Autolayout之 关于+requiresConstraintBasedLayout方法作用

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constraint-based layout engages lazily when someone tries to use it (e.g., adds a constraint to a view). If you do all of your constraint set up in -updateConstraints, you might never even receive updateConstraints if no one makes a constraint. To fix this chicken and egg problem, override this method to return YES if your view needs the window to use constraint-based layout.

意思就是基于约束的布局是懒触发的,只有在添加了约束的情况下,系统才会自动调用 -updateConstraints 方法,如果把所有的约束放在 updateConstraints中,那么系统将会不知道你的布局方式是基于约束的,所以 重写+requiresConstraintBasedLayout 返回YES就是明确告诉系统:虽然我之前没有添加约束,但我确实是基于约束的布局!这样可以保证系统一定会调用 -updateConstraints 方法 从而正确添加约束.

参考文章:The Mystery of the +requiresConstraintBasedLayout