以编程方式设置约束时应用程序崩溃答案

【问题标题】：App crashes when setting constraint programatically以编程方式设置约束时应用程序崩溃
【发布时间】：2021-12-26 07:27:46
【问题描述】：

我正在务实地创建一个 toast 视图（使用 SO 的一些帮助）并对其设置垂直约束。当我尝试在约束中使用动态值时，应用程序崩溃并显示以下消息：

Unable to parse constraint format: 
Expected a ')' at the end of a constraint modifier, or a ',' before another constraint modifier 
V:|-(>=66.0-[toastView(==40)]-68-| 
           ^

吐司代码：

func showToastWithMessage(_ message : String, _ bottomBarHeight: CGFloat?) {
        guard let delegate = UIApplication.shared.delegate as? AppDelegate, let window = delegate.window else {
            return
        }
        if let toast = window.subviews.first(where: { $0 is UILabel && $0.tag == -1001 }) {
            toast.removeFromSuperview()
        }
        
        let toastView = UILabel()
        toastView.backgroundColor = UIColor.black.withAlphaComponent(0.7)
        toastView.textColor = UIColor.white
        toastView.textAlignment = .center
        toastView.font = UIFont.systemFont(ofSize: 14.0, weight: .medium)
        toastView.text = message
        toastView.numberOfLines = 0
        toastView.alpha = 0
        toastView.translatesAutoresizingMaskIntoConstraints = false
        toastView.tag = -1001
        
        window.addSubview(toastView)
        
        let horizontalCenterContraint: NSLayoutConstraint = NSLayoutConstraint(item: toastView, attribute: .centerX, relatedBy: .equal, toItem: window, attribute: .centerX, multiplier: 1, constant: 0)
        
        let widthContraint: NSLayoutConstraint = NSLayoutConstraint(item: toastView, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .width, multiplier: 1, constant: (self.frame.size.width-25) )
        
        var bannerPlacement = 300.0
        
        if let height = bottomBarHeight {
            bannerPlacement = height + 10.0
        }
        
        let verticalContraint: [NSLayoutConstraint] = NSLayoutConstraint.constraints(withVisualFormat: "V:|-(>=\(bannerPlacement)-[toastView(==40)]-68-|", options: [.alignAllCenterX, .alignAllCenterY], metrics: nil, views: ["toastView": toastView])
        
        NSLayoutConstraint.activate([horizontalCenterContraint, widthContraint])
        NSLayoutConstraint.activate(verticalContraint)
        
        UIView.animate(withDuration: 0.5, delay: 0, options: .curveEaseIn, animations: {
            toastView.alpha = 1
        }, completion: nil)
        
        DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(3), execute: {
            UIView.animate(withDuration: 0.5, delay: 0, options: .curveEaseIn, animations: {
                toastView.alpha = 0
            }, completion: { finished in
                toastView.removeFromSuperview()
            })
        })
    }

应用程序在“V:|-(>=(bannerPlacement)-[toastView(==40)]-68-|”上崩溃，原因是创建verticalConstraint对象时(bannerPlacement)。如果我在那里放一个静态值，一切正常，但在我的情况下，值是动态的。如何在其中添加变量？

提前致谢！

【问题讨论】：

我冒昧地正确格式化了错误消息，它揭示了有关问题的其他信息。

标签： ios swift uiview

【解决方案1】：

你没有具体说明你想用可视化格式表达什么来实现和表达，所以我只能专注于形式语法问题。

显然，括号是不平衡的。这就是 iOS 所抱怨的。

您很可能在66.0 之后缺少右括号：

代替：

V:|-(>=66.0-[toastView(==40)]-68-|

表达式应如下所示：

V:|-(>=66.0)-[toastView(==40)]-68-|

【讨论】：