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今天在做工程内存泄漏检查时发现SnapKit本身存在的内存泄漏，官方更新了新的bate版，但是更新后始终编译不过：提示错误：

Undefined symbols for architecture arm64:
"protocol witness table for SnapKit.ConstraintItem : SnapKit.ConstraintRelatableTarget in SnapKit",     referenced from:
   XXX.UserViewController.(setupUI () -> ()).(closure #4) in UserViewController.o
"SnapKit.ConstraintViewDSL.bottomMargin.getter : SnapKit.ConstraintItem", referenced from:
   XXX.UserViewController.(setupUI () -> ()).(closure #4) in UserViewController.o
"SnapKit.ConstraintViewDSL.makeConstraints (closure : (make : SnapKit.ConstraintMaker) -> ()) ->     ()", referenced from:
  XXX.UserViewController.setupUI** () -> () in UserViewController.o

在GitHub上提问后对方建议清空一下DerivedData文件夹，清空之后，Bingo！ 问题解决！
DerivedData文件夹的路径为：

  /Users/用户/Library/Developer/Xcode/DerivedData

另外，这个缓存文件夹还是要经常清理的，太占存储空间啦！！

注：文中如有任何错误，请各位批评指正！