视图控制对象的生命周期与内存过低警告

1. 视图控制对象应该在什么时候载入视图？

    When the view controller receive the  view message, the view  controller begin to creat the view, if the veiw object does not exist.

    If you send the message of view in view controller init method, this leads the pattern deferred mechanism does not work。

    This  problem is seem not serious, but this situation is very serious when considering  low memory warnings.

2. when syetem is not much memory to use, the  system will send low  memmory warnings. Receiving the warning, release the not use, and easy rebulit resourece.