当键盘出现的时候，如何让UITextField自动上移

对于iPhone界面控件的操作应该算是开发中必备的能力。键盘出现的时候上移相关的控件算是常见的需求，但是从这么多人问这个问题就可以看出，还是有很多人对这些需求的实现方式有疑问。

对于这个问题，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，来源自stackoverflow。

-(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];}