源码AutoLayoutShorthand，Auto Layout Shorthand (ALS)是创建或者添加Cocoa Auto Layout约束的可替代系统，感觉有点像CSS，功能比较强大，且没有HTML令人沮丧的默认布局模式。
ALS介绍示例:
[iconView als_addConstraints:@{
@"left ==": als_superview,
@"width ==": @(kIconWidth),
@"top ==": als_superview,
@"height ==": @(kIconHeight),
}];
alternative system for creating and adding Cocoa Auto Layout constraints
