removeConstraint 无效

Question

相关代码：

[self.scrollView removeConstraint:self.introduceTitleLabelTopConstraint];
NSLayoutConstraint* constraint = [NSLayoutConstraint constraintWithItem:self.introduceTitleLabel attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.scrollView attribute:NSLayoutAttributeTop multiplier:1 constant:0];
[self.scrollView addConstraint:constraint];

相关日志：
其中倒数第二行是我新增的约束，最后一行是我删除的约束，这个日志为什么还要打印我已经删掉的约束？

2016-09-18 12:03:15.206 GamePlatform[48552:616181] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "<NSLayoutConstraint:0x7f8612190d90 UIImageView:0x7f86126062c0.top == UIView:self.noticeContainer.top>",
    "<NSLayoutConstraint:0x7f86121c60b0 UIView:self.noticeBackgroundView.top == UIImageView:0x7f86126062c0.top + 15>",
    "<NSLayoutConstraint:0x7f86121c6100 UIView:self.noticeContainer.bottom == UIView:self.noticeBackgroundView.bottom>",
    "<NSLayoutConstraint:0x7f861260b0a0 UIView:self.noticeContainer.top == UIScrollView:self.scrollView.top>",
    "<NSLayoutConstraint:0x7f86105e2550 ZLIconLabel:self.introduceTitleLabel.top == UIScrollView:self.scrollView.top>",
    "<NSLayoutConstraint:0x7f86121c0550 ZLIconLabel:self.introduceTitleLabel.top == UIView:self.noticeContainer.bottom>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7f86121c60b0 UIView:self.noticeBackgroundView.top == UIImageView:0x7f86126062c0.top + 15>

还有个问题就是，虽然界面上显示是正确的，为什么会出现 'Unable to simultaneously satisfy constraints.' 错误？

而且提示的 breaking constraint 跟我要修改的约束没有任何关联。

Answer 1

backButton.translatesAutoresizingMaskIntoConstraints = false
constLabelTop.active = false
NSLayoutConstraint(item: label, attribute: .CenterY, relatedBy: .Equal, toItem: backButton, attribute: .CenterY,multiplier: 1, constant: 0).active = true
self.view.layoutIfNeeded()
https://stackoverflow.com/que...