drain：
In a reference-counted environment, releases and pops the receiver; in a garbage-collected environment, triggers garbage collection if the memory allocated since the last collection is greater than the current threshold.release
Decrements the receiver’s reference count. (required)在一个garbage collected(垃圾收集)环境里，release不做任何操作。 NSAutoreleasePool因此提供了一个 drain 方法，它在reference-counted(引用计数)环境中的行为和调用release一样, 但是在一个garbage collected(垃圾收集)环境中则触发garbage
collection(垃圾收集)动作 (if the memory allocated since the last collection is greater than the current threshold)。 因此通常你应该使用drain而不是release去释放一个autorelease pool。