小编Dhr*_*yay的帖子

我正在尝试添加一个按钮并以编程方式设置约束,但我一直收到此错误,无法弄清楚我的代码有什么问题.我在这里看了其他问题但是在我的情况下它们并没有太大帮助.

    btn.setTitle("mybtn", for: .normal)
    btn.setTitleColor(UIColor.blue, for: .normal)
    btn.backgroundColor = UIColor.lightGray
    view.addSubview(btn)
    btn.translatesAutoresizingMaskIntoConstraints = false

    let left = NSLayoutConstraint(item: btn, attribute: .leftMargin, relatedBy: .equal, toItem: view, attribute: .leftMargin, multiplier: 1.0, constant: 0)

    let right = NSLayoutConstraint(item: btn, attribute: .rightMargin, relatedBy: .equal, toItem: view, attribute: .rightMargin, multiplier: 1.0, constant: 0)

    let top = NSLayoutConstraint(item: btn, attribute: .top, relatedBy: .equal, toItem: topLayoutGuide, attribute: .bottom, multiplier: 1.0, constant: 0)

    btn.addConstraints([left, right, top])