image.png
一个常见的情景是：
用 AutoLayout 使用 UIScrollView，使得 UIScrollView 的子视图能撑开 UIScrollView，而不用手动设置 contentSize，因为有时候子视图的内容是不固定的，从而高度也不固定。
如何做？（以垂直滑动举例说明）
- 如果是 UILabel，宽度可以根据 scrollView 来设置。高度不必管，它会自己撑开。
NSLayoutConstraint.activate([
contentLabel.topAnchor.constraint(equalTo: titleLabel.bottomAnchor, constant: 10),
contentLabel.leftAnchor.constraint(equalTo: titleLabel.leftAnchor),
contentLabel.rightAnchor.constraint(equalTo: titleLabel.rightAnchor),
contentLabel.widthAnchor.constraint(equalTo: scrollView.widthAnchor, constant: -20),
])
-
所有子视图都必须设置 translatesAutoresizingMaskIntoConstraints 为 NO。因为一个 View 如果是用代码创建的，那它的translatesAutoresizingMaskIntoConstraints 默认为 YES，为 YES 的含义是：这个 View 现在使用AutoResizing 进行布局，AutoLayout会根据 autoresizing mask，给这个 View 创建一系列「同等行为」的约束（the system creates a set of constraints that duplicate the behavior specified by the view’s autoresizing mask），这时候开发者就不能写自己的 AutoLayout 约束了，冲突了。
所以，要把translatesAutoresizingMaskIntoConstraints 设置为 NO，写自己的 AutoLayout 约束，这样才能动态地计算View 的位置和大小。鱼与熊掌不可兼得
最后一个子视图（最底或者最右，当然也可能是最左和最上，此处以最底示范）添加「跟 ScrollView底部的约束」，这样才能撑开ScrollView
bottomLabel.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor, constant: -10)
完整可运行代码：
import UIKit
class ViewController: UIViewController {
let scrollView = UIScrollView(frame: .zero)
let titleLabel = UILabel(frame: .zero)
let contentLabel = UILabel(frame: .zero)
let bottomLabel = UILabel(frame: .zero)
override func viewDidLoad() {
super.viewDidLoad()
view.backgroundColor = UIColor.white
setupSubviews()
}
}
extension ViewController {
func setupSubviews() {
// 1. 设置 scrollView 的约束
view.addSubview(scrollView)
scrollView.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activate([
scrollView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
scrollView.leftAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leftAnchor, constant: 20),
scrollView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
scrollView.rightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.rightAnchor)
])
// 2.添加子控件，设置内部约束
titleLabel.text = "===========titleLabel==========="
scrollView.addSubview(titleLabel)
titleLabel.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activate([
titleLabel.topAnchor.constraint(equalTo: scrollView.topAnchor, constant: 10),
titleLabel.leftAnchor.constraint(equalTo: scrollView.leftAnchor, constant: 10),
titleLabel.rightAnchor.constraint(equalTo: scrollView.rightAnchor, constant: -10)
])
contentLabel.text = "将此处替换为你自己的字符串（超过一屏），即可看见滑动效果"
contentLabel.numberOfLines = 0
scrollView.addSubview(contentLabel)
contentLabel.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activate([
contentLabel.topAnchor.constraint(equalTo: titleLabel.bottomAnchor, constant: 10),
contentLabel.leftAnchor.constraint(equalTo: titleLabel.leftAnchor),
contentLabel.rightAnchor.constraint(equalTo: titleLabel.rightAnchor),
contentLabel.widthAnchor.constraint(equalTo: scrollView.widthAnchor, constant: -20),
])
scrollView.addSubview(bottomLabel)
bottomLabel.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activate([
bottomLabel.topAnchor.constraint(equalTo: contentLabel.bottomAnchor, constant: 10),
bottomLabel.leftAnchor.constraint(equalTo: titleLabel.leftAnchor),
bottomLabel.rightAnchor.constraint(equalTo: titleLabel.rightAnchor),
// 3.最后一个 subview,添加跟 scrollView 的约束
bottomLabel.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor, constant: -10)
])
}
}
参考文档：
