iOS – 查找视图的顶级约束？

我试图在代码中找到视图的顶级约束.
在故事板中添加了顶级约束,我不想使用IBOutlet.

在以下代码中记录firstAttribute的值似乎总是返回NSLayoutAttributeHeight类型的约束.任何想法如何可靠地找到代码中的视图的顶级约束？

NSLayoutConstraint *topConstraint;

for (NSLayoutConstraint *constraint in self.constraints) {
    if (constraint.firstAttribute == NSLayoutAttributeTop) {
        topConstraint = constraint;
        break;
    }
}

解决方法

而不是迭代self.constraints,你应该遍历self.superview.constraints.

自我约束只包含与视图相关的限制(例如,高度和宽度约束).

这是一个代码示例：

- (void)awakeFromNib
{
  [super awakeFromNib];

  if (!self.topConstraint) {
    [self findTopConstraint];
  }
}

- (void)findTopConstraint
{
  for (NSLayoutConstraint *constraint in self.superview.constraints) {
    if ([self isTopConstraint:constraint]) {
      self.topConstraint = constraint;
      break;
    }
  }
}

- (BOOL)isTopConstraint:(NSLayoutConstraint *)constraint
{
  return  [self firstItemMatchesTopConstraint:constraint] ||
          [self secondItemMatchesTopConstraint:constraint];
}

- (BOOL)firstItemMatchesTopConstraint:(NSLayoutConstraint *)constraint
{
  return constraint.firstItem == self && constraint.firstAttribute == NSLayoutAttributeTop;
}

- (BOOL)secondItemMatchesTopConstraint:(NSLayoutConstraint *)constraint
{
  return constraint.secondItem == self && constraint.secondAttribute == NSLayoutAttributeTop;
}