Ios 如何根据SnapKit中superview高度的大小按比例设置边偏移

以下是我试图用SnapKit实现的目标，但由于明显的原因，它没有编译

contentView.snp.makeConstraints { (make) in
   make.top.equalTo(scrollView).offset(scrollView.snp.height * 0.2) //ERROR - Binary operator cannot be applied to type ConstraintItem and Double 
} 

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我会这样做：

contentView.snp.makeConstraints { make in
    make.height.equalToSuperview().dividedBy(2)
    make.left.right.bottom.equalToSuperview()
}