为什么不记录约束标识符？

Question

我在chid视图控制器上设置了4个约束：

        childController.view.translatesAutoresizingMaskIntoConstraints = false
        heightConstraint = childController.view.heightAnchor.constraint(equalToConstant: 800.0)
        leadingConstraint = childController.view.leadingAnchor.constraint(equalTo: self.view.leadingAnchor)
        trailingConstraint = childController.view.trailingAnchor.constraint(equalTo: self.view.trailingAnchor)
        topViewUpperConstraint = childController.view.topAnchor.constraint(equalTo: bottomLayoutGuide.topAnchor, constant: -44)
        topViewUpperConstraint.identifier = "topViewUpperConstraint"

        NSLayoutConstraint.activate([heightConstraint, leadingConstraint, trailingConstraint, topViewUpperConstraint])

我的子视图控制器中有一个表视图，我希望它仅在我的子视图控制器topViewUpperContraint.constant相对于其bottomLayoutGuide.topAnchor superView时可滚动}等于-500。

我在UIScrollViewDelegate方法中遍历子视图控制器的约束，以找到标识为"topViewUpperConstraint"的约束，如下所示：

func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
    let viewConstraints = view.constraints
    print("*** The number of constraint on the main view is: \(viewConstraints.count)")

    for constraint in viewConstraints{
        print("*** Constraint identifer is: \(constraint.identifier)")
        if constraint.identifier == "topViewUpperConstraint" {
            if constraint.constant == -500{
                tableView.isScrollEnabled = true
            } else {
                tableView.isScrollEnabled = false
            }
        }
    }
}

但是，在子视图控制器的约束集中，标识符"topViewUpperConstraint"没有任何约束。此外，在主视图上设置了13个约束，其中我只设置了4个。你能解释一下我在这里缺少什么吗？

Answer 1

激活约束时，会将其添加到约束中引用的两个视图的公共祖先的 constraints 属性中。在您的示例中，您将视图的顶部约束到bottomLayoutGuide的顶部，因此约束将添加到视图层次结构中的共同祖先，这可能是您视图的超级视图。

有13个限制条件，因为自动布局正在为您不会看到的内容设置约束，并且没有像 bottomLayoutGuide 那样明确添加。< / p>

如果您在OS view.constraints循环而不添加任何自己的约束

ViewController

您将看到for constraint in view.constraints { print("Item1: \(constraint.firstItem), Item2: \(constraint.secondItem)") } 个约束：

8

包括*** The number of constraint on the main view is: 8 Item1: <_UILayoutGuide: 0x7fcedcd07e20; frame = (0 0; 0 20); hidden = YES; layer = <CALayer: 0x608000025020>>, Item2: nil Item1: <_UILayoutGuide: 0x7fcedcd07e20; frame = (0 0; 0 20); hidden = YES; layer = <CALayer: 0x608000025020>>, Item2: Optional(<UIView: 0x7fcedcd07a70; frame = (0 0; 414 736); autoresize = W+H; layer = <CALayer: 0x608000024fc0>>) Item1: <_UILayoutGuide: 0x7fcedce05530; frame = (0 736; 0 0); hidden = YES; layer = <CALayer: 0x600000024ca0>>, Item2: nil Item1: <_UILayoutGuide: 0x7fcedce05530; frame = (0 736; 0 0); hidden = YES; layer = <CALayer: 0x600000024ca0>>, Item2: Optional(<UIView: 0x7fcedcd07a70; frame = (0 0; 414 736); autoresize = W+H; layer = <CALayer: 0x608000024fc0>>) Item1: <UIView: 0x7fcedcd07a70; frame = (0 0; 414 736); autoresize = W+H; layer = <CALayer: 0x608000024fc0>>, Item2: nil Item1: <UIView: 0x7fcedcd07a70; frame = (0 0; 414 736); autoresize = W+H; layer = <CALayer: 0x608000024fc0>>, Item2: nil Item1: <UIView: 0x7fcedcd07a70; frame = (0 0; 414 736); autoresize = W+H; layer = <CALayer: 0x608000024fc0>>, Item2: nil Item1: <UIView: 0x7fcedcd07a70; frame = (0 0; 414 736); autoresize = W+H; layer = <CALayer: 0x608000024fc0>>, Item2: nil 本身的4个约束， topLayoutGuide 和 bottomLayoutGuide 各2个。