Keep autolayout constraints active status on device rotation(在设备旋转时保持自动布局约束处于活动状态)

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问题描述

我注意到，当我以编程方式更新我的自动布局约束时，所有更改都会在我旋转屏幕时还原.

I noticed that when I update my autolayout constraints programmatically, all changes are reverted when I rotate the screen.

重现问题:

带有 UIView 和 2 个约束的基本 Storyboard 界面:

Basic Storyboard interface with UIView and 2 constraints:

宽度等于 superview.width(乘数 1)激活
宽度等于 superview.width(乘数 1/2)已禁用

使用 IBOutlet 创建和链接这两个约束

Create and link these 2 constraints with IBOutlet

对我来说似乎是一个错误.

Seems like a bug to me.

你怎么看?

截图:

故事板:

约束 #1:

约束 #2:

Size Classes

Installed 是指 Size Classes 安装，而不是 active/inactive.

Size Classes

Installed refers to Size Classes installation, not to active/inactive.

您必须以编程方式创建另一个约束，并激活/停用那个一个.这是因为您无法更改约束的乘数(我可以更改乘数属性NSLayoutConstraint?)，你也不能修改大小类(activateConstraints: 和 deactivateConstraints: 在 IB 中创建的约束旋转后不持久).

You must create another constraint programmatically, and activate/deactivate that one. This is because you cannot change the multiplier of a constraint (Can i change multiplier property for NSLayoutConstraint?), nor can you tinker with Size Classes (activateConstraints: and deactivateConstraints: not persisting after rotation for constraints created in IB).

有几种方法可以做到这一点.在下面的示例中，我创建了一个 x1 约束的副本，其中包含一个乘数或 1/2.然后我在两者之间切换:

There are a few ways to do so. In the example below, I create a copy of your x1 constraint, with a multiplier or 1/2. I then toggle between the two:

@IBOutlet var fullWidthConstraint: NSLayoutConstraint!
var halfWidthConstraint: NSLayoutConstraint!

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    halfWidthConstraint = NSLayoutConstraint(item: fullWidthConstraint.firstItem,
        attribute: fullWidthConstraint.firstAttribute,
        relatedBy: fullWidthConstraint.relation,
        toItem: fullWidthConstraint.secondItem,
        attribute: fullWidthConstraint.secondAttribute,
        multiplier: 0.5,
        constant: fullWidthConstraint.constant)
    halfWidthConstraint.priority = fullWidthConstraint.priority
}

@IBAction func changeConstraintAction(sender: UISwitch) {
    if sender.on {
        NSLayoutConstraint.deactivateConstraints([fullWidthConstraint])
        NSLayoutConstraint.activateConstraints([halfWidthConstraint])
    } else {
        NSLayoutConstraint.deactivateConstraints([halfWidthConstraint])
        NSLayoutConstraint.activateConstraints([fullWidthConstraint])
    }
}

<小时>

在 iOS 9+、Xcode 7+ 上测试.

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