Question

我在界面构建器中有一个约束，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，则在viewDidLoad中，常量是默认值。调用willTransitionToTraitCollection或viewWillTransitionToSize都不会被调用，但在某些时候，约束会更新为紧凑值，因为稍后它是正确的。约束在哪里由UIViewController更新？这不是updateViewConstraints。

更新：建议的viewDidLayoutSubviews和traitCollectionDidChange方法不是常量更改的位置：

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指出我正确的方向。

答案是约束在进入视图层次结构后在视图的第一个layoutSubviews中更新。所以在你的视图控制器中你可以这样做：

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()
}

Answer 2

traitCollectionDidChange:是您可以捕捉大小类更改的地方，或者父视图的viewDidLayoutSubviews是您可以检测到任何自动布局更改的地方

更新大小类的约束在哪里？

2 个答案: