（没有逐字逐词翻译，希望能够理解，如果有不对的，恳请指正）
UIButton subview
when the button is not in the window hierarchy will need to send layoutIfNeeded to
the button before retrieving layout information (such as button.titleLabel.frame) to ensure that the
layout values are up to date.For example, if you had something like this:
举例如下，如果iOS8.3以前，你这样写代码：
UIButton *button = [UIButton buttonWithType:UIButtonTypeSystem]; |
// code that sets up the button, but doesn’t yet add it to a window |
CGRect titleFrame = button.titleLabel.frame; |
// code that relies on the correct value for titleFrame |
UIButton *button = [UIButton buttonWithType:UIButtonTypeSystem]; |
// code that sets up the button, but doesn’t yet add it to a window |
[button layoutIfNeeded]; // This is also safe pre-iOS 8.3
|
CGRect titleFrame = button.titleLabel.frame; |
// code that relies on the correct value for titleFrame |
iOS SDK Release Notes for iOS 8.3 Beta 4 节选（UIKit）
原文：http://blog.csdn.net/workresource/article/details/45071081