ios - 以编程方式添加前导约束会使应用程序崩溃

Question

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])
    }

Answer 1

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 anchor

let 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 。