(取消)激活约束是如何在内部工作的?

一种选择是重写 updateViewConstraints() .在那里,您可以激活/停用约束,而不必发出警告。

沿着这些线:

@IBAction func doUp(_ sender: Any) {
    flag = !flag
    self.view.setNeedsUpdateConstraints()
}

override func updateViewConstraints() {
    constraint1.isActive = flag
    constraint2.isActive = !flag
    super.updateViewConstraints()
}

另一种选择是激活这两个约束,但赋予它们不同的优先级。然后更改优先级,而不是激活/停用:

    // in viewDidLoad, or wherever you created the constraints
    constraint1.priority = .defaultHigh
    constraint2.priority = .defaultLow

    constraint1.isActive = true
    constraint2.isActive = true

然后,点击按钮:

    @IBAction func doUp(_ sender: Any) {
        flag = !flag
        constraint1.priority = flag ? .defaultHigh : .defaultLow
        constraint2.priority = flag ? .defaultLow : .defaultHigh
    }