Swift 3 |以编程方式添加约束不会使我的子视图居中

Question

我尝试编写一个 UIImageView 扩展，它在我的 ImageView 中间显示一个 ActivityIndicator，我尝试了以下方法，请注意我的 ImageView 对高度 (100) 和宽度 (100) 有限制：

public extension UIImageView {
    public func downloadedFrom(url: URL) {

        let activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: UIActivityIndicatorViewStyle.gray)
    activityIndicator.startAnimating()

        let leftSpaceConstraint = NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutAttribute.left, relatedBy: NSLayoutRelation.equal, toItem: self, attribute: NSLayoutAttribute.left, multiplier: 1, constant: 40)

        let topSpaceConstraint = NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutAttribute.top, relatedBy: NSLayoutRelation.equal, toItem: self, attribute: NSLayoutAttribute.top, multiplier: 1, constant: 40)

        let widthConstraint = NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 20)

        let heightConstraint = NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 20)

        self.addSubview(activityIndicator)

        self.addConstraints([leftSpaceConstraint, topSpaceConstraint, widthConstraint, heightConstraint])
    }
}

我也尝试设置 leadingSpaceConstraint / trailingSpaceConstraint / horizontalConstraint / verticalConstraint 但我的代码没有效果，我设置的每个约束都没有改变任何东西。我的 activityIndicator 像这样留在左上角：

我不明白为什么？在 viewDidLoad 中调用方法 "downloadedFrom" 是这样的：

myImageView.downloadedFrom(url: imageUrl)

我哪里出错了？ TY

Answer 1

您正在将您的 imageView 从左侧放在父 UIView 的顶部。它需要取决于 x-axis 和 y-axis 的父 UIImageView 中间的坐标。此外，将 translatesAutoresizingMaskIntoConstraints 设置为 false。看下面的代码：

     public extension UIImageView {
            public func downloadedFrom(url: URL) {

            let activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: UIActivityIndicatorViewStyle.gray)
            activityIndicator.startAnimating()
            activityIndicator.translatesAutoresizingMaskIntoConstraints = false

            let leftSpaceConstraint = NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: self, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0)

            let topSpaceConstraint = NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: self, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0)

            let widthConstraint = NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 20)

            let heightConstraint = NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 20)

            self.addSubview(activityIndicator)

            self.addConstraints([leftSpaceConstraint, topSpaceConstraint, widthConstraint, heightConstraint])
            }
        }

Swift 3 |以编程方式添加约束不会使我的子视图居中

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