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

@interface ViewController()@property(weak, nonatomic) IBOutlet NSLayoutConstraint * bottomCons;//textV距離屏幕底部的約束，默認設置為0

@end

• (void) viewDidLoad { [super viewDidLoad];

  // 監(jiān)聽鍵盤 將要 彈出和隱藏
  [[NSNotificationCenter defaultCenter] addObserver: self selector: @selector(keyboardWillShowOrHide: ) name: UIKeyboardWillShowNotification object: nil];

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

}

/** 第三方鍵盤將要顯示時， 此方法會被調用 3次，鍵盤y坐標依次為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
即將隱藏時，拿到的鍵盤高度是292，所以代碼中直接寫死為 0
*/

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

  //注意，字典里存放的都是對象，要把對象轉為CGRect結構體，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: 修改底部視圖距離底部的間距,

  之前下面方法放在keyboardWillChangeFrame：方法中，導致第3次292時，_bottomCons.constant == 0不成立，于是_bottomCons.constant被重新賦值為0！這就是bug原因。解決辦法就是監(jiān)聽將要顯示和將要消失兩個通知！
  // _bottomCons.constant = (_bottomCons.constant == 0 ? endFrame.size.height :
  // 0);
  // 修改底部視圖距離底部的間距
  if ([note.name isEqualToString: @"UIKeyboardWillHideNotification"]) {
  _bottomCons.constant = 0;
  } else { // willShow
  _bottomCons.constant = keyboardFrame.size.height;
  }

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

@end