使用Method swizzling (也就是运行时交换两个方法的imp ，实现重写方法)

贴上资源。很简单

https://gist.github.com/rudyjahchan/2191796  

http://itony.me/592.html

http://stackoverflow.com/questions/5960049/callback-method-before-deallocating-any-object

http://justsee.iteye.com/blog/2019539

http://stackoverflow.com/questions/16811461/method-swizzling-for-alloc