Refactor implementation pieces in prep for #230.

This splits each implementation piece into a separate object
so that it is easier to combine them.

lib/rspec/mocks/message_expectation.rb

@@ -25,7 +25,9 @@ def initialize(error_generator, expectation_ordering, expected_from, method_doub
         @failed_fast = nil
         @eval_context = nil
         @implementation = implementation
-        @values_to_return = nil
+
+        @initial_implementation_logic = nil
+        @terminal_implementation_logic = nil
       end
 
       # @private
@@ -78,8 +80,7 @@ def and_return(*values, &implementation)
           # TODO: deprecate `and_return { value }`
           @implementation = implementation
         else
-          @values_to_return = values
-          @implementation = build_implementation
+          self.terminal_implementation_logic = AndReturnImplementation.new(values)
         end
       end
 
@@ -154,7 +155,7 @@ def and_throw(*args)
       def and_yield(*args, &block)
         yield @eval_context = Object.new if block
         @args_to_yield << args
-        @implementation = build_implementation
+        self.initial_implementation_logic = AndYieldImplementation.new(@args_to_yield, @eval_context, @error_generator)
         self
       end
 
@@ -418,7 +419,7 @@ def increase_actual_received_count!
         @actual_received_count += 1
       end
 
-      protected
+    private
 
       def failed_fast?
         @failed_fast
@@ -435,12 +436,20 @@ def set_expected_received_count(relativity, n)
                                    end
       end
 
-      private
+      def initial_implementation_logic=(logic)
+        @initial_implementation_logic = logic
+        update_implementation
+      end
 
-      def build_implementation
-        Implementation.new(
-          @values_to_return, @args_to_yield,
-          @eval_context, @error_generator
+      def terminal_implementation_logic=(logic)
+        @terminal_implementation_logic = logic
+        update_implementation
+      end
+
+      def update_implementation
+        @implementation = Implementation.new(
+          @initial_implementation_logic,
+          @terminal_implementation_logic
         ).method(:call)
       end
     end
@@ -462,29 +471,16 @@ def negative_expectation_for?(message)
       end
     end
 
-    # Represents a configured implementation. Takes into account
-    # `and_return` and `and_yield` instructions.
+    # Handles the implementation of an `and_yield` declaration.
     # @private
-    class Implementation
-      def initialize(values_to_return, args_to_yield, eval_context, error_generator)
-        @values_to_return = values_to_return
+    class AndYieldImplementation
+      def initialize(args_to_yield, eval_context, error_generator)
         @args_to_yield = args_to_yield
         @eval_context = eval_context
         @error_generator = error_generator
       end
 
-      def call(*args_to_ignore, &block)
-        default_return_value = perform_yield(&block)
-        return default_return_value unless @values_to_return
-
-        if @values_to_return.size > 1
-          @values_to_return.shift
-        else
-          @values_to_return.first
-        end
-      end
-
-      def perform_yield(&block)
+      def call(*args_to_ignore, block)
         return if @args_to_yield.empty? && @eval_context.nil?
 
         @error_generator.raise_missing_block_error @args_to_yield unless block
@@ -498,5 +494,34 @@ def perform_yield(&block)
         value
       end
     end
+
+    # Handles the implementation of an `and_return` implementation.
+    # @private
+    class AndReturnImplementation
+      def initialize(values_to_return)
+        @values_to_return = values_to_return
+      end
+
+      def call(*args_to_ignore, block)
+        if @values_to_return.size > 1
+          @values_to_return.shift
+        else
+          @values_to_return.first
+        end
+      end
+    end
+
+    # Represents a configured implementation. Takes into account
+    # any number of sub-implementations.
+    # @private
+    class Implementation
+      def initialize(*implementations)
+        @implementations = implementations.compact
+      end
+
+      def call(*args, &block)
+        @implementations.map { |i| i.call(*args, block) }.last
+      end
+    end
   end
 end