小编And*_*rei的帖子

我一直在尝试以纯代码（不使用情节提要）构建自适应布局并开始运行。我使用布局锚来设置约束，并利用traitCollectionDidChange方法在各种约束集和其他接口更改之间进行切换。我使用switch语句使用相应的约束集调用相应的方法。

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
    super.traitCollectionDidChange(previousTraitCollection)

    switch (traitCollection.horizontalSizeClass, traitCollection.verticalSizeClass) {
    case (.regular, .regular):
        setupRegularRegular()

    case (.compact, .compact):
        setupCompactCompact()

    case (.regular, .compact):
        setupRegularCompact()

    case (.compact, .regular):
        setupCompactRegular()

    default: break
    }

}

为了进行测试，我仅更改了一个约束，即centerYAnchor.constraint。在纵向模式中，常量为-150，在横向模式中，我希望将其更改为50。对于iPad，我将常量设置为0。

bigLabel.centerYAnchor.constraint(equalTo: view.centerYAnchor, constant: -150).isActive = true

在iPhone和iPad之间切换时，效果很好。但是，如果您开始将iPhone从纵向旋转到横向，则布局系统将失败，并显示：

[LayoutConstraints] Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want. 
Try this: 
    (1) look at each constraint and try to figure out which you don't expect; 
    (2) find …