Application Lifecycle
After application:didFinishLaunchingWithOptions:, then what?
Application enters a “run loop” repeatedly doing the following ...
An autorelease pool is created (more on this in a moment)
Application waits for events (touch, timed event, I/O, etc.)
Events are dispatched through UIKit objects and often on to your objects (via delegates, etc.)
When all is done, the screen is updated (appropriate drawRect: methods are called)
The autorelease pool is drained
Rinse, repeat.
注：两次autorelease 对象会导致自动释放池最终两次release 该对象，可能会使程序crash。
