iphone release与drain

In a garbage-collected environment, there is no need for autorelease pools. You may, however, write a framework that is designed to work in both a garbage-collected and reference-counted environment. In this case, you can use autorelease pools to hint to the collector that collection may be appropriate. In a garbage-collected environment, sending a drain message to a pool triggers garbage collection if necessary; release, however, is a no-op. In a reference-counted environment, drain has the same effect as release. Typically, therefore, you should use drain instead of release.

在引用计数方式下，release与drain的效果是一样的。

在垃圾回收环境中，release是一个空操作。因此，NSAutoreleasePool提供了drain方法，在引用计数环境中，该方法的作用等同于调用release，但在垃圾回收环境中，它会触发垃圾回收（如果自上次垃圾回收以来分配的内存大于当前的阈值）。因此，在通常情况下，您应该使用drain而不是release来销毁自动释放池。