如何以编程方式使用自动布局将活动指示器定位到其超级视图的中心？

为什么以下代码无法将活动指示器定位到其超级视图的中心：

UIActivityIndicatorView *activityIndicator = [[UIActivityIndicatorView alloc]
       initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];

[self.mysuperview addSubview:activityIndicator];
[activityIndicator addConstraints:[NSLayoutConstraint 
                   constraintsWithVisualFormat:@"|-(>=20)-[view(==100)]-(>=20)-|"
                   options:NSLayoutFormatAlignAllCenterX | NSLayoutFormatAlignAllCenterY
                   metrics:nil
                   views:@{@"view" : self.mysuperview}]];

活动指示器位于左上角的某个位置，绝对不是中心。

================= 更新： 找到解决方案：我必须在创建指标后关闭自动调整大小约束，然后关闭给定工作的所有解决方案：

 [activityIndicator setTranslatesAutoresizingMaskIntoConstraints:NO];

我在@Vignesh 给出的链接上找到了它，所以我接受他/她的回答。

以下四个 Swift 5 / iOS 12 代码示例展示了如何居中UIActivityIndicatorView在 - 的里面UIView of a UIViewController with 自动布局.

所有样品都会产生相同的结果，但根据您的需求和品味，您可以选择其中之一。

If your UIActivityIndicatorView的超级视图不是self.view，你只需更换每个self.view用你自己的（未包装的）调用superview.

1. NSLayoutConstraint初始化样式

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let indicatorView = UIActivityIndicatorView(style: .gray)
        indicatorView.isHidden = false
        indicatorView.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(indicatorView)

        // Auto layout
        let horizontalConstraint = NSLayoutConstraint(item: indicatorView,
                                                      attribute: .centerX,
                                                      relatedBy: .equal,
                                                      toItem: self.view,
                                                      attribute: .centerX,
                                                      multiplier: 1,
                                                      constant: 0)
        let verticalConstraint = NSLayoutConstraint(item: indicatorView,
                                                    attribute: .centerY,
                                                    relatedBy: .equal,
                                                    toItem: self.view,
                                                    attribute: .centerY,
                                                    multiplier: 1,
                                                    constant: 0)
        NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint])
        /*
         // You can replace NSLayoutConstraint activate(_:) call with the following lines:
         self.view.addConstraint(horizontalConstraint)
         self.view.addConstraint(verticalConstraint)
         */
    }

}

2. UIViewAutoresizing style

Springs 和 Struts 将在运行时转换为相应的自动布局约束。

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let indicatorView = UIActivityIndicatorView(style: .gray)
        indicatorView.isHidden = false
        indicatorView.translatesAutoresizingMaskIntoConstraints = true // default is true
        self.view.addSubview(indicatorView)

        // Springs and struts
        indicatorView.center = CGPoint(x: self.view.bounds.midX, y: self.view.bounds.midY)
        indicatorView.autoresizingMask = [
            .flexibleLeftMargin,
            .flexibleRightMargin,
            .flexibleTopMargin,
            .flexibleBottomMargin
        ]
    }

}

3.视觉格式语言风格

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let indicatorView = UIActivityIndicatorView(style: .gray)
        indicatorView.isHidden = false
        indicatorView.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(indicatorView)

        // Auto layout
        let views = ["superview": self.view!, "indicatorView": indicatorView]
        let horizontalConstraints = NSLayoutConstraint
            .constraints(withVisualFormat: "H:[superview]-(<=0)-[indicatorView]",
                         options: .alignAllCenterY,
                         metrics: nil,
                         views: views)
        let verticalConstraints = NSLayoutConstraint
            .constraints(withVisualFormat: "V:[superview]-(<=0)-[indicatorView]",
                         options: .alignAllCenterX,
                         metrics: nil,
                         views: views)
        self.view.addConstraints(horizontalConstraints)
        self.view.addConstraints(verticalConstraints)
    }

}

4. NSLayoutAnchor风格（需要 iOS 9）

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let indicatorView = UIActivityIndicatorView(style: .gray)
        indicatorView.isHidden = false
        indicatorView.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(indicatorView)

        // Auto layout
        let horizontalConstraint = indicatorView
            .centerXAnchor.constraint(equalTo: self.view.centerXAnchor)
        let verticalConstraint = indicatorView
            .centerYAnchor.constraint(equalTo: self.view.centerYAnchor)
        NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint])
        /*
         // You can replace NSLayoutConstraint activate(_:) call with the following lines:
         self.view.addConstraint(horizontalConstraint)
         self.view.addConstraint(verticalConstraint)
         */
    }

}