未为约束准备视图层次结构：无法查找问题

Question

以下是正在使用的约束。

  

＆lt; NSLayoutConstraint：0x7fcfdd405bc0 UILabel：0x7fcfdd405780'ACTIVITY'.centerY == UIView：0x7fcfdb677b60.centerY＆gt;视图层次结构没有为约束准备：

footer!.frame = CGRectMake(0, 0, tableView.frame.size.width, FOOTER_HEIGHT)


let activityLabel = UILabel() 
activityLabel.text = "SomeText"
activityLabel.translatesAutoresizingMaskIntoConstraints = false

let CenterConstraint = NSLayoutConstraint(item: activityLabel, attribute: .CenterX, relatedBy: .Equal, toItem: footerView, attribute: .CenterX, multiplier: 1, constant: 0)
let CenterConstraint = NSLayoutConstraint(item: activityLabel, attribute: .CenterY, relatedBy: .Equal, toItem: footerView, attribute: .CenterY, multiplier: 1, constant: 0)

footer!.addConstraints([xxCenterConstraint,yyCenterConstraint])

footer!.addSubview(activityLabel)

let notificationLabel = UILabel()
notificationLabel.text = "9"
notificationLabel.textAlignment = .Center
notificationLabel.backgroundColor = customBlueColor 
footerView!.addSubview(notificationLabel)
notificationLabel.translatesAutoresizingMaskIntoConstraints = false

let width = NSLayoutConstraint(item: notificationLabel, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: NOTIFICATION_LABEL_HEIGHT)
let height = NSLayoutConstraint(item: notificationLabel, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: NOTIFICATION_LABEL_HEIGHT)
let yCenterConstraint = NSLayoutConstraint(item: notificationLabel, attribute: .CenterY, relatedBy: .Equal, toItem: footerView, attribute: .CenterY, multiplier: 1, constant: 0)
let leadingConstraint1 = NSLayoutConstraint(item: activityLabel, attribute: .Trailing, relatedBy: .Equal, toItem: notificationLabel, attribute: .Leading, multiplier: 1, constant:-LEADING_SPACE)
footer!.addConstraints([width,height,yCenterConstraint,leadingConstraint1])

notificationLabel.layer.cornerRadius = NOTIFICATION_LABEL_HEIGHT/2
notificationLabel.clipsToBounds = true

Answer 1

在视图属于同一视图层次结构之前，您无法添加约束。在addConstraints行之前移动addSubview行。