我正在努力了解如何以编程方式添加约束。到目前为止,我的代码是这样的:
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
//addViewStandard()
addConstraintsView()
}
func addConstraintsView() {
let someView = UIView(frame: CGRect.zero)
someView.backgroundColor = UIColor.blue
// I want to mimic a frame set of CGRect(x: 20, y: 50, width: 50, height: 50)
let widthConstraint = NSLayoutConstraint(item: someView, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 50)
let heightConstraint = NSLayoutConstraint(item: someView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 50)
let leadingConstraint = NSLayoutConstraint(item: someView, attribute: .leading, relatedBy: .equal, toItem: view, attribute: .leading, multiplier: 1.0, constant: 20)
someView.translatesAutoresizingMaskIntoConstraints = false
someView.addConstraints([widthConstraint, heightConstraint, leadingConstraint])
view.addSubview(someView)
}
}
现在,当我运行该应用程序时,它会因为前导约束而崩溃。错误消息是“无法使用未准备好约束的 View 层次结构来设置布局”。我在这里做错了什么?我应该将约束添加到对象(本例中的蓝色框)还是将它们添加到它的父 View ?
编辑:
代码更改后我有:
func addConstraintsView() {
let someView = UIView(frame: CGRect.zero)
someView.backgroundColor = UIColor.blue
view.addSubview(someView)
someView.translatesAutoresizingMaskIntoConstraints = false
let widthConstraint = NSLayoutConstraint(item: someView, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 50)
let heightConstraint = NSLayoutConstraint(item: someView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 50)
let leadingConstraint = NSLayoutConstraint(item: someView, attribute: .leading, relatedBy: .equal, toItem: view, attribute: .left, multiplier: 1.0, constant: 20)
someView.addConstraints([widthConstraint, heightConstraint])
view.addConstraints([leadingConstraint])
}
最佳答案
首先,
view.addSubview(someView)
someView.translatesAutoresizingMaskIntoConstraints = false
应该在约束阶段之前;您必须在将 someView 添加到其父 View 后应用约束。
此外,如果您的目标是 iOS 9,我建议您使用像
这样的布局 anchorlet widthConstraint = someView.widthAnchor.constraint(equalToConstant: 50.0)
let heightConstraint = someView.heightAnchor.constraint(equalToConstant: 50.0)
let leadingConstraint = someView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 20.0)
NSLayoutConstraint.activate([widthConstraint, heightConstraint, leadingConstraint])
这样您就不必担心将约束应用于哪个 View 。
最后(并消除您的疑问),如果您不能使用布局 anchor ,您应该将前导约束添加到父 View ,而不是 View 。
关于ios - 以编程方式添加前导约束会使应用程序崩溃,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/42157211/