答案：调用下layoutIfNeeded后，系统会触发更新，但不一定保证何时能更新。
如果要强制获得Autolayout的frame可以给UIView增加如下的Category方法：
- (CGFloat)my_layoutHeight {
return [self systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
}
- (CGFloat)my_layoutWidth {
return [self systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].width;
}
- (CGFloat)my_layoutHeight {
return [self systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
}
- (CGFloat)my_layoutWidth {
return [self systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].width;
}
- (CGFloat)my_layoutHeight {
return [self systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
}
- (CGFloat)my_layoutWidth {
return [self systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].width;
}