Swift 如何让 UIKit 挤压你的视图，使其适合屏幕

我们可以向第一个视图添加多个高度约束，只要它们不冲突.

所以，我们想告诉第一个视图100的高度为88点.我们将为该约束提供一个低于所需优先级的优先级，以便自动布局可以在必要时打破它:

let firstHeight: NSLayoutConstraint = firstView.heightAnchor.constraint(equalToConstant: 88.0)
firstHeight.priority = .required - 1

我们不希望其ever小于88点，因此我们将为其提供第二个小于或等于88的所需高度约束:

firstView.heightAnchor.constraint(lessThanOrEqualToConstant: 88.0)

并且，为了防止第五个视图超出安全区域底部，我们将给它一个小于或等于的约束:

fifthView.bottomAnchor.constraint(lessThanOrEqualTo: view.safeAreaLayoutGuide.bottomAnchor)

下面是一个完整的示例:

class FiveViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let firstView = UIView()
        firstView.backgroundColor = .red
        let secondView = UIView()
        secondView.backgroundColor = .cyan
        let thirdView = UIView()
        thirdView.backgroundColor = .yellow
        let fourthView = UIView()
        fourthView.backgroundColor = .green
        let fifthView = UIView()
        fifthView.backgroundColor = .orange

        [firstView, secondView, thirdView, fourthView, fifthView].forEach { v in
            v.translatesAutoresizingMaskIntoConstraints = false
            view.addSubview(v)
        }

        // we want the first view to TRY to be 88-points tall
        let firstHeight: NSLayoutConstraint = firstView.heightAnchor.constraint(equalToConstant: 88.0)
        // but with less-than-required priority so it can shrink
        firstHeight.priority = .required - 1
        
        
        NSLayoutConstraint.activate([
            
            firstView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
            firstView.leftAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leftAnchor),
            firstView.rightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.rightAnchor),

            // don't do this
            //firstView.heightAnchor.constraint(equalToConstant: 88),

            firstHeight,
            
            // we don't want the view heights to be Greater-Than 88-points
            firstView.heightAnchor.constraint(lessThanOrEqualToConstant: 88.0),

            secondView.topAnchor.constraint(equalTo: firstView.bottomAnchor),
            secondView.leftAnchor.constraint(equalTo: firstView.leftAnchor),
            secondView.rightAnchor.constraint(equalTo: firstView.rightAnchor),
            secondView.heightAnchor.constraint(equalTo: firstView.heightAnchor),
            
            thirdView.topAnchor.constraint(equalTo: secondView.bottomAnchor),
            thirdView.leftAnchor.constraint(equalTo: firstView.leftAnchor),
            thirdView.rightAnchor.constraint(equalTo: firstView.rightAnchor),
            thirdView.heightAnchor.constraint(equalTo: firstView.heightAnchor),
            
            fourthView.topAnchor.constraint(equalTo: thirdView.bottomAnchor),
            fourthView.leftAnchor.constraint(equalTo: firstView.leftAnchor),
            fourthView.rightAnchor.constraint(equalTo: firstView.rightAnchor),
            fourthView.heightAnchor.constraint(equalTo: firstView.heightAnchor),
            
            fifthView.topAnchor.constraint(equalTo: fourthView.bottomAnchor),
            fifthView.leftAnchor.constraint(equalTo: firstView.leftAnchor),
            fifthView.rightAnchor.constraint(equalTo: firstView.rightAnchor),
            fifthView.heightAnchor.constraint(equalTo: firstView.heightAnchor),
            
            // we want the fifth view Bottom to never extend below the safe area Bottom
            fifthView.bottomAnchor.constraint(lessThanOrEqualTo: view.safeAreaLayoutGuide.bottomAnchor),
            
        ])
    }
}

结果是: