http://studyroom.g.hatena.ne.jp/tily/20101012/1286888791

other_class_instance_spec.rb

class HerClass
  def run; end
end

class MyClass
  def initialize; @her = HerClass.new end
  def exec;       @her.run            end
end

describe MyClass, "execメソッドが実行されたとき："  do

  # from http://d.hatena.ne.jp/kmrshntr/20100726/1280149431
  it "動く (けど、読みづらい気がする)" do
    @her_mock = mock("her")
    HerClass.should_receive(:new).and_return(@her_mock)
    @my = MyClass.new
    @her_mock.should_receive(:run)
    @my.exec
  end

  it "動かない (should_receive が Mock オブジェクトを返さないため)" do
    HerClass.should_receive(:new).and_return(
      mock("her").should_receive(:run)
    )
    @my = MyClass.new
    @my.exec
  end

  it "動く (Object#tap をつかって Mock オブジェクトをつかまえるやり方)" do
    HerClass.should_receive(:new).and_return(
      mock("her").tap {|m| m.should_receive(:run) }
    )   
    @my = MyClass.new
    @my.exec
  end 

  it "動かない (mock() がブロック引数をとればこう書けるんだけどな…)" do
    HerClass.should_receive(:new).and_return(
      mock("her") {|m| m.should_receive(:run) }
    )
    @my = MyClass.new
    @my.exec
  end

end