Constraint must contain a first layout item
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报错提示
Terminating app
due to uncaught exception ‘NSInvalidArgumentException’, reason:
‘* +[NSLayoutConstraint constraintWithItem:attribute:relatedBy:
toItem:attribute:multiplier:constant:]:
Constraint must contain a first layout item’
原因
添加约束的时候，toItem:view。view为nil，view不能为空，加不了约束。可以断点检查下是否为空。Xcode的断点调试真的很完善，用过其他ide就知道了。
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