论坛里的少年们好，有两个问题想要请教：我先用注释部分向ud(userDefaults)里写入数据，然后将其注释重新编译运行。多次运行，打印的结果都如下 1.看文档说，ud返回的类型为immutable，但是如果用mutable类型接受objectForKey的返回值没有警告，而且竟然可以进行mutable特有的操作。2.再次从ud取数据,结果是受刚才mutable操作的影响。(但是实际上沙盒目录文件不受影响，我就想是不是从ud中取数据不一定走的硬盘)，谢谢 @ibuick
如果在23-24行间加入synchronize plist文件还是不受影响
我的理解
userDefault会在初始化的时候读取文件(一个plist) 这里读取的所有NSArray 和 NSDictionary都是immutable的
但是userDefault本身也是一个Dictionary 所以在内存中时 你可以set mutable对象进去 只要在内存里都是不受限制的
但是在synchronize时 所有的mutable对象都会被当成immutable来写 所以 在之前初始化的时候 都会被读成immutable的
所以 当你用的时候 最好这样用
[[APP.userDefaults objectForKey:kUserDefaultUser] mutableCopy];
At runtime, you use an NSUserDefaults object to read the defaults that your application uses from a user’s defaults database. NSUserDefaults caches the information to avoid having to open the user’s defaults database each time you need a default value. The synchronize method, which is automatically invoked at periodic intervals, keeps the in-memory cache in sync with a user’s defaults database.
Values returned from NSUserDefaults are immutable, even if you set a mutable object as the value. For example, if you set a mutable string as the value for "MyStringDefault", the string you later retrieve using stringForKey: will be immutable.
A defaults database is created automatically for each user. The NSUserDefaults class does not currently support per-host preferences. To do this, you must use the CFPreferences API (see Preferences Utilities Reference). However, NSUserDefaults correctly reads per-host preferences, so you can safely mix CFPreferences code with NSUserDefaults code.
文档似乎写得很清楚了