iOS键盘弹出遮挡输入框问题

当键盘弹出时有可能遮挡住输入框，直接给出一个简单的解决方案
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    //注册键盘弹起与收起通知
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillShow:)
                                                 name:UIKeyboardWillShowNotification
                                               object:nil];
    
}
-(void)keyboardWillShow:(NSNotification *)note
{
    NSDictionary *info = [note userInfo];
    CGSize keyboardSize = [[info objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue].size;
    _keyBoardHeight = keyboardSize.height;
    
    
    CGRect frame = _userNameTextfield.frame;
    int offset = frame.origin.y + 32 - (self.view.frame.size.height -_keyBoardHeight);
    NSTimeInterval animationDuration = 0.30f;
    [UIView beginAnimations:@"ResizeForKeyboard" context:nil];
    [UIView setAnimationDuration:animationDuration];
        //将视图的Y坐标向上移动offset个单位，以使下面腾出地方用于软键盘的显示
    if(offset > 0)
        self.view.frame = CGRectMake(0.0f, -offset, self.view.frame.size.width, self.view.frame.size.height);
    [UIView commitAnimations];
}  
//当用户按下return键或者按回车键，keyboard消失
-(BOOL)textFieldShouldReturn:(UITextField *)textField
{
    [textField resignFirstResponder];
    return YES;
}
 
//输入框编辑完成以后，将视图恢复到原始状态
-(void)textFieldDidEndEditing:(UITextField *)textField
{
    NSTimeInterval animationDuration = 0.30f;
    [UIView beginAnimations:@"ResizeForKeyboard" context:nil];
    [UIView setAnimationDuration:animationDuration];
    self.view.frame =CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height);
    [UIView commitAnimations];
}