VFL是什么就不多说，在这里主要记录下在项目使用VFL时遇到的一些问题！
1、多控件的水平布局问题
这里设置了水平约束，以及控件的宽高约束，且三个控件的centerY是相等的
// 这里有三个控件，需要让它们处于水平布局状态
addSubview(previousBtn)
addSubview(textLabel)
addSubview(nextBtn)
var cons = NSLayoutConstraint
.constraints(withVisualFormat: "H:|-[previousBtn(30)]-20-[textLabel]-20-[nextBtn(30)]-|",
options: .alignAllCenterY,
metrics: nil,
views: views)
cons += NSLayoutConstraint
.constraints(withVisualFormat: "V:[previousBtn(30)]",
options: [],
metrics: nil,
views: views)
cons += NSLayoutConstraint
.constraints(withVisualFormat: "V:[nextBtn(30)]",
options: [],
metrics: nil,
views: views)
2、控件居中问题
上面的三个控件需要设置垂直居中，（由于水平方向上已经有相关的约束，所以可以不设置），我这里只设置textLabel的垂直约束（那个都可以，因为他们的centerY已经有相等约束）
这里的实现有两套方案
A、VFL约束与Apple封装的约束混合使用
let views: [String: Any] = ["previousBtn": previousBtn,
"textLabel": textLabel,
"nextBtn": nextBtn]
添加垂直居中约束
var cons = NSLayoutConstraint
.constraints(withVisualFormat: "H:|-[previousBtn(30)]-20-[textLabel]-20-[nextBtn(30)]-|",
options: .alignAllCenterY,
metrics: nil,
views: views)
cons += NSLayoutConstraint
.constraints(withVisualFormat: "V:[previousBtn(30)]",
options: [],
metrics: nil,
views: views)
cons += NSLayoutConstraint
.constraints(withVisualFormat: "V:[nextBtn(30)]",
options: [],
metrics: nil,
views: views)
// 混合约束
cons += [NSLayoutConstraint.init(item: textLabel,
attribute: .centerY,
relatedBy: .equal,
toItem: self,
attribute: .centerY,
multiplier: 1,
constant: 0)]
addConstraints(cons)
B、纯VFL约束。
最后必须同时添加水平居中约束和垂直居中约束，否则会报错
let views: [String: Any] = ["previousBtn": previousBtn,
"textLabel": textLabel,
"nextBtn": nextBtn,
"superview": self]
var cons = NSLayoutConstraint
.constraints(withVisualFormat: "H:|-[previousBtn(30)]-20-[textLabel]-20-[nextBtn(30)]-|",
options: .alignAllCenterY,
metrics: nil,
views: views)
cons += NSLayoutConstraint
.constraints(withVisualFormat: "V:[previousBtn(30)]",
options: [],
metrics: nil,
views: views)
cons += NSLayoutConstraint
.constraints(withVisualFormat: "V:[nextBtn(30)]",
options: [],
metrics: nil,
views: views)
// VFL设置居中 必须设置水平居中和垂直居中
cons += NSLayoutConstraint
.constraints(withVisualFormat: "H:[textLabel]-(<=0)-[superview]",
options: .alignAllCenterY,
metrics: nil,
views: views)
cons += NSLayoutConstraint
.constraints(withVisualFormat: "V:[textLabel]-(<=0)-[superview]",
options: .alignAllCenterX,
metrics: nil,
views: views)
addConstraints(cons)
