Autolayout

• 约束优先级问题
  • 使用Masonry

约束优先级问题

使用Masonry

一行两个Label，距离屏幕左右边距各15，测试不同文字内容长度时Label的显示情况，以及通过设置优先级后Label的显示情况来透彻理解约束优先级问题。

• ContentHuggingPriority表示当前的Label的内容不想被拉伸;
• 默认HuggingPriority == 250
• 值越大越不被拉伸(最大1000)

• ContentCompressionResistancePriority表示当前的Label的内容不想被收缩;
• 默认CompressionResistancePriority == 750
• 值越大越不被收缩(最大1000)

@interface ViewController ()

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

@end

#pragma mark - Getters and Setters
- (UILabel *)leftLabel {
    if (_leftLabel == nil) {
        _leftLabel = [[UILabel alloc] init];
        _leftLabel.text = @"左边";
        _leftLabel.textColor = [UIColor redColor];
        _leftLabel.backgroundColor = [UIColor lightGrayColor];
    }
    return _leftLabel;
}

- (UILabel *)rightLabel {
    if (_rightLabel == nil) {
        _rightLabel = [[UILabel alloc] init];
        _rightLabel.text = @"右边";
        _rightLabel.textColor = [UIColor blueColor];
        _rightLabel.backgroundColor = [UIColor lightGrayColor];
    }
    return _rightLabel;
}

#pragma mark - Life Cycle
- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self setupUI];
}

#pragma mark - SetupUI
- (void)setupUI {
    
    [self.view addSubview:self.leftLabel];
    [self.view addSubview:self.rightLabel];
    
    [self.leftLabel mas_makeConstraints:^(MASConstraintMaker *make) {
        make.left.equalTo(self.view).offset(16);
        make.top.equalTo(self.view).offset(50);
    }];
    [self.rightLabel mas_makeConstraints:^(MASConstraintMaker *make) {
        make.right.equalTo(self.view).offset(-16);
        make.centerY.equalTo(self.leftLabel);
    }];
}

文字内容足够少、居左右两侧显示

按照上面默认的即可。

文字内容足够少、但需要一方拉伸

[self.leftLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.left.equalTo(self.view).offset(16);
    make.top.equalTo(self.view).offset(50);
    make.right.equalTo(self.rightLabel.mas_left).offset(-10);
}];
[self.rightLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.right.equalTo(self.view).offset(-16);
    make.centerY.equalTo(self.leftLabel);
}];

[self.leftLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.left.equalTo(self.view).offset(16);
    make.top.equalTo(self.view).offset(50);
}];
[self.rightLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.right.equalTo(self.view).offset(-16);
    make.centerY.equalTo(self.leftLabel);
    make.left.equalTo(self.leftLabel.mas_right).offset(10);
}];

或者按照如下方式设置

/**
 * 设置左侧 Label 不被拉伸(即右边的被拉伸)
 * 默认情况下:HuggingPriority == 250;
 * 就要设置 leftLabel.HuggingPriority > 250
 * 或者     rightLabel.HuggingPriority < 250
 * UILayoutPriorityRequired == 1000;
 * 这里只要是 HuggingPriority > 250 即可，写成 251 也是同样的效果;
 */
[self.leftLabel setContentHuggingPriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];
//    UILayoutPriorityFittingSizeLevel == 50;
//    [self.rightLabel setContentHuggingPriority:UILayoutPriorityFittingSizeLevel forAxis:UILayoutConstraintAxisHorizontal];

[self.leftLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.left.equalTo(self.view).offset(16);
    make.top.equalTo(self.view).offset(50);
    make.right.equalTo(self.rightLabel.mas_left).offset(-10);
}];
[self.rightLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.right.equalTo(self.view).offset(-16);
    make.centerY.equalTo(self.leftLabel);
    make.left.equalTo(self.leftLabel.mas_right).offset(10);
}];

文字内容足够多、右侧被压缩

只要设置leftLabel的right距rightLabel的mas_left间距即可。需要谁不被压缩，就设置谁距离另外一个被压缩的的间距即可。

如果左右相互之间的间距都设置的话，那么默认右侧Label被压缩。

[self.leftLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.left.equalTo(self.view).offset(16);
    make.top.equalTo(self.view).offset(50);
    make.right.equalTo(self.rightLabel.mas_left).offset(-10);
}];
[self.rightLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.right.equalTo(self.view).offset(-16);
    make.centerY.equalTo(self.leftLabel);
}];

文字内容足够多、左侧被压缩

设置rightLabel的left距leftLabel的mas_right间距即可。需要谁不被压缩，就设置谁距离另外一个被压缩的的间距即可。

[self.leftLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.left.equalTo(self.view).offset(16);
    make.top.equalTo(self.view).offset(50);
}];
[self.rightLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.right.equalTo(self.view).offset(-16);
    make.centerY.equalTo(self.leftLabel);
    make.left.equalTo(self.leftLabel.mas_right).offset(10);
}];

或者通过ContentCompressionResistancePriority设置

/**
 * 设置右侧 Label 不被压缩(即左侧的被压缩)
 * 默认情况下:ContentCompressionResistancePriority == 750;
 * 就要设置 rightLabel.ContentCompressionResistancePriority > 750
 * 或者     leftLabel.ContentCompressionResistancePriority < 750
 * UILayoutPriorityRequired == 1000;
 * 这里只要是 ContentCompressionResistancePriority > 750 即可，写成 751 也是同样的效果;
 */
[self.rightLabel setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];
//    UILayoutPriorityDefaultLow == 250;
[self.leftLabel setContentCompressionResistancePriority:UILayoutPriorityDefaultLow forAxis:UILayoutConstraintAxisHorizontal];

[self.leftLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.left.equalTo(self.view).offset(16);
    make.top.equalTo(self.view).offset(50);
    make.right.equalTo(self.rightLabel.mas_left).offset(-10);
}];
[self.rightLabel mas_makeConstraints:^(MASConstraintMaker *make) {
    make.right.equalTo(self.view).offset(-16);
    make.centerY.equalTo(self.leftLabel);
    make.left.equalTo(self.leftLabel.mas_right).offset(10);
}];