ios - 使用视觉格式约束时， subview 未显示在 ScrollView 中

Question

我不明白为什么蓝色 View 没有显示，请看看我的代码。当我使用 view.addSubView(mapView) 时，它会显示在主视图中，但是当我执行 scrollView.addSubView(mapView) 时，它不会显示。一切似乎都是正确的约束。我不确定为什么它不起作用。

import UIKit



class practiceViewController: UIViewController,UIScrollViewDelegate {
    override func viewDidLoad() {
        var allConstraint = [NSLayoutConstraint]()

        let scrollView:UIScrollView = {
            let v = UIScrollView()
            v.backgroundColor = .red
            v.translatesAutoresizingMaskIntoConstraints = false
            return v
        }()

        let mapView:UIView = {
            let v = UIView()
            v.backgroundColor = .blue
            v.translatesAutoresizingMaskIntoConstraints = false
            return v
        }()

        //views
        let views = [
            "scrollView":scrollView,
            "mapView":mapView
        ]
        //add views
        view.addSubview(scrollView)
        scrollView.addSubview(mapView)

       // constraints
        let scrollviewHorizontalConstraint = NSLayoutConstraint.constraints(withVisualFormat: "H:|[scrollView]|", options: [], metrics: nil, views: views)
        let scrollviewVerticleConstraint = NSLayoutConstraint.constraints(withVisualFormat: "V:|[scrollView]|", options: [], metrics: nil, views: views)

        allConstraint += scrollviewVerticleConstraint
        allConstraint += scrollviewHorizontalConstraint

        let mapViewHorizontalConstraint = NSLayoutConstraint.constraints(withVisualFormat: "H:|[mapView]|", options: [], metrics: nil, views: views)
        let mapViewVerticleConstraint = NSLayoutConstraint.constraints(withVisualFormat: "V:|[mapView]|", options: [], metrics: nil, views: views)

        allConstraint += mapViewHorizontalConstraint
        allConstraint += mapViewVerticleConstraint

        NSLayoutConstraint.activate(allConstraint)
    }

}

Answer 1

您的 map View 需要宽度和高度。 ScrollView 不知道内容大小是多少，直到您设置内容的大小，因此 ScrollView 默认为零。尝试这样的事情:

let mapViewHorizontalConstraint = NSLayoutConstraint.constraints(withVisualFormat: "H:|[mapView(200)]|", options: [], metrics: nil, views: views)
let mapViewVerticleConstraint = NSLayoutConstraint.constraints(withVisualFormat: "V:|[mapView(200)]|", options: [], metrics: nil, views: views)