自动布局

• 约束优先级：在Autolayout中每个约束都有一个优先级，优先级的范围是1 ~ 1000；创建一个约束，默认的优先级是1000，是最高的级别
• Content hugging priority:该优先级表示一个控件抗被拉伸的优先权；优先级越高，越不容易被拉伸，默认是250
• **Content Compression Resistance Priority：该优先级表示一个控件抗被压缩的优先级。优先级越高，越不容易被压缩，默认是750

- (UILayoutPriority)contentHuggingPriorityForAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);
- (void)setContentHuggingPriority:(UILayoutPriority)priority forAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);

- (UILayoutPriority)contentCompressionResistancePriorityForAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);
- (void)setContentCompressionResistancePriority:(UILayoutPriority)priority forAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);

在控件的初始化中进行设置

[leftLabel setContentCompressionResistancePriority:UILayoutPriorityDefaultLow forAxis:UILayoutConstraintAxisHorizontal];

[rightLabel setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];