in different blocks, a painful experience.

This change should make it possible to extract methods from blocks
into the context around the block, and have those extracted helper
methods still able to call methods on the DSL object.

lib/docile/execution.rb

@@ -22,8 +22,13 @@ module Docile
           value_from_block = block_context.instance_variable_get(ivar)
           proxy_context.instance_variable_set(ivar, value_from_block)
         end
+
         proxy_context.instance_exec(*args, &block)
       ensure
+        if block_context.respond_to?(:__docile_undo_fallback__)
+          block_context.send(:__docile_undo_fallback__)
+        end
+
         block_context.instance_variables.each do |ivar|
           value_from_dsl_proxy = proxy_context.instance_variable_get(ivar)
           block_context.instance_variable_set(ivar, value_from_dsl_proxy)

lib/docile/fallback_context_proxy.rb

@@ -38,6 +38,31 @@ module Docile
     def initialize(receiver, fallback)
       @__receiver__ = receiver
       @__fallback__ = fallback
+
+      # Enables calling DSL methods from helper methods in the block's context
+      unless fallback.respond_to?(:method_missing)
+        # NOTE: There's no {#define_singleton_method} on Ruby 1.8.x
+        singleton_class = (class << fallback; self; end)
+
+        # instrument {#method_missing} on the block's context to fallback to
+        # the DSL object. This allows helper methods in the block's context to
+        # contain calls to methods on the DSL object.
+        singleton_class.
+          send(:define_method, :method_missing) do |method, *args, &block|
+            if receiver.respond_to?(method.to_sym)
+              receiver.__send__(method.to_sym, *args, &block)
+            else
+              super(method, *args, &block)
+            end
+          end
+
+        # instrument a helper method to remove the above instrumentation
+        singleton_class.
+          send(:define_method, :__docile_undo_fallback__) do
+            singleton_class.send(:remove_method, :method_missing)
+            singleton_class.send(:remove_method, :__docile_undo_fallback__)
+          end
+      end
     end
 
     # @return [Array<Symbol>]  Instance variable names, excluding

spec/docile_spec.rb

@@ -117,6 +117,94 @@ describe Docile do
       end
     end
 
+    context "when block's context has helper methods which call DSL methods" do
+      class BlockContextWithHelperMethods
+        def initialize(array_as_dsl)
+          @array_as_dsl = array_as_dsl
+        end
+
+        # Classic dynamic programming factorial, using the methods of {Array}
+        # as a DSL to implement it, via helper methods {#calculate_factorials}
+        # and {#save_factorials} which are defined in this class, so therefore
+        # outside the block.
+        def factorial_as_dsl_against_array(n)
+          Docile.dsl_eval(@array_as_dsl) { calculate_factorials(n) }.last
+        end
+
+        # Uses the helper method {#save_factorials} below.
+        def calculate_factorials(n)
+          (2..n).each { |i| save_factorial(i) }
+        end
+
+        # Uses the methods {Array#push} and {Array#at} as a DSL from a helper
+        # method defined in the block's context. Successfully calling this
+        # proves that we can find helper methods from outside the block, and
+        # then find DSL methods from inside those helper methods.
+        def save_factorial(i)
+          push(i * at(i - 1))
+        end
+      end
+
+      subject { context.method(:factorial_as_dsl_against_array) }
+
+      let(:context) { BlockContextWithHelperMethods.new(array_as_dsl) }
+
+      let(:array_as_dsl) { [1, 1] }
+
+      it "finds DSL methods within helper method defined in block's context" do
+        # see https://en.wikipedia.org/wiki/Factorial
+        [
+          [0,                  1],
+          [1,                  1],
+          [2,                  2],
+          [3,                  6],
+          [4,                 24],
+          [5,                120],
+          [6,                720],
+          [7,              5_040],
+          [8,             40_320],
+          [9,            362_880],
+          [10,         3_628_800],
+          [11,        39_916_800],
+          [12,       479_001_600],
+          [13,     6_227_020_800],
+          [14,    87_178_291_200],
+          [15, 1_307_674_368_000]
+        ].each do |n, expected_factorial|
+          array_as_dsl.replace([1, 1])
+          expect(subject.call(n)).to eq expected_factorial
+        end
+      end
+
+      it "removes fallback instrumentation from the DSL object after block" do
+        expect { subject.call(5) }.
+          not_to change { context.respond_to?(:method_missing) }.
+          from(false)
+      end
+
+      it "removes method to remove fallbacl from the DSL object after block" do
+        expect { subject.call(5) }.
+          not_to change { context.respond_to?(:__docile_undo_fallback__) }.
+          from(false)
+      end
+
+      context "when helper methods call methods that are undefined" do
+        let(:array_as_dsl) { "not an array" }
+
+        it "raises NoMethodError" do
+          expect { subject.call(5) }.
+            to raise_error(NoMethodError, /undefined method `at' /)
+        end
+
+        it "removes fallback instrumentation from the DSL object after block" do
+          expect { subject.call(5) rescue nil }.
+            not_to change { context.respond_to?(:method_missing) }.
+            from(false)
+        end
+      end
+    end
+
+    context "when DSL have NoMethod error inside" do
       class DSLWithNoMethod
         def initialize(b); @b = b; end
         attr_accessor :b
@@ -125,10 +213,10 @@ describe Docile do
         end
       end
 
-    context "when DSL have NoMethod error inside" do
+      it "raise NoMethodError error from nil" do
-      it "raise error from nil" do
         Docile.dsl_eval(DSLWithNoMethod.new(nil)) do
-          expect { push_element }.to raise_error(NoMethodError, /undefined method `push' (for|on) nil:NilClass/)
+          expect { push_element }.
+            to raise_error(NoMethodError, /undefined method `push' (for|on) nil:NilClass/)
         end
       end
     end