- (void)layoutIfNeeded
Description
Lays out the subviews immediately.
Use this method to force the layout of subviews before drawing. Using the view that receives the message as the root view, this method lays out the view subtree starting at the root.
- (void)setNeedsLayout
Description
Invalidates the current layout of the receiver and triggers a layout update during the next update cycle.
Call this method on your application’s main thread when you want to adjust the layout of a view’s subviews. This method makes a note of the request and returns immediately. Because this method does not force an immediate update, but instead waits for the next update cycle, you can use it to invalidate the layout of multiple views before any of those views are updated. This behavior allows you to consolidate all of your layout updates to one update cycle, which is usually better for performance.
以上描述來自蘋果官方文檔晦譬，下面是總結(jié)：
- layoutIfNeeded
- 用于子控件包括父控件的frame設(shè)置完以后,調(diào)用該方法,會(huì)立即布局
- setNeedsLayout
- 用于還未設(shè)置父子控件的frame時(shí),調(diào)用該方法,會(huì)在其frame被設(shè)置后,即恰當(dāng)?shù)臅r(shí)刻,自動(dòng)完成布局
