iOS之'mutating method sent to immutable object'

今天在代码测试中发现以下代码：

// 更新本地缓存
NSMutableDictionary *gesture = (NSMutableDictionary *)[[NSUserDefaults standardUserDefaults] objectForKey:Lock_Gesture_Dic];
[gesture setValue:self.access_token forKey:Lock_Access_Token];

执行后错误信息如下：

2016-05-06 15:12:18.566[5054:105571] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFDictionary setObject:forKey:]: mutating method sent to immutable object'

*** First throw call stack:

(

0 CoreFoundation 0x000000010f312d85 __exceptionPreprocess + 165

1 libobjc.A.dylib 0x000000010ed86deb objc_exception_throw + 48

2 CoreFoundation 0x000000010f312cbd +[NSException raise:format:] + 205

3 CoreFoundation 0x000000010f239133 -[__NSCFDictionary setObject:forKey:] + 99

4 Proj 0x0000000109ef641b +[AppDelegate aaaaa] + 3083

原来：userDefaults objectForKey: 返回的是一个不可变对象（immutable object），有人可能会想到创建一个可变对象或进行类型强制转换，
其实这是不被推荐的，官方建议写法：

NSMutableDictionary *gesture = [(NSMutableDictionary*) [[NSUserDefaults standardUserDefaults] objectForKey:Lock_Gesture_Dic] mutableCopy];