带有键盘类型数字键盘的键盘在UITextField iOS上出现错误

Question

单个UITextField的应用程序在我保持键盘打开文本字段时会出现以下错误，并切换到另一个应用程序，同时在该应用程序中打开键盘。

错误：

Cannot snapshot view (<UIKeyboardImpl: 0x10040aa20; frame = (0 0; 414 226); layer = <CALayer: 0x174030d20>>) with afterScreenUpdates:NO, because the view is not in a window. Use afterScreenUpdates:YES.

以上错误会隐藏文本字段中的键盘。

这是我的代码

@implementation ViewController

BOOL isKeyboardOnScreen;
CGFloat currentKeyboardHeight;

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    UITapGestureRecognizer *tapRecognizer = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(closeKeyboard)];
    [tapRecognizer setNumberOfTapsRequired:1];
    tapRecognizer.cancelsTouchesInView = YES;
    [self.view addGestureRecognizer:tapRecognizer];
}

- (void)closeKeyboard {
    [self.view endEditing:YES];
}

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillShow:)
                                                 name:UIKeyboardWillShowNotification
                                               object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillHide:)
                                                 name:UIKeyboardWillHideNotification
                                               object:nil];

}

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];

    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIKeyboardWillShowNotification object:nil];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIKeyboardWillHideNotification object:nil];
}

#pragma mark - keyboard movements

- (void)keyboardWillShow:(NSNotification *)notification {
    if (!isKeyboardOnScreen) {
        isKeyboardOnScreen = YES;
        CGSize keyboardSize = [[[notification userInfo] objectForKey:UIKeyboardFrameBeginUserInfoKey] CGRectValue].size;
        if ([txtPhone isFirstResponder]) {
            [UIView animateWithDuration:0.0 animations:^{
                CGRect f = loginView.frame; //self.view.frame;
                f.origin.y = f.origin.y-keyboardSize.height;
                loginView.frame = f;
                currentKeyboardHeight = keyboardSize.height;
            }];
        }
    }
}

- (void)keyboardWillHide:(NSNotification *)notification {
    if (isKeyboardOnScreen) {
        isKeyboardOnScreen = NO;
        if ([txtPhone isFirstResponder]) {
            [UIView animateWithDuration:0.0 animations:^{
                CGRect f = loginView.frame; //self.view.frame;
                f.origin.y = f.origin.y+currentKeyboardHeight;
                loginView.frame = f;
            }];
        }
    }
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}

@end

我也尝试使用UIApplicationDidEnterBackgroundNotification，但没有工作。