篇幅一，介绍了为什么可以通过mas_key 定位问题。如下：
https://www.jianshu.com/p/710f742ca71a

如果一个布局很多的页面，突然出现这种问题呢？比如下面的，

    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "<MASLayoutConstraint:0x1c08a0a80 UILabel:0x112a1c600.left == UIView:0x112a1b830.left + 5>",
    "<MASLayoutConstraint:0x1c08a1080 UILabel:0x112a1d1e0.left == UIView:0x112a1b830.left + 5>",
    "<MASLayoutConstraint:0x1c08a1140 UILabel:0x112a1d1e0.right == UILabel:0x112a1c600.left - 5>"
)

信息只有UILabel UIView,我们可以通过runtime,获取到ivar 信息，然后手动给ivar 设置mas_key 值。

代码如下：

uint32_t ivarCount;
    Ivar *ivars = class_copyIvarList([self class], &ivarCount);
    
    if(ivars)
    {
        for(uint32_t i=0; i<ivarCount; i++)
        {
            Ivar ivar = ivars[i];
            
            id pointer = object_getIvar(self, ivar);
            
            if ([pointer isKindOfClass:[UIView class]]) {
                NSString *name = [NSString stringWithUTF8String:ivar_getName(ivar)];
                UIView *v = (UIView *)pointer;
                v.mas_key = name;
            }
        }
        
        free(ivars);
    }
    

通过判断ivar 是不是UIView 的子类，然后设置mas_key 的值为ivar的name.这样子提示信息就很详细了。

可以提取一个全局的函数出来

/*
 * 设置ivar 的 mas_key 值为 对应的名字比如

 @interface PBTTestClass : NSObject
 @property (nonatomic, strong) UIButton *payButton;
 @end

 PBTTestClass *c = [PBTTestClass new];
 autoSetClassMas_key(c);
 
  就设置 c.payButton.mas_key = @"_payButton";
 *
 */

static void autoSetClassMas_key(NSObject *instance) {
    
    uint32_t ivarCount;
    Ivar *ivars = class_copyIvarList(instance.class, &ivarCount);
    if(ivars)
    {
        for(uint32_t i=0; i<ivarCount; i++)
        {
            Ivar ivar = ivars[i];
            id pointer = object_getIvar(instance, ivar);
            if ([pointer isKindOfClass:[UIView class]]) {
                NSString *name = [NSString stringWithUTF8String:ivar_getName(ivar)];
                UIView *v = (UIView *)pointer;
                v.mas_key = name;
            }
        }
        free(ivars);
    }
}