mas_updateConstraints没有消除砌体中的约束

Question

我在UITableViewCell中使用Masonry，该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图像url，则显示或隐藏它。如果imageView被隐藏，我想将contentLabel到cell.contentView.bottom设置为另一个值，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);
    }];
  }

但我总是收到下面的错误消息：

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.

mas_updateConstraints似乎没有删除旧的约束，而是添加了一个新的约束，两者相互冲突。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，但它可能会帮助其他人：

我的问题主要如下：

我首先给UIView:的一个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"更新"此约束

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

这保留了原来的约束，并添加了一个与第一个约束冲突的新约束。

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)); 

…冲突结束了

显然，iOS Masonry无法将UIView定义的约束更新为NSNumber定义的约束。