Ios 如何打开已编程的UIButton（swift4）

我正在尝试使用编程约束创建一个按钮。我尽量不使用故事板。我打开按钮时遇到问题。我该怎么做

import UIKit

var aa: [NSLayoutConstraint] = []

class ViewController: UIViewController {

    var btn: UITextField!

    override func viewDidLoad() {
        super.viewDidLoad()

        self.view.addSubview(btn)
        let leadingc2 = btn.widthAnchor.constraint(equalToConstant: 80)
        let trailingC2 = btn.heightAnchor.constraint(equalToConstant: 50)
        let topc2 = btn.centerXAnchor.constraint(equalTo: self.view.centerXAnchor, constant: -50)
        let bottomc2 = btn.centerYAnchor.constraint(equalTo: self.view.centerYAnchor, constant: -250)

        aa = [leadingc2,trailingC2,topc2,bottomc2]

        NSLayoutConstraint.activate(aa)
    }
}

---

你不需要打开它。在使用它之前，您需要实例化它

override func viewDidLoad() {
    super.viewDidLoad()

    btn = UITextField() // Create the button like this before using it.
    self.view.addSubview(btn)

    btn.translatesAutoresizingMaskIntoConstraints = false
    let leadingc2 = btn.widthAnchor.constraint(equalToConstant: 80)
    let trailingC2 = btn.heightAnchor.constraint(equalToConstant: 50)
    let topc2 = btn.centerXAnchor.constraint(equalTo: self.view.centerXAnchor, constant: -50)
    let bottomc2 = btn.centerYAnchor.constraint(equalTo: self.view.centerYAnchor, constant: -250)

    aa = [leadingc2,trailingC2,topc2,bottomc2]

    NSLayoutConstraint.activate(aa)

}

使用

声明的任何变量将强制展开，这意味着如果您忘记创建实例并使用变量，它将抛出错误并使您的应用程序崩溃。
使用以下代码：

import UIKit

class ViewController: UIViewController {

  override func viewDidLoad() {
    super.viewDidLoad()

    view.addSubview(button)
    setupConstraints()
  }

  lazy var button: UIButton = {
    let button = UIButton(type: .system)
    button.setTitle("Button", for: .normal)
    button.backgroundColor = .blue
    //your action here
    button.addTarget(self, action: #selector(buttonAction), for: .touchUpInside)
    return button
  }()

  private func setupConstraints() {
    button.translatesAutoresizingMaskIntoConstraints = false
    let top = NSLayoutConstraint(item: button, attribute: .top, relatedBy: .equal, toItem: view, attribute: .top, multiplier: 1, constant: 100)
    let left = NSLayoutConstraint(item: button, attribute: .left, relatedBy: .equal, toItem: view, attribute: .left, multiplier: 1, constant: 50)
    let right = NSLayoutConstraint(item: button, attribute: .right, relatedBy: .equal, toItem: view, attribute: .right, multiplier: 1, constant: -50)
    let height = NSLayoutConstraint(item: button, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 50)
    view.addConstraints([top, left, right, height])
  }

  @objc func buttonAction() {
    print("Button has pressed")
  }

}

当使用button函数无法将“（）->”类型的值转换为指定类型的“UIButton”@AlexKolovatov时，我遇到了此错误，是哪个约束导致了此错误？您刚刚忘记添加此行。btn.translatesAutoresizingMaskIntoConstraints=false@AlexKolovatov是的，但答案不是约束。它用于按钮崩溃，因为它没有实例。我只是添加了OP的其他代码，这样他就能更好地理解在哪里做。但谢谢你指出这一点。如果你想的话，你可以用那一行编辑答案。当然你是对的。但我记得当我还是个初学者时，每一个错误都让我头疼：）