自动布局之-NSLayoutConstraint

view.translatesAutoresizingMaskIntoConstraints = NO;

/**
 设置约束

 @param view1 指定需要添加约束的视图一
 @param attr1 指定视图一需要约束的属性
 @param relation 指定视图一和视图二添加约束的关系
 @param view2 指定视图一依赖关系的视图二；可为nil
 @param attr2 指定视图一所依赖的视图二的属性，若view2=nil，该属性设置 NSLayoutAttributeNotAnAttribute
 @param multiplier 系数   情况一和二为亲测
                 情况一：设置A视图的高度 = A视图高度 * multiplier + constant；此时才会起作用；
                 情况二：设置A视图和其他视图的关系或 toItem=nil，multiplier设置不等于0即可，若等于0会crash；
 @param c 常量
 @return 返回生成的约束对象
 */
+ (instancetype)constraintWithItem:(id)view1 attribute:(NSLayoutAttribute)attr1 relatedBy:(NSLayoutRelation)relation toItem:(nullable id)view2 attribute:(NSLayoutAttribute)attr2 multiplier:(CGFloat)multiplier constant:(CGFloat)c;

// 设置 redView 的宽 = 100.f * 1.f
    NSLayoutConstraint *redViewWidthConstraint =
    [NSLayoutConstraint constraintWithItem:self.redView
                                 attribute:NSLayoutAttributeWidth
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:nil
                                 attribute:NSLayoutAttributeNotAnAttribute
                                multiplier:1.f
                                  constant:100.f];
    // 设置 redView 的高 = 100.f * 1.f
    NSLayoutConstraint *redViewHeightConstraint =
    [NSLayoutConstraint constraintWithItem:self.redView
                                 attribute:NSLayoutAttributeHeight
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:nil
                                 attribute:NSLayoutAttributeNotAnAttribute
                                multiplier:1.f
                                  constant:100.f];
    // 设置 redView 的前面(左边) = self.view.leading + 20.f
    NSLayoutConstraint *redViewleadingConstraint =
    [NSLayoutConstraint constraintWithItem:self.redView
                                 attribute:NSLayoutAttributeLeading
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:self.view
                                 attribute:NSLayoutAttributeLeading
                                multiplier:1.f
                                  constant:20.f];
    
    // 设置 redView 的顶部
    NSLayoutConstraint *redViewTopConstraint =
    [NSLayoutConstraint constraintWithItem:self.redView
                                 attribute:NSLayoutAttributeTop
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:self.view
                                 attribute:NSLayoutAttributeTop
                                multiplier:1.f
                                  constant:30.f];
    [self.view addConstraint:redViewWidthConstraint];
    [self.view addConstraint:redViewHeightConstraint];
    [self.view addConstraint:redViewleadingConstraint];
    [self.view addConstraint:redViewTopConstraint];

?举个栗子??:
     [NSLayoutConstraint constraintWithItem:aView
     attribute:NSLayoutAttributeHeight
     relatedBy:NSLayoutRelationEqual
     toItem:aView
     attribute:NSLayoutAttributeWidth
     multiplier:2.f
     constant:100.f];
     等价于  aView的高 = aView的宽 * 2.f + 100.f;

     [NSLayoutConstraint constraintWithItem:self.redView
     attribute:NSLayoutAttributeWidth | NSLayoutAttributeHeight
     relatedBy:NSLayoutRelationEqual
     toItem:nil
     attribute:NSLayoutAttributeNotAnAttribute
     multiplier:1.f
     constant:100.f];

/**
 Description

 @param format VFL语句，举个栗子：@"H:|-[_redView]-padding-|"
 @param opts 枚举参数，默认写0
 @param metrics 字典，key值为VFL语句中的变量 padding，value为指定的值: @{@"padding" : @50}
 @param views VFL语句中使用到的视图
 */

+ (NSArray<__kindof NSLayoutConstraint *> *)constraintsWithVisualFormat:(NSString *)format options:(NSLayoutFormatOptions)opts metrics:(nullable NSDictionary<NSString *,id> *)metrics views:(NSDictionary<NSString *, id> *)views;

举个栗子：
NSArray *constraints1 =
    [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[_redView]-padding-|"
                                            options:0
                                            metrics:@{@"padding" : @50}
                                              views:NSDictionaryOfVariableBindings(_redView)];
    NSArray *constraint2 =
    [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-height-[_redView(height)]"
                                            options:0
                                            metrics:@{@"height" : @20}
                                              views:NSDictionaryOfVariableBindings(_redView)];
    [self.view addConstraints:constraints1];
    [self.view addConstraints:constraint2];

UILabel *label = [[UILabel alloc] init];
    label.translatesAutoresizingMaskIntoConstraints = NO;
    label.text = @"Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~Label~~";
    label.textColor = [UIColor blackColor];
    label.backgroundColor = [UIColor redColor];
    label.numberOfLines = 0;
    [self.view addSubview:label];
    
    NSArray *labelConstraintsH =
    [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-20-[label]-20-|"
                                            options:0
                                            metrics:nil
                                              views:NSDictionaryOfVariableBindings(label)];
    NSArray *labelConstraintsV =
    [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-30-[label]"
                                            options:0
                                            metrics:nil
                                              views:NSDictionaryOfVariableBindings(label)];
    [self.view addConstraints:labelConstraintsH];
    [self.view addConstraints:labelConstraintsV];