在 iPad 上，用 modalPresentationStyle = UIModalPresentationFormSheet 方式推出一个 viewController，这时这个 viewController 不会响应 resignFirstResponder，其他样式的 modalPresentationStyle 没有问题。苹果一个开发在开发者论坛说这是个 feature，不是 bug，devforums.apple.com

Was your view by any chance presented with the UIModalPresentationFormSheet style? To avoid frequent in-and-out animations, the keyboard will sometimes remain on-screen even when there is no first responder. This is not a bug.

就算不是 bug 也很恼人，有人给出了解决方法 devforums.apple.com。新建一个 UINavigationController category，禁掉 disablesAutomaticKeyboardDismissal：

- (BOOL)disablesAutomaticKeyboardDismissal
{
    return NO;
}

然后把 viewController 挂在 UINavigationController 下即可：

MyViewController *myViewController = [[MyViewController alloc] initWithNibName:@"MyViewController" bundle:nil];
UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:myViewController];
theNavigationController.modalPresentationStyle = UIModalPresentationFormSheet;
[self presentModalViewController:theNavigationController animated:YES];

SO 参考: