title: what's different with windowShouldClose and windowWillClose?
body: <p>From the names , one is  will  , one is should  ,
  When I implement windowShouldClose  it worked fine.
  When I implement windowWillClose    it worked fine.</p>
  
  <p>But , when I implement two of them  , only windowShouldClose work  , why?</p>