Under that scenario, when the next time this view controller was appear again, the UIKit will be smart enough to restore the previous view state if it‘s not dealloced.
http://stackoverflow.com/questions/23075747/why-the-keyboard-was-automatically-popped-out-when-the-viewcontroller-was-pushed/23121140#23121140
界面自动弹出键盘的问题
原文：http://my.oschina.net/michael1986/blog/416991
