iPad keyboard will not dismiss if modal view controller presentation style is UIModalPresentationFor

在ipad上present 一个 navigation controller 时，present后弹出的keyboard，不能够 dismiss 掉。即使 textfield resignFirstResponder，keyboard也不会消失。
Problem:
在iPad keyboard上发现一个问题，当viewController在navigation controller下以模态视图弹出，并且navigationController.modalPresentationStyle = UIModalPresentationFormSheet时，点击done按钮，软键盘不撤销，即使 textfield resignFirstResponder了。

Solution:
当模态视图在 UINavigationControlle下显示，我们需要设置 navigation controller的disablesAutomaticKeyboardDismissal属性，我们可以通过添加类别实现这个。

File: UINavigationController+KeyboardDismiss.h
#import 
@interface UINavigationController (KeyboardDismiss)
- (BOOL)disablesAutomaticKeyboardDismissal;
@end
File: UINavigationController+KeyboardDismiss.m
#import "UINavigationController+KeyboardDismiss.h"
@implementation UINavigationController(KeyboardDismiss)
- (BOOL)disablesAutomaticKeyboardDismissal
{
    return NO;
}
@end
在需要用到的ViewController文件中导入创建的类别即可。