要點：

UIScrollView是依靠與其子視圖（subview）之間的約束來確定ContentSize的大小缘挽，如果不設(shè)置好子view的寬高度約束的話掖肋，就會造成UISCrollView顯示異常油啤。

對于UIScrollView的subview來說撒蟀，它的leading/trailing/top/bottom的space是相對于UIScrollView的contentSize而不是bounds來確定的虽风，換句話說：UIScrollView與其subview之間相對位置的約束并不會直接用于frame的計算,而是會轉(zhuǎn)化為對ContentSize的計算咆瘟。(摘自
:https://blog.csdn.net/longshihua/java/article/details/78441466)

錯誤用法

沒有指定greenView的 寬高嚼隘， 無法顯示scrollView

    let scrollView = UIScrollView()
    scrollView.delegate = self
    scrollView.backgroundColor = .blue
    view.addSubview(scrollView)
    scrollView.snp.makeConstraints { (make) in
        make.edges.equalToSuperview()
    }

    let greenView = UIView()
    greenView.backgroundColor = .green
    scrollView.addSubview(greenView)
    greenView.snp.makeConstraints { (make) in
        make.leading.trailing.equalToSuperview()
        make.top.equalToSuperview()
        make.bottom.equalToSuperview()
    }

2、對subviews指定寬高袒餐，讓最底部的view設(shè)置對scrollView的約束飞蛹，可顯示，但是會有布局異常灸眼。

  let scrollView = UIScrollView()
    scrollView.delegate = self
    scrollView.backgroundColor = .blue
    view.addSubview(scrollView)
    scrollView.snp.makeConstraints { (make) in
        make.edges.equalToSuperview()
    }

    let greenView = UIView()
    greenView.backgroundColor = .green
    scrollView.addSubview(greenView)
    greenView.snp.makeConstraints { (make) in
        make.top.leading.equalToSuperview()
        make.width.equalTo(300)
        make.height.equalTo(100)
    }

    let redView = UIView()
    redView.backgroundColor = .red
    scrollView.addSubview(redView)
    redView.snp.makeConstraints { (make) in
        make.leading.equalToSuperview()
        make.top.equalTo(greenView.snp.bottom)
        make.width.equalTo(300)
        make.height.equalTo(1000)
        make.bottom.equalToSuperview() // 不添加底部的約束 則無法滾動
    }

正確用法

對scrollView 添加contentView卧檐，讓subviews添加對contentView的約束，而contentView只需處理contentSize的寬度

    let scrollView = UIScrollView()
    scrollView.delegate = self
    scrollView.backgroundColor = .blue
    view.addSubview(scrollView)
    scrollView.snp.makeConstraints { (make) in
        make.edges.equalToSuperview()
    }
    
    contentView = UIView()
    contentView.backgroundColor = .yellow
    scrollView.addSubview(contentView)
    contentView.snp.makeConstraints { (make) in
        make.edges.equalToSuperview()
        make.width.equalTo(300) 
        // 設(shè)置contentSize的 width值
    }
    
    let redView = UIView()
    redView.backgroundColor = .red
    contentView.addSubview(redView)
    redView.snp.makeConstraints { (make) in
        make.leading.top.trailing.equalToSuperview()
        make.height.equalTo(400)
    }
    
    let greenView = UIView()
    greenView.backgroundColor = .green
    contentView.addSubview(greenView)
    greenView.snp.makeConstraints { (make) in
        make.leading.trailing.equalToSuperview()
        make.height.equalTo(600)
        make.top.equalTo(redView.snp.bottom)
        make.bottom.equalToSuperview() 
        // 固定底部約束焰宣，設(shè)置好contentSize的 height值
    }