UIView 子类中的 self.removeFromSuperview()

Question

背景： 我创建了一个 UIView (helloView) 的子类来在我的 MainViewController 视图上显示一些文本和一个按钮。 这个子类在MainVC中初始化viewDidLoad并添加到视图中：view.addSubview(helloView)

目标：helloView 有一个 UIButton (continueButton)，目标为 dimissHelloView。这个方法简单地调用self.removeFromSuperView()，以便简单地在屏幕上获取helloView。

问题：用户与按钮的交互会破坏代码并返回 nil 指针异常（在展开 Optional 值时意外发现 nil）。尽管在 SO 和 Google 上搜索了几个小时，但我还是找不到通过其自己的按钮子视图关闭视图的正确方法。

以前的故障排除：调用removeFromSuperview时superview似乎是nil，但如果我使用断点并在控制台中调用它不是。

代码：忽略属性和约束部分，按钮创建和目标动作在底部。

import UIKit

class HelloView: UIView {

    var helloText: String = ""
    var continueButton: UIButton!

    override init(frame: CGRect) {
        super.init(frame: frame)

        didLoad()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)

        didLoad()
    }

    func didLoad(){

        self.translatesAutoresizingMaskIntoConstraints = false
        setContinueButton()
        self.addSubview(continueButton)
    }

    override func didMoveToSuperview() {
        setConstraints()
    }

    func setConstraints(){

        //remove any previous constraints
        for constraint in superview!.constraints {
            if constraint.firstItem as! UIView == self && constraint.secondItem as? UIView == superview {
                superview!.removeConstraint(constraint)
            }
        }

        // Hello View basic constraints
        let xAxisConstraint = NSLayoutConstraint(item: self, attribute: .CenterX, relatedBy: .Equal, toItem: superview, attribute: .CenterX, multiplier: 1, constant: 0)
        let yAxisConstraint = NSLayoutConstraint(item: self, attribute: .CenterY, relatedBy: .Equal, toItem: superview, attribute: .CenterY, multiplier: 1, constant: 0)

        // check device orientation and add constraints consequently
        if UIDeviceOrientationIsLandscape(UIDevice.currentDevice().orientation) {
            let widthConstraint = NSLayoutConstraint(item: self, attribute: .Width, relatedBy: .Equal, toItem: superview, attribute: .Width, multiplier: 1, constant: -100)
            let heightConstraint = NSLayoutConstraint(item: self, attribute: .Height, relatedBy: .Equal, toItem: superview, attribute: .Height, multiplier: 1, constant: -150)
            superview!.addConstraints([xAxisConstraint, yAxisConstraint, widthConstraint, heightConstraint])
        }
        if UIDeviceOrientationIsPortrait(UIDevice.currentDevice().orientation){
            let widthConstraint = NSLayoutConstraint(item: self, attribute: .Width, relatedBy: .Equal, toItem: superview, attribute: .Width, multiplier: 1, constant: -100)
            let heightConstraint = NSLayoutConstraint(item: self, attribute: .Height, relatedBy: .Equal, toItem: superview, attribute: .Height, multiplier: 1, constant: -300)
            superview!.addConstraints([xAxisConstraint, yAxisConstraint, widthConstraint, heightConstraint])
        }

        // Continue Button constraints
        let centerXConstraint = NSLayoutConstraint(item: continueButton, attribute: .CenterX, relatedBy: .Equal, toItem: self, attribute: .CenterX, multiplier: 1, constant: 0)
        let bottomConstraint = NSLayoutConstraint(item: continueButton, attribute: .Bottom, relatedBy: .Equal, toItem: self, attribute: .Bottom, multiplier: 1, constant: -8)
        self.addConstraints([centerXConstraint, bottomConstraint])
    }

    func setContinueButton(){
        continueButton = UIButton(frame: CGRect(x: 0, y: 0, width: 100, height: 20))
        continueButton.setTitle("Continue", forState: .Normal)
        continueButton.setTitleColor(UIColor.whiteColor(), forState: .Normal)
        continueButton.addTarget(self, action: #selector(HelloView.dismissHelloView), forControlEvents: .TouchUpInside)
        continueButton.translatesAutoresizingMaskIntoConstraints = false
    }

    func dismissHelloView(){
        print(superview!)
        self.removeFromSuperview()
    }

}

Answer 1

当您从其父视图中删除视图时，didMoveToSuperview 将被调用（父视图将为 nil）。你的setConstraints 函数是从didMoveToSuperview 调用的，它强制解开superview，它为nil，导致崩溃。可选项的全部意义在于它迫使您考虑如果某些东西为零会发生什么。

无论如何，在您的情况下，您不希望在 superview 变为 nil 时发生任何事情，所以我会在 didMoveToSuperview 的顶部放置这样的内容：

guard superview != nil else { return }