VFL是什么就不多說，在這里主要記錄下在項目使用VFL時遇到的一些問題！

1、多控件的水平布局問題

這里設(shè)置了水平約束，以及控件的寬高約束，且三個控件的centerY是相等的

// 這里有三個控件，需要讓它們處于水平布局狀態(tài)
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、控件居中問題

上面的三個控件需要設(shè)置垂直居中，（由于水平方向上已經(jīng)有相關(guān)的約束，所以可以不設(shè)置），我這里只設(shè)置textLabel的垂直約束（那個都可以，因?yàn)樗麄兊腸enterY已經(jīng)有相等約束）
這里的實(shí)現(xiàn)有兩套方案

A、VFL約束與Apple封裝的約束混合使用

let views: [String: Any] = ["previousBtn": previousBtn,
                          "textLabel": textLabel,
                          "nextBtn": nextBtn]

添加垂直居中約束

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)

// 混合約束                   
cons += [NSLayoutConstraint.init(item: textLabel,
                                  attribute: .centerY,
                                  relatedBy: .equal,
                                  toItem: self,
                                  attribute: .centerY,
                                  multiplier: 1,
                                  constant: 0)]
addConstraints(cons)

B、純VFL約束。

最后必須同時添加水平居中約束和垂直居中約束，否則會報錯

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設(shè)置居中 必須設(shè)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)

Enjoy :)