iOS - 查找视图的顶部约束? [英] iOS - Find top constraint for a view?

问题描述

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

I am trying to find the top constraint of the view in code. The top constraint is added in storyboard, and I don't want to use an IBOutlet.

在以下代码中记录 firstAttribute 的值似乎总是返回 NSLayoutAttributeHeight 类型的约束.知道如何在代码中可靠地找到视图的最高约束吗?

Logging the value of the firstAttribute in the following code seems to always return a constraint of type NSLayoutAttributeHeight. Any idea how I could reliably find a top constraint of a view in code?

NSLayoutConstraint *topConstraint;

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

推荐答案

您应该遍历 self.superview.constraints，而不是遍历 self.constraints.

Instead of iterating through self.constraints, you should iterate through self.superview.constraints.

self.constraints 仅包含与视图相关的约束(例如高度和宽度约束).

The self.constraints only contain constraints related to just the view (e.g. height and width constraints).

下面是代码示例:

- (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;
}

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