iOS Masonry 设置布局优先级。

Masonry 中设置布局优先级需要使用系统方法：

设置抗拉伸性。值越低越容易被拉伸

- (void)setContentHuggingPriority:(UILayoutPriority)priority forAxis:(UILayoutConstraintAxis)axis;

实例代码:

    [A setContentHuggingPriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];
    [B setContentHuggingPriority:UILayoutPriorityDefaultLow forAxis:UILayoutConstraintAxisHorizontal];

由于B的抗拉伸值低，故需要拉伸时，优先拉伸B

设置抗压缩性。 值越低，越容易被压缩

- (void)setContentCompressionResistancePriority:(UILayoutPriority)priority forAxis:(UILayoutConstraintAxis)axis

实例代码

    [A setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];
    [B setContentCompressionResistancePriority:UILayoutPriorityDefaultLow forAxis:UILayoutConstraintAxisHorizontal];

由于B的抗压缩值低。故需要压缩时，优先压缩B