define WeakObj(o) __weak typeof(o) weak##o = o;

@interface ViewController()@property(weak, nonatomic) IBOutlet NSLayoutConstraint * bottomCons;//textV距离屏幕底部的约束，默认设置为0

@end

• (void) viewDidLoad { [super viewDidLoad];

  // 监听键盘 将要 弹出和隐藏
  [[NSNotificationCenter defaultCenter] addObserver: self selector: @selector(keyboardWillShowOrHide: ) name: UIKeyboardWillShowNotification object: nil];

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

}

/** 第三方键盘将要显示时， 此方法会被调用 3次，键盘y坐标依次为0，226，292:
2016-08-04 14:50:40.810 YZInputViewDemo[883:260581] 0.000000
2016-08-04 14:50:40.976 YZInputViewDemo[883:260581] 226.000000
2016-08-04 14:50:41.001 YZInputViewDemo[883:260581] 292.000000
即将隐藏时，拿到的键盘高度是292，所以代码中直接写死为 0
*/

• (void) keyboardWillShowOrHide: (NSNotification * ) note {

  //注意，字典里存放的都是对象，要把对象转为CGRect结构体，使用CGRectValue方法。
  CGRect keyboardFrame = [note.userInfo[UIKeyboardFrameEndUserInfoKey] CGRectValue];
  CGFloat duration = [note.userInfo[UIKeyboardAnimationDurationUserInfoKey] floatValue];

  WeakObj(self);
  NSLog(@"%f", keyboardFrame.size.height);

  warning: 修改底部视图距离底部的间距,

  之前下面方法放在keyboardWillChangeFrame：方法中，导致第3次292时，_bottomCons.constant == 0不成立，于是_bottomCons.constant被重新赋值为0！这就是bug原因。解决办法就是监听将要显示和将要消失两个通知！
  // _bottomCons.constant = (_bottomCons.constant == 0 ? endFrame.size.height :
  // 0);
  // 修改底部视图距离底部的间距
  if ([note.name isEqualToString: @"UIKeyboardWillHideNotification"]) {
  _bottomCons.constant = 0;
  } else { // willShow
  _bottomCons.constant = keyboardFrame.size.height;
  }

  [UIView animateWithDuration: duration animations: ^{ [weakself.view layoutIfNeeded];
  }];
  }

@end