iOS开发笔记--[__NSArrayI addObjectsFromArray:]: unrecognized selector错误

操作NSMutableArray数字增加和删除时候出现崩溃，报错[__NSArrayI addObjectsFromArray:]: unrecognized selector。

找了一个小时，发“__NSArrayI”这个提示比较重要。查了一下发现

__NSArrayI is a code-word for an immutable array - that is, a "regular" NSArray which you cannot change.

__NSArrayM is a code-word for a mutable array - that is, NSMutableArray. In NSMutableArray, you can add and remove items.

NSArrayM才是NSMutableArray.

奇怪，我明明是声明的是NSMutableArray. 怎么突然变成了NSArray.