iOS-当键盘出现的时候，如何让UITextField自动上移

在我们的开发中，大多数情况下都需要用到文本框输入，当键盘出现的时候，如何让UITextField自动上移这个算是常见需求，对此我也参考了一些唐巧等一些大牛的资料。
主要是通过增加对键盘出现和消失的相应的Notification，然后在键盘出现和消息的时候，通过设置相关控件的frame来实现。相关代码如下：

-(void)textFieldDidBeginEditing:(UITextField *)sender
{
    if ([sender isEqual:_textField])
    {
        //move the main view, so that the keyboard does not hide it.
        if  (self.view.frame.origin.y >= 0)
        {
            [self setViewMovedUp:YES];
        }
    }
}

//method to move the view up/down whenever the keyboard is shown/dismissed
-(void)setViewMovedUp:(BOOL)movedUp
{
    [UIView beginAnimations:nil context:NULL];
    [UIView setAnimationDuration:0.5]; // if you want to slide up the view
    
    CGRect rect = self.view.frame;
    if (movedUp)
    {
        // 1. move the view's origin up so that the text field that will be hidden come above the keyboard
        // 2. increase the size of the view so that the area behind the keyboard is covered up.
        rect.origin.y -= kOFFSET_FOR_KEYBOARD;
        rect.size.height += kOFFSET_FOR_KEYBOARD;
    }
    else
    {
        // revert back to the normal state.
        rect.origin.y += kOFFSET_FOR_KEYBOARD;
        rect.size.height -= kOFFSET_FOR_KEYBOARD;
    }
    self.view.frame = rect;
    
    [UIView commitAnimations];
}

- (void)keyboardWillShow:(NSNotification *)notif
{
    //keyboard will be shown now. depending for which textfield is active, move up or move down the view appropriately
    
    if ([_textField isFirstResponder] && self.view.frame.origin.y >= 0)
    {
        [self setViewMovedUp:YES];
    }
    else if (![_textField isFirstResponder] && self.view.frame.origin.y < 0)
    {
        [self setViewMovedUp:NO];
    }
}

- (void)viewWillAppear:(BOOL)animated
{
    // register for keyboard notifications
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWillShow:)
                                                 name:UIKeyboardWillShowNotification object:self.view.window];
}

- (void)viewWillDisappear:(BOOL)animated
{
    // unregister for keyboard notifications while not visible.
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIKeyboardWillShowNotification object:nil];
}