Masonry优先级

#import "ViewController.h"
#import <Masonry/Masonry.h>

@interface ViewController ()

@property (nonatomic,strong) UILabel *leftLabel;
@property (nonatomic,strong) UILabel *middleLabel;
@property (nonatomic,strong) UILabel *rightLabel;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self.view addSubview:self.leftLabel];
    [self.view addSubview:self.middleLabel];
    [self.view addSubview:self.rightLabel];
    [self p_addMasonry];
}

- (void)p_addMasonry {
    [self.leftLabel mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.mas_equalTo(150);
        make.left.mas_equalTo(0);
        make.height.mas_equalTo(30);
    }];
    
    [self.middleLabel mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.mas_equalTo(150);
        make.left.mas_equalTo(self.leftLabel.mas_right);
        make.height.mas_equalTo(30);
        make.right.mas_lessThanOrEqualTo(self.rightLabel.mas_left);
    }];
    
    [self.rightLabel mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.mas_equalTo(150);
        make.right.mas_equalTo(0);
        make.height.mas_equalTo(30);
        make.left.mas_greaterThanOrEqualTo(self.leftLabel.mas_right);
    }];
    
    [_middleLabel setContentCompressionResistancePriority:UILayoutPriorityDefaultLow forAxis:UILayoutConstraintAxisHorizontal];
}

- (UILabel *)leftLabel {
    if (!_leftLabel) {
        _leftLabel = [[UILabel alloc] init];
        _leftLabel.textAlignment = NSTextAlignmentCenter;
        _leftLabel.textColor = [[UIColor redColor] colorWithAlphaComponent:0.3];
        _leftLabel.backgroundColor = [[UIColor purpleColor] colorWithAlphaComponent:0.1];
        _leftLabel.font = [UIFont fontWithName:@"Futura-Medium" size:17];
        _leftLabel.text = @"hello world!";
    }
    return _leftLabel;
}

- (UILabel *)middleLabel {
    if (!_middleLabel) {
        _middleLabel = [[UILabel alloc] init];
        _middleLabel.textAlignment = NSTextAlignmentCenter;
        _middleLabel.textColor = [[UIColor greenColor] colorWithAlphaComponent:0.3];
        _middleLabel.backgroundColor = [[UIColor yellowColor] colorWithAlphaComponent:0.2];
        _middleLabel.font = [UIFont fontWithName:@"Futura-Medium" size:17];
        _middleLabel.text = @"Futura-Medium";
    }
    return _middleLabel;
}

- (UILabel *)rightLabel {
    if (!_rightLabel) {
        _rightLabel = [[UILabel alloc] init];
        _rightLabel.textAlignment = NSTextAlignmentCenter;
        _rightLabel.textColor = [UIColor colorWithRed:49.0/255.0 green:186.0/255.0 blue:81.0/255.0 alpha:1];
        _rightLabel.backgroundColor = [[UIColor orangeColor] colorWithAlphaComponent:0.1];
        _rightLabel.font = [UIFont fontWithName:@"Futura-Medium" size:17];
        _rightLabel.text = @"Futura-Medium===============";
    }
    return _rightLabel;
}

@end

理论
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，默认的优先级是最高的1000
Content Hugging Priority: 该优先级表示一个控件抗被拉伸的优先级。优先级越高，越不容易被拉伸，默认是251。
Content Compression Resistance Priority: 该优先级和上面那个优先级相对应，表示一个控件抗压缩的优先级。优先级越高，越不容易被压缩，默认是750


所以默认情况下两边的label的 Content Hugging和 Content Compression优先级都是一样的，为了让右边的label完全显示，那么我们需要增大右边label的抗压缩级，或者减小左边label的抗压缩级，总之是得让右边的抗压缩级大于左边的label，这样才能让右边的label内容优先显示。


所以默认情况下两边的label的Content Hugging和Content Compression优先级都是一样的，为了让右边的label完全显示，那么我们需要增大右边label的抗压缩级，或者减小左边label的抗压缩级，总之是得让右边的抗压缩级大于左边的label，这样才能让右边的label内容优先显示。

UIView中关于Content Hugging 和 Content Compression Resistance的方法有：

- (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);
在初始化label里面添加代码：

[leftLabel setContentCompressionResistancePriority:UILayoutPriorityDefaultLow forAxis:UILayoutConstraintAxisHorizontal];
或者

[rightLabel setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];