代码片段：动态加子控件的约束

for (NSInteger i = 0; i < self.scrollViews.count; i++) {
        
        UIScrollView * currentS = self.scrollViews[i];
        UIScrollView * previousS = i > 0 ? self.scrollViews[i-1] : nil;
        [currentS mas_makeConstraints:^(MASConstraintMaker *make) {
            
            make.top.bottom.width.height.equalTo(self.scrollView);
            if (previousS) {
                
                make.left.equalTo(previousS.mas_right);
            } else {
                
                make.left.equalTo(self.scrollView);
            }
        }];
    }
// 另外一种判断

for (NSInteger i = 0; i < self.scrollViews.count; i++) {
        
        UIScrollView * currentS = self.scrollViews[i];
        [currentS mas_makeConstraints:^(MASConstraintMaker *make) {
            
            make.top.bottom.width.height.equalTo(self.scrollView);
            if (i == 0) {
                
                make.left.equalTo(self.scrollView);
            } else {
                
                make.left.equalTo(self.scrollViews[i-1].mas_right);
            }
        }];
    }