View+MASAdditions.m
查找兩個view的公共父視圖Superview；
第一個view是自身self，第二個view是傳進來的view；
第一層遍歷，找到第二個view的superView；
第二層遍歷，找到第一個view的superView，并判斷和第二個view的superView是否相等，
若相等賦值給closestCommonSuperview；
如果closestCommonSuperview有值了，就跳出循環了；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
查找數組中的子view的公共superView;
首先遍歷這個數組，只有UIView的class才繼續遍歷；
前一個view(previousView)不為空，查找和previousView的公共父視圖；
循環依次進行下去；
- (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;
}