View+MASAdditions.m
查找两个view的公共父视图Superview；
第一个view是自身self，第二个view是传进来的view；
第一层遍历，找到第二个view的superView；
第二层遍历，找到第一个view的superView，并判断和第二个view的superView是否相等，
若相等赋值给closestCommonSuperview；
如果closestCommonSuperview有值了，就跳出循环了；先跳出里面的while，在外面的while；
- (instancetype)mas_closestCommonSuperview:(MAS_VIEW *)view {
    MAS_VIEW *closestCommonSuperview = nil;

    MAS_VIEW *secondViewSuperview = view;
    while (!closestCommonSuperview && secondViewSuperview) {
        MAS_VIEW *firstViewSuperview = self;
        while (!closestCommonSuperview && firstViewSuperview) {
            if (secondViewSuperview == firstViewSuperview) {
                closestCommonSuperview = secondViewSuperview;
            }
            firstViewSuperview = firstViewSuperview.superview;
        }
        secondViewSuperview = secondViewSuperview.superview;
    }
    return closestCommonSuperview;
}


NSArray+MASAdditions.m
查找数组中的子view的公共superView;
首先遍历这个数组，只有UIView的class才继续遍历；
前一个view(previousView)不为空，查找和previousView的公共父视图；
循环依次进行下去；
- (MAS_VIEW *)mas_commonSuperviewOfViews
{
    MAS_VIEW *commonSuperview = nil;
    MAS_VIEW *previousView = nil;
    for (id object in self) {
        if ([object isKindOfClass:[MAS_VIEW class]]) {
            MAS_VIEW *view = (MAS_VIEW *)object;
            if (previousView) {
                commonSuperview = [view mas_closestCommonSuperview:commonSuperview];
            } else {
                commonSuperview = view;
            }
            previousView = view;
        }
    }
    NSAssert(commonSuperview, @"Can't constrain views that do not share a common superview. Make sure that all the views in this array have been added into the same view hierarchy.");
    return commonSuperview;
}