使用布局时适配UILabel文字

宽度不够时，可以被压缩 

[_label setContentCompressionResistancePriority:UILayoutPriorityFittingSizeLevel forAxis:UILayoutConstraintAxisHorizontal];

抱紧 类似于sizeToFit 

[_label setContentHuggingPriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];

不可以被压缩，尽量显示完整 

[_label setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];

• 控件抗压缩的优先级。优先级越高，越不容易被压缩：

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

• 控件抗拉伸的优先级。优先级越高，越不容易被拉伸：

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

•   UILayoutConstraintAxis布局方向

typedef NS_ENUM(NSInteger, UILayoutConstraintAxis) {
    UILayoutConstraintAxisHorizontal = 0,    //横向
    UILayoutConstraintAxisVertical = 1       //纵向
};

• UILayoutPriority优先级： 

static const UILayoutPriority UILayoutPriorityRequired API_AVAILABLE(ios(6.0)) = 1000;
// A required constraint.  Do not exceed this.
static const UILayoutPriority UILayoutPriorityDefaultHigh API_AVAILABLE(ios(6.0)) = 750;
// This is the priority level with which a button resists compressing its content.
static const UILayoutPriority UILayoutPriorityDragThatCanResizeScene API_AVAILABLE(macCatalyst(13.0)) = 510;
// This is the appropriate priority level for a drag that may end up resizing the window's scene.
static const UILayoutPriority UILayoutPrioritySceneSizeStayPut API_AVAILABLE(macCatalyst(13.0)) = 500;
// This is the priority level at which the window's scene prefers to stay the same size.
// It's generally not appropriate to make a constraint at exactly this priority. You want to be higher or lower.
static const UILayoutPriority UILayoutPriorityDragThatCannotResizeScene API_AVAILABLE(macCatalyst(13.0)) = 490;
// This is the priority level at which a split view divider, say, is dragged.
// It won't resize the window's scene.
static const UILayoutPriority UILayoutPriorityDefaultLow API_AVAILABLE(ios(6.0)) = 250;
// This is the priority level at which a button hugs its contents horizontally.
static const UILayoutPriority UILayoutPriorityFittingSizeLevel API_AVAILABLE(ios(6.0)) = 50;
// When you send -[UIView systemLayoutSizeFittingSize:], the size fitting most closely to the target size (the argument) is computed.
// UILayoutPriorityFittingSizeLevel is the priority level with which the view wants to conform to the target size in that computation.
// It's quite low.
// It is generally not appropriate to make a constraint at exactly this priority.
// You want to be higher or lower.