[英]Adding a subview and position it with constraints programmatically
我在视图控制器上添加了一个使用IB的UIView
。 我们称之为redView
。
然后我在代码中使用自动布局约束固定了它的所有四个边,当我运行它时,它看起来像预期的那样。
现在我想以编程方式将UILabel
添加到此视图中,并使用自动布局约束将其放置在中心。
下面是我到目前为止的代码。
import UIKit
class ViewController: UIViewController {
@IBOutlet private var redView: UIView!
override func viewDidLoad() {
super.viewDidLoad()
redView.setTranslatesAutoresizingMaskIntoConstraints(false)
let leadingConstraint = NSLayoutConstraint(item: redView, attribute: .Leading, relatedBy: .Equal, toItem: view, attribute: .Leading, multiplier: 1, constant: 0)
let trailingConstraint = NSLayoutConstraint(item: redView, attribute: .Trailing, relatedBy: .Equal, toItem: view, attribute: .Trailing, multiplier: 1, constant: 0)
let topConstraint = NSLayoutConstraint(item: redView, attribute: .Top, relatedBy: .Equal, toItem: view, attribute: .Top, multiplier: 1, constant: 0)
let bottomConstraint = NSLayoutConstraint(item: redView, attribute: .Bottom, relatedBy: .Equal, toItem: view, attribute: .Bottom, multiplier: 1, constant: 0)
view.addConstraints([leadingConstraint, trailingConstraint, topConstraint, bottomConstraint])
let label = UILabel()
label.text = "Auto Layout Exercise"
redView.addSubview(label)
let xCenterConstraint = NSLayoutConstraint(item: label, attribute: .CenterX, relatedBy: .Equal, toItem: redView, attribute: .CenterX, multiplier: 1, constant: 0)
let yCenterConstraint = NSLayoutConstraint(item: label, attribute: .CenterY, relatedBy: .Equal, toItem: redView, attribute: .CenterY, multiplier: 1, constant: 0)
let leadingConstraint1 = NSLayoutConstraint(item: label, attribute: .Leading, relatedBy: .Equal, toItem: redView, attribute: .Leading, multiplier: 1, constant: 20)
let trailingConstraint1 = NSLayoutConstraint(item: label, attribute: .Trailing, relatedBy: .Equal, toItem: redView, attribute: .Trailing, multiplier: 1, constant: -20)
redView.addConstraints([xCenterConstraint, yCenterConstraint, leadingConstraint1, trailingConstraint1])
}
}
问题是标签没有出现在redView
。 谁能告诉我我在这里缺少什么?
谢谢。
我很确定你还需要设置:
label.setTranslatesAutoresizingMaskIntoConstraints(false)
否则我认为约束不适用于标签。
Swift 2/3的注意事项
它已更改为布尔属性: UIView.translatesAutoresizingMaskIntoConstraints
而不是以前:
label.setTranslatesAutoresizingMaskIntoConstraints(false)
就是现在:
label.translatesAutoresizingMaskIntoConstraints = false
您需要为每个UIControl执行此操作以分配NSConstraints
label.translatesAutoresizingMaskIntoConstraints = false
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