UI键盘回避和自动布局

鉴于 iOS 6 中对自动布局的关注以及 Apple 工程师的推荐（查看 WWDC 2012 视频）我们不再直接操作视图的框架，那么如何仅使用自动布局和 NSLayoutConstraint 来避免键盘呢？

Update

这看起来是一个合理的解决方案：键盘敏感布局示例 http://www.think-in-g.net/ghawk/blog/2012/09/practicing-auto-layout-an-example-of-keyboard-sensitive-layout/ (GitHub 源） https://github.com/ghawkgu/keyboard-sensitive-layout但我看到的一个潜在问题是，当用户旋转设备并且键盘已经在屏幕上时会发生什么？

这篇博文很棒，但我想提出一些改进建议。首先，您可以注册来观察帧变化，因此您不需要注册来观察显示和隐藏通知。其次，您应该将键盘的 CGRect 从屏幕坐标转换为视图坐标。最后，您可以复制 iOS 用于键盘本身的精确动画曲线，以便键盘和跟踪视图同步移动。

将所有这些放在一起，您将得到以下结果：

@interface MyViewController ()
// This IBOutlet holds a reference to the bottom vertical spacer
// constraint that positions the "tracking view",i.e., the view that
// we want to track the vertical motion of the keyboard
@property (weak, nonatomic) IBOutlet NSLayoutConstraint *bottomVerticalSpacerConstraint;
@end


@implementation MyViewController
-(void)viewDidLoad
{
  [super viewDidLoad];
  // register for notifications about the keyboard changing frame
  [[NSNotificationCenter defaultCenter] addObserver:self
                                           selector:@selector(keyboardWillChangeFrame:)
                                               name:UIKeyboardWillChangeFrameNotification
                                             object:self.view.window];
}

-(void)keyboardWillChangeFrame:(NSNotification*)notification
{
  NSDictionary * userInfo = notification.userInfo;
  UIViewAnimationCurve animationCurve  = [userInfo[UIKeyboardAnimationCurveUserInfoKey] intValue];
  NSTimeInterval duration = [userInfo[UIKeyboardAnimationDurationUserInfoKey] doubleValue];

  // convert the keyboard's CGRect from screen coords to view coords
  CGRect kbEndFrame = [self.view convertRect:[[userInfo objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue]
                                    fromView:self.view.window];
  CGRect kbBeginFrame = [self.view convertRect:[[userInfo objectForKey:UIKeyboardFrameBeginUserInfoKey] CGRectValue]
                                      fromView:self.view.window];
  CGFloat deltaKeyBoardOrigin = kbEndFrame.origin.y - kbBeginFrame.origin.y;

  // update the constant factor of the constraint governing your tracking view
  self.bottomVerticalSpacerConstraint.constant -= deltaKeyBoardOrigin;
  // tell the constraint solver it needs to re-solve other constraints.
  [self.view setNeedsUpdateConstraints];

  [UIView beginAnimations:nil context:NULL];
  [UIView setAnimationDuration:duration];
  [UIView setAnimationCurve:animationCurve];
  [UIView setAnimationBeginsFromCurrentState:YES];
  // within this animation block, force the layout engine to apply 
  // the new layout changes immediately, so that we
  // animate to that new layout. We need to use old-style
  // UIView animations to pass the curve type.
  [self.view layoutIfNeeded];
  [UIView commitAnimations];
}

-(void)dealloc {
  [[NSNotificationCenter defaultCenter] removeObserver:self
                                                  name:UIKeyboardWillChangeFrameNotification
                                                object:nil];
}
@end

只要您在键盘打开时不改变方向，这就会起作用。

这是一个答案如何模仿iOS 7上的键盘动画向数字键盘添加“完成”按钮？ https://stackoverflow.com/questions/18837166/how-to-mimic-keyboard-animation-on-ios-7-to-add-done-button-to-numeric-keyboar展示了如何正确模仿键盘动画曲线。

关于所有这些基于通知的解决方案要注意的最后一件事：如果应用程序中的其他屏幕也使用键盘，它们可能会产生意想不到的效果，因为只要视图控制器没有被释放，它仍然会收到通知，即使它的视图已卸载。解决此问题的一种方法是在通知处理程序中放置一个条件，以确保它仅在视图控制器位于屏幕上时才运行。