我需要未知元素的最高约束

Question

我有一个应用程序，它通过单击按钮从服务器下载图像。下载图像后，我创建一个新的 imageView 并将其添加到我的 contentView(UIView) 中。我需要创建约束——每个新的图像视图都需要来自前一个图像视图的顶部约束

func addNewImageToTheScrollView(img: UIImage?) {
    if let imageResponse = img {
        let imageView = UIImageView(image: imageResponse.crop(rect: CGRect(x: 0, y: imageResponse.size.height/2, width: self.contentView.frame.width, height: 200)))
        self.contentView.addSubview(imageView)

        imageView.translatesAutoresizingMaskIntoConstraints = false
        let x = NSLayoutConstraint(item: imageView, attribute: .centerX, relatedBy: .equal, toItem: self.contentView, attribute: .centerX, multiplier: 1.0, constant: 0)
        let y = NSLayoutConstraint(item: imageView, attribute: .top, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 30)
        let width = NSLayoutConstraint(item: imageView, attribute: .width, relatedBy: .equal, toItem: self.contentView, attribute: .width, multiplier: 1.0, constant: 0)
        let height = NSLayoutConstraint(item: imageView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 150)

        self.contentView.addConstraints([x, y])
        imageView.addConstraints([width, height])
    }
}

如果我注释约束代码，它会工作正常，除非每个新的 imageView 都在同一个地方，在 View 的顶部。现在有了这个约束代码，我在下载

后遇到了这样的代码问题

2017-07-02 14:50:01.018 ImageFromServerTest[11516:1080948] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'NSLayoutConstraint for >: A multiplier of 0 or a nil second item together with a location for the first attribute creates an illegal constraint of a location equal to a constant. Location attributes must be specified in pairs.'

Answer 1

我相信你在这里遇到了问题：

let y = NSLayoutConstraint(item: imageView, attribute: .top, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 30)

"toItem" 参数应该有一些值。第一个参数也应该是 imageView.topAnchor。可能应该看起来像这样：

let y = NSLayoutConstraint(item: imageView.topAnchor, attribute: .top, relatedBy: .equal, toItem: self.contentView.topAnchor, attribute: .notAnAttribute, multiplier: 1.0, constant: 30)

Answer 2

无论何时使用滚动视图，都有两条经验法则：-

给scrollView相对于superview的leading,trailing,bottom,top约束，即self.view

 @IbOutlet weak var scrollView: UIScrollView!

    let leading = NSLayoutConstraint(item: scrollView, attribute: .leading, relatedBy: .equal, toItem: self.view, attribute: .leadingMargin, multiplier: 1.0, constant: 0)

    let trailing = NSLayoutConstraint(item: scrollView, attribute: .trailing, relatedBy: .equal, toItem: self.view, attribute: .trailingMargin, multiplier: 1.0, constant: 0)

    let top = NSLayoutConstraint(item: scrollView, attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1.0, constant: 0)

    let bottom = NSLayoutConstraint(item: scrollView, attribute: .bottom, relatedBy: .equal, toItem: self.view, attribute: .bottom, multiplier: 1.0, constant: 0)

相对于 scrollView 向 contentView 添加约束

@IbOutlet weak var contentView: UIView!

 let leading = NSLayoutConstraint(item: contentView, attribute: .leading, relatedBy: .equal, toItem: scrollView, attribute: .leading, multiplier: 1.0, constant: 0)
 let trailing = NSLayoutConstraint(item: contentView, attribute: .trailing, relatedBy: .equal, toItem: scrollView, attribute: .trailing, multiplier: 1.0, constant: 0)

 let top = NSLayoutConstraint(item: contentView, attribute: .top, relatedBy: .equal, toItem: scrollView, attribute: .top, multiplier: 1.0, constant: 0)

  //increase the constant according to how much long you need the scrollview to be
 let bottom = NSLayoutConstraint(item: contentView, attribute: .bottom, relatedBy: .equal, toItem: scrollView, attribute: .bottom, multiplier: 1.0, constant: 0)

现在添加关于 contentView 的子视图约束（标签、图像）

例如-您收到了第一张图片，因此我们将在您的函数之外维护一个 UIImageView 数组。

var imageViews = [UIImageViews]() //declared outside the function


    //received an image here
        var imageView = UIImageView() // define the frame according to yourself using frame init method
        imageView.image = image

    if imageViews.isEmpty { // first image view 
    //add constraints to first image view

    let x = NSLayoutConstraint(item: imageView, attribute: .centerX, relatedBy: .equal, toItem: contentView, attribute: .centerX, multiplier: 1.0, constant: 0)
    let y = NSLayoutConstraint(item: imageView, attribute: .top, relatedBy: .equal, toItem: contentView, attribute: .top, multiplier: 1.0, constant: 30)
    let width = NSLayoutConstraint(item: imageView, attribute: .width, relatedBy: .equal, toItem: self.contentView, attribute: .width, multiplier: 1.0, constant: 0)
    let height = NSLayoutConstraint(item: imageView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 150)
    }

else { //second, third, fourth image view... so on

    let x = NSLayoutConstraint(item: imageView, attribute: .centerX, relatedBy: .equal, toItem: contentView, attribute: .centerX, multiplier: 1.0, constant: 0)
    let y = NSLayoutConstraint(item: imageView, attribute: .top, relatedBy: .equal, toItem: imageViews[imageViews.count - 1], attribute: .bottom, multiplier: 1.0, constant: 30)

    let width = NSLayoutConstraint(item: imageView, attribute: .width, relatedBy: .equal, toItem: self.contentView, attribute: .width, multiplier: 1.0, constant: 0)
    let height = NSLayoutConstraint(item: imageView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 150)
}

      imageViews.append(imageView) 
}

希望您现在知道如何处理这个问题了。如果有超过 4 或 5 个图像视图，您可能需要检查数组的数量并相应地增加 scrollView 的 contentView。你可以使用 self.scrollView.contentSize = CGSize(width, height)

我需要未知元素的最高约束

I need top constraint of unknow element

constraints

ios

nslayoutconstraint

swift