add a rough Spy::Instance#instead

lib/spy/instance.rb

@@ -21,6 +21,7 @@ def initialize(spied, original)
       @around_procs = []
       @call_history = []
       @strategy = Strategy.factory_build(self)
+      @instead = nil
     end
 
     def name
@@ -67,6 +68,10 @@ def after(&block)
       self
     end
 
+    def instead(&block)
+      @instead = block
+    end
+
     private
 
     def extract_visibility

lib/spy/instance/api/internal.rb

@@ -80,11 +80,15 @@ def build_method_call(receiver, *args, &block)
         end
 
         def call_and_record(receiver, args, opts = {}, &block)
-          record = opts[:record] || build_method_call(receiver, *args, &block)
-          @call_history << record
+          if @instead
+            @instead.call build_method_call(receiver, *args, &block)
+          else
+            record = opts[:record] || build_method_call(receiver, *args, &block)
+            @call_history << record
 
-          result = call_original(receiver, *args, &block)
-          record.result = result
+            result = call_original(receiver, *args, &block)
+            record.result = result
+          end
         end
 
         def call_original(receiver, *args, &block)

test/instead_test.rb

@@ -0,0 +1,74 @@
+require 'test_helper'
+
+class InsteadTest < Minitest::Spec
+  describe 'Spy::Instance#instead' do
+    after do
+      Spy.restore(:all)
+    end
+
+    it 'will run the instead block and not the original call' do
+      o = Object.new
+      o.instance_eval do
+        def incr; total += 1; end
+        def total; @total ||= 0; end
+        def total=(n); @total = n; end
+      end
+
+      Spy.on(o, :incr).instead do |mc|
+        mc.receiver.total += 2
+      end
+
+      o.incr
+
+      assert_equal 2, o.total
+    end
+
+    it 'will return the value returned by the block' do
+      o = Object.new
+      o.instance_eval do
+        def name; 'josh'; end
+      end
+
+      Spy.on(o, :name).instead { 'penny' }
+
+      assert_equal 'penny', o.name
+    end
+
+    it 'plays nicely with Spy::Instance#when' do
+      o = Object.new
+      o.instance_eval do
+        def value=(n); @value = n; end
+        def value; @value; end
+        def name; 'josh'; end
+      end
+
+      Spy.on(o, :name).when { o.value == 0 }.instead { 'penny' }
+
+      o.value = 0
+      assert_equal 'penny', o.name
+
+      o.value = 1
+      assert_equal 'josh', o.name
+    end
+
+    it 'allows multiple whens and insteads' do
+      skip 'havent implemented multiple whens yet'
+
+      o = Object.new
+      o.instance_eval do
+        def value=(n); @value = n; end
+        def value; @value; end
+        def name; 'josh'; end
+      end
+
+      Spy.on(o, :name).when { o.value == 0 }.instead { 'penny' }
+      Spy.on(o, :name).when { o.value == 1 }.instead { 'lauren' }
+
+      o.value = 0
+      assert_equal 'penny', o.name
+
+      o.value = 1
+      assert_equal 'lauren', o.name
+    end
+  end
+end