UIScrollView AutoLayout

最近改一个BUG，遇到了一个使用AutoLayout让内容撑开UIScrollView的场景；
试了半天才搞定撑开和可以滚动，特此记录一下。

代码如下：

//
//  ViewController.swift
//  ScrollViewAutoLayoutDemo
//
//  Created by Crazy凡 on 2019/4/2.
//  Copyright © 2019 kongkaikai. All rights reserved.
//

import UIKit
import SnapKit

class ViewController: UIViewController {
    let scrollView = UIScrollView()
    let testlabel = UILabel()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        view.addSubview(scrollView)
        scrollView.addSubview(testlabel)
        scrollView.snp.makeConstraints { (maker) in
            maker.center.equalToSuperview()
            maker.width.lessThanOrEqualTo(UIScreen.main.bounds.width - 60)
            maker.height.lessThanOrEqualTo(UIScreen.main.bounds.height - 80).priority(.required)

            // 保障UIScrollView可以被撑开，low 是为了避免约束冲突
            maker.height.equalTo(testlabel.snp.height).priority(.low)
        }

        testlabel.numberOfLines = 0
        testlabel.snp.makeConstraints { (maker) in
            maker.top.left.bottom.right.equalToSuperview()
            maker.width.equalToSuperview()

            // 保障可以正确的计算contentSize
            maker.height.greaterThanOrEqualToSuperview()
        }
        testlabel.text = "测试字符串，多写一些测试撑开效果"
    }
}