无法以编程方式添加 top 和 centerX 约束

Question

let imageView = UIImageView(image: UIImage(named: "Loading"))
imageView.frame = CGRect(x: 100, y: 100, width: 200, height: 200)
imageView.center = self.view.center
self.view.addSubview(imageView)

let xConstraint = NSLayoutConstraint(item: imageView, attribute: .centerX, relatedBy: .equal, toItem: self.view, attribute: .centerX, multiplier: 1, constant: 0)
let verticalSpace = NSLayoutConstraint(item: imageView, attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .bottom, multiplier: 1, constant: 100)
NSLayoutConstraint.activate([xConstraint, verticalSpace])

self.rotate(imageView: imageView, aCircleTime: 1)


let titleLabel = UILabel()
titleLabel.text = "Loading picker route"
titleLabel.textAlignment = .center
self.view.addSubview(imageView)


NSLayoutConstraint(item: titleLabel, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: self.view.frame.size.width).isActive = true
NSLayoutConstraint(item: titleLabel, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 44).isActive = true
NSLayoutConstraint(item: titleLabel, attribute: .centerX, relatedBy: .equal, toItem: imageView , attribute: .centerX, multiplier: 1, constant: 0).isActive = true
NSLayoutConstraint(item: titleLabel, attribute: .top, relatedBy: .equal, toItem: imageView, attribute: .bottom, multiplier: 1, constant: 100).isActive = true

最后两行崩溃，报告如下

由于未捕获的异常“NSGenericException”而终止应用程序， 原因：'无法使用锚激活约束 <0x2839a8540><0x2839fc000>

Answer 1

你需要添加标签

titleLabel.textAlignment = .center
self.view.addSubview(titleLabel)

加上这两行

imageView.translatesAutoresizingMaskIntoConstraints = false
titleLabel.translatesAutoresizingMaskIntoConstraints = false

编辑：

let imageView = UIImageView(image: UIImage(named: "Loading"))
imageView.frame = CGRect(x: 100, y: 100, width: 200, height: 200)
imageView.center = self.view.center
self.view.addSubview(imageView)

let xConstraint = NSLayoutConstraint(item: imageView, attribute: .centerX, relatedBy: .equal, toItem: self.view, attribute: .centerX, multiplier: 1, constant: 0)
let verticalSpace = NSLayoutConstraint(item: imageView, attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1, constant: 100)
let width = NSLayoutConstraint(item: imageView, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 200)
let height =  NSLayoutConstraint(item: imageView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 200)

NSLayoutConstraint.activate([xConstraint, verticalSpace,width,height])

self.rotate(imageView: imageView, aCircleTime: 1)

let titleLabel = UILabel()
titleLabel.text = "Loading picker route"
titleLabel.textAlignment = .center

self.view.addSubview(titleLabel)
 

imageView.translatesAutoresizingMaskIntoConstraints = false
titleLabel.translatesAutoresizingMaskIntoConstraints = false


NSLayoutConstraint(item: titleLabel, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: self.view.frame.size.width).isActive = true
NSLayoutConstraint(item: titleLabel, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 44).isActive = true
NSLayoutConstraint(item: titleLabel, attribute: .centerX, relatedBy: .equal, toItem: imageView , attribute: .centerX, multiplier: 1, constant: 0).isActive = true
NSLayoutConstraint(item: titleLabel, attribute: .top, relatedBy: .equal, toItem: imageView, attribute: .bottom, multiplier: 1, constant: 100).isActive = true