ios - 尺寸等级的约束在哪里更新？

Question

我在界面生成器中有一个约束，它有一个默认值和一个紧凑的宽度值。我将此约束链接到 IBOutlet，因此我可以在运行时获取代码中的常量值。如果我在 iPhone 4S 模拟器上运行，在 viewDidLoad 中常量是默认值。 willTransitionToTraitCollection 或 viewWillTransitionToSize 都没有被调用，但在某些时候约束被更新为紧凑值，因为稍后它是正确的。 UIViewController 在哪里更新约束？这不是 updateViewConstraints。

更新:建议的viewDidLayoutSubviews 和traitCollectionDidChange 方法也不是常量改变的地方:

override func viewDidLayoutSubviews() {
    println("Before viewDidLayoutSubviews: \(self.navControllerBottomSpaceConstraint.constant)")
    super.viewDidLayoutSubviews()
    println("After viewDidLayoutSubviews: \(self.navControllerBottomSpaceConstraint.constant)")
}

override func traitCollectionDidChange(previousTraitCollection: UITraitCollection?) {
    println("Before traitCollectionDidChange: \(self.navControllerBottomSpaceConstraint.constant)")
    super.traitCollectionDidChange(previousTraitCollection)
    println("After traitCollectionDidChange: \(self.navControllerBottomSpaceConstraint.constant)")
}

输出:

Before viewDidLayoutSubviews: 20.0
After viewDidLayoutSubviews: 20.0
Before viewDidLayoutSubviews: 20.0
After viewDidLayoutSubviews: 20.0
Before viewDidLayoutSubviews: 20.0
After viewDidLayoutSubviews: 20.0
Before viewDidLayoutSubviews: 20.0
After viewDidLayoutSubviews: 20.0
Before traitCollectionDidChange: 20.0
After traitCollectionDidChange: 20.0
Before viewDidLayoutSubviews: 0.0
After viewDidLayoutSubviews: 0.0
Before viewDidLayoutSubviews: 0.0
After viewDidLayoutSubviews: 0.0

Answer 1

感谢 Matt 和 Sega-Zero 为我指明了正确的方向。

答案是在 View 进入 View 层次结构后，约束在 View 的第一个 layoutSubviews 中更新。所以在你的 View Controller 中你可以这样做:

override func viewDidLayoutSubviews() {
    if self.propertyCalculatedFromConstraints == nil && self.view.window != nil {
        // constraints and sizes are now valid
        self.propertyCalculatedFromConstraints = self.constraint.constant
    }
    super.viewDidLayoutSubviews()
}

override func viewDidDisappear() {
    self.propertyCalculatedFromConstraints = nil
    super.viewDidDisappear()
}