mas_updateConstraints没有消除砌体中的约束

Question

我在我的UITableViewCell中使用UITableViewCell，这个单元格有两个子视图，一个是contentLabel，另一个是imageView。虽然imageView并不总是存在，但如果单元格有一个映像url，则呈现它或隐藏它。如果imageView是隐藏的，我希望将contentLabel设置为cell.contentView.bottom为另一个值，如下所示：

  [_contentLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.left.equalTo(self.contentView.mas_left);
    make.top.equalTo(self.contentView.mas_bottom).offset(20);
    make.right.equalTo(self.contentView.mas_right).offset(-6);
    _bottomConstraint = make.bottom.equalTo(_imageView.mas_top).offset(-14);
  }];

  [_imageView mas_makeConstraints:^(MASConstraintMaker *make) {
    make.left.equalTo(_contentLabel.mas_left);
    make.bottom.equalTo(self.contentView.mas_bottom).offset(-14);
  }];

  if (tweet.imageUrl) {
    _imageView.hidden = NO;
    [_imageView sd_setImageWithURL:tweet.imageUrl placeholderImage:[UIImage imageNamed:@"loading"] options:0];
  } else {
    _imageView.hidden = YES;
    [_imageView sd_setImageWithURL:NULL];
  }

  if (_imageView.hidden) {
    [_contentLabel mas_updateConstraints:^(MASConstraintMaker *make) {
      make.bottom.equalTo(_imageView.mas_top).offset(0);
    }];
  }

但我总是在下面得到错误信息：

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. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<MASLayoutConstraint:0x7fef9cd178d0 UILabel:0x7fef9cd437e0.bottom == UIImageView:0x7fef9cd26260.top - 14>",
    "<MASLayoutConstraint:0x7fef9caf5430 UILabel:0x7fef9cd437e0.bottom == UIImageView:0x7fef9cd26260.top>"
)

Will attempt to recover by breaking constraint 
<MASLayoutConstraint:0x7fef9caf5430 UILabel:0x7fef9cd437e0.bottom == UIImageView:0x7fef9cd26260.top>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

这似乎是mas_updateConstraints并没有删除旧的约束，而是添加了一个新的约束，这两个约束相互冲突。那么，如何在运行时动态更新约束值呢？

Answer 1

也许这不是你的问题，但它可能会帮助别人：

我的问题主要有以下几点：

我首先为UIView的子类分配了一个高度约束：

make.height.equalTo(label); // label is another subclass of UIView

后来，我尝试使用以下方法“更新”此约束：

make.height.equalTo(@(newHeight)); // newHeight is a CGFloat  

这留下了原来的约束，并添加了一个与第一个约束相冲突的新约束。

然后，我将第一个约束的分配更改为：

CGFloat labelHeight = label.frame.size.height;
make.height.equalTo(@(labelHeight)); 

…冲突就消失了。

显然，iOS砌体不能用UIView定义的约束更新NSNumber定义的约束。