如何将UIActivityIndi​​catorView集中在UICollectionViewCell中？

Question

我试图在下载图像时在每个集合视图单元格中放置一个UIActivityIndi​​catorView。我把它出现在每个细胞中，但它拒绝自我中心。它停留在左上角。我怎样才能让它正确居中？

以下是我的表现：

extension UIView {

    func showActivityIndicator(onView: UIView, withIndicator: UIActivityIndicatorView) {
        withIndicator.frame = CGRect(x: onView.frame.midX - 20, y: onView.frame.midY - 20, width: 40, height: 40)
        withIndicator.activityIndicatorViewStyle = .whiteLarge
        withIndicator.center = onView.center
        onView.addSubview(withIndicator)
        withIndicator.startAnimating()
    }
}

我在cellForItemAtIndexPath中调用该函数，如：

showActivityIndicator(onView: cell.contentView, withIndicator: activityInd)

但是我所做的一切都不会从左上角移动它。有什么建议吗？

Answer 1

试试这个

withIndicator.center = CGPointMake(self.frame.size.width/2, self.frame.size.height/2);

Answer 2

您需要添加约束以使其居中。例如，使用NSLayoutAnchor。

您需要将translatesAutoresizingMaskIntoConstraints设置为false，以设置约束。并在将其添加到视图后设置约束（代码注释中的提示）：

extension UIView {

    func showActivityIndicator(onView: UIView, withIndicator: UIActivityIndicatorView) {
        withIndicator.frame = CGRect(x: onView.frame.midX - 20, y: onView.frame.midY - 20, width: 40, height: 40)
        withIndicator.activityIndicatorViewStyle = .whiteLarge

        // set translatesAutoresizingMaskIntoConstraints to false to set your constraints
        withIndicator.translatesAutoresizingMaskIntoConstraints = false
        onView.addSubview(withIndicator)
        withIndicator.startAnimating()

        // add the constraints to center the indicator
        withIndicator.centerXAnchor.constraint(equalTo: onView.centerXAnchor).isActive = true
        withIndicator.centerYAnchor.constraint(equalTo: onView.centerYAnchor).isActive = true
    }

}

Answer 3

我建议使用约束（又称自动布局）：

indicator.translatesAutoresizingMaskIntoConstraints = false
"your cell".addSubview(indicator)

let widthConstraint = NSLayoutConstraint(item: indicator, attribute: .Width, relatedBy: .Equal,
                                                 toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 250)

let heightConstraint = NSLayoutConstraint(item: indicator, attribute: .Height, relatedBy: .Equal,
                                                  toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 100)

let xConstraint = NSLayoutConstraint(item: indicator, attribute: .CenterX, relatedBy: .Equal, toItem: self.tableView, attribute: .CenterX, multiplier: 1, constant: 0)

let yConstraint = NSLayoutConstraint(item: indicator, attribute: .CenterY, relatedBy: .Equal, toItem: self.tableView, attribute: .CenterY, multiplier: 1, constant: 0)

NSLayoutConstraint.activateConstraints([widthConstraint, heightConstraint, xConstraint, yConstraint])