如何以编程方式覆盖约束？

Question

最初我将容器的高度设置为 75。但是viewWillLoad我想更新这个值并设置为width / 5。

为此，我尝试：

containerHeight = NSLayoutConstraint(item: container, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: width / 5)
container.addConstraint(containerHeight)

还：

    containerHeight.constant = width / 5

但两种变体都不会更新我的约束。它仍然返回给我 75. 怎么了？

Answer 1

如果您已经在故事板中创建了一个现有约束，您可以添加一个IBOutlet并更改其常量，不要创建一个新的...

然后把你的代码放进去 viewDidLayoutSubviews()

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()

    constraint.constant = 10 // Just to test and see it really works 75 -> 10

    self.view.layoutIfNeeded() // If iOS 7

    // I think constraints are updated after this function and you wont see a change in the frame.height until it has ran. Put your print in an async and you'll see the updated frame.
    dispatch_async(dispatch_get_main_queue()) {
        print(view.frame.height)
    }
}