setNeedsLayout和layoutIfNeeded

- (void)setNeedsLayout;

Invalidates the current layout of the receiver and triggers a layout update during the next update cycle.

- (void)layoutIfNeeded;

Lays out the subviews immediately, if layout updates are pending.

-setNeedsLayout方法： 标记为需要重新布局，异步调用layoutIfNeeded刷新布局，不立即刷新，但layoutSubviews一定会被调用
    -layoutIfNeeded方法： 如果，有需要刷新的标记，立即调用layoutSubviews进行布局（如果没有标记，不会调用layoutSubviews）

https://blog.csdn.net/meegomeego/article/details/39890385

https://blog.csdn.net/flytomyskyone/article/details/42709345