当数组为空时遍历数组容易出这样的问题， -[NSNull countByEnumeratingWithState:objects:count:]: unrecognized selector sent to instance 0x3e4e85e0
解决办法就是在遍历数组之前先要判断
if (![array isKindOfClass:[NSNull class]])
{
for循环
}
-[NSNull countByEnumeratingWithState:objects:count:]:
原文：http://www.cnblogs.com/cui-cui/p/5802380.html
