来自 Apple Docs：从 NSUserDefaults 返回的值是不可变的，即使您将可变对象设置为值。例如，如果您将可变字符串设置为“MyStringDefault”的值，那么您稍后使用stringForKey 检索的字符串将是不可变的。
苹果文档一直这么说。在实践中，字典和数组一直是可变的，尽管 Apple 发出警告，只要您使用了同步。Mountain Lion 的不同之处在于，现在，如果您读/写深度嵌套的字典，那些深度嵌套的对象不会保存到 NSUserDefaults。
它们甚至可能看起来像是已保存，因为您可以在退出应用程序之前立即读取这些值。巧妙地，当您重新启动时，它们并不存在。
更糟糕的是，制作 mutableCopy 并不能解决问题。只有制作 mutableCopyDeepPropertyList 才能解决问题（请参阅下面的解决方案）
在 Mountain Lion 之前，这样的代码可以工作，尽管文档建议它不应该
NSMutableDictionary *parentDict = [[NSUserDefaults standardUserDefaults] objectForKey:@"parentDict"];
NSLog( @"starting up... %@", parentDict );
if ( !parentDict )
{
NSMutableDictionary *childDict = [NSMutableDictionary dictionaryWithObject: @"1" forKey: @"MyNumber1"];
parentDict = [NSMutableDictionary dictionaryWithObject:childDict forKey: @"childDict"];
[[NSUserDefaults standardUserDefaults] setObject: parentDict forKey: @"parentDict"];
[[NSUserDefaults standardUserDefaults] synchronize];
NSLog( @"first time run... %@", parentDict );
exit(0);
}
NSMutableDictionary *childDict = [parentDict objectForKey: @"childDict"];
[childDict removeObjectForKey:@"MyNumber2"];
[childDict setObject: @"2" forKey: @"MyNumber2"];
[[NSUserDefaults standardUserDefaults] setObject: parentDict forKey: @"parentDict"];
[[NSUserDefaults standardUserDefaults] synchronize];
// Now read the value back to verify it:
parentDict = [[NSUserDefaults standardUserDefaults] objectForKey:@"parentDict"];
NSLog( @"exiting... %@", parentDict );
exit(0);
第一次运行：
2013-07-26 18:01:55.064 Mbox Director-[Debug][15391:303] 启动...（空）2013-07-26 18:01:55.210 Mbox Director-[Debug][15391:303]第一次运行... { childDict = { MyNumber1 = 1; }; }
第二次运行（一切看起来都正确）：
2013-07-26 18:02:54.999 Mbox Director-[Debug][15510:303] 启动... { childDict = { MyNumber1 = 1; }; } 2013-07-26 18:02:55.000 Mbox Director-[Debug][15510:303] 正在退出... { childDict = { MyNumber1 = 1; MyNumber2 = 2; }; }
山狮第 3 次运行的结果（注意，启动时缺少 MyNumber2 ......）：
2013-07-26 17:39:48.760 Mbox Director-[Debug][15047:303] 启动... { childDict = { MyNumber1 = 1; }; } 2013-07-26 17:39:48.760 Mbox Director-[Debug][15047:303] 正在退出... { childDict = { MyNumber1 = 1; MyNumber2 = 2; }; }
Lion 中的结果：第 3 次运行（注意，MyNumber2 已保存...）：2013-07-26 17:36:23.886 Mbox Director-[Debug][17013:120b] 正在启动... {
childDict = { MyNumber1 = 1 ; MyNumber2 = 2; }; } 2013-07-26 17:36:23.938 Mbox Director-[Debug][17013:120b] 正在退出... { childDict = { MyNumber1 = 1; MyNumber2 = 2; }; }
// This function makes a deep mutable copy. NSDictionary and NSArray mutableCopy does not create a DEEP mutableCopy.
// We accomplish a deep copy by first serializing the dictionary
// to a property list, and then unserializing it to a guaranteed deep copy.
// It requires that your array is serializable, of course.
// This method seems to be more bulletproof than some of the other implementations
// available on the web.
//
// Follows copy rule... you are responsible for releasing the returned object.
// Returns nil if not serializable!
id mutableCopyFromPlist( id plist )
{
NSError *error = nil;
@try
{
#ifdef MAC_OS_X_VERSION_10_6
NSData *binData = [NSPropertyListSerialization dataWithPropertyList:plist
format:NSPropertyListBinaryFormat_v1_0
options:0
error:&error];
NSString *errorString = [error localizedDescription];
#else
NSString *errorString = nil;
NSData *binData = [NSPropertyListSerialization dataFromPropertyList:plist
format:NSPropertyListBinaryFormat_v1_0
errorDescription:&errorString];
#endif
if (errorString || !binData )
{
DLogErr( @"error serializing property list %@", errorString );
}
else
{
#ifdef MAC_OS_X_VERSION_10_6
NSError *error = nil;
id deepCopy = [NSPropertyListSerialization
propertyListWithData:binData
options:NSPropertyListMutableContainersAndLeaves
format:NULL
error:&error];
errorString = [error localizedDescription];
#else
id deepCopy = [NSPropertyListSerialization
propertyListFromData:binData
mutabilityOption:NSPropertyListMutableContainersAndLeaves
format:NULL
errorDescription:&errorString];
#endif
[deepCopy retain]; // retain this so that we conform to the 'copy rule'... our function name contains the work 'Copy'
if (errorString)
{
DLogErr( @"error serializing property list %@", errorString );
}
else
{
return deepCopy;
}
}
}
@catch (NSException *exception )
{
DLogErr( @"error serializing property list %@", [error localizedDescription] );
}
return nil; // couldn't make a deep copy... probably not serializable
}
@implementation NSDictionary (VNSDictionaryCategory)
// This function makes a deep mutable copy. NSDictionary's mutableCopy does not create a DEEP mutableCopy.
// We accomplish a deep copy by first serializing the dictionary
// to a property list, and then unserializing it to a guaranteed deep copy.
// It requires that your dictionary is serializable, of course.
// This method seems to be more bulletproof than some of the other implementations
// available on the web.
//
// Follows copy rule... you are responsible for releasing the returned object.
// Returns nil if not serializable!
-(NSMutableDictionary *)mutableCopyDeepPropertyList
{
return mutableCopyFromPlist( self );
}
@end
#pragma mark -
@implementation NSArray (VNSArrayCategory)
// This function makes a deep mutable copy. NSDictionary's mutableCopy does not create a DEEP mutableCopy.
// We accomplish a deep copy by first serializing the dictionary
// to a property list, and then unserializing it to a guaranteed deep copy.
// It requires that your array is serializable, of course.
// This method seems to be more bulletproof than some of the other implementations
// available on the web.
//
// Follows copy rule... you are responsible for releasing the returned object.
// Returns nil if not serializable!
-(NSMutableArray *)mutableCopyDeepPropertyList
{
return mutableCopyFromPlist( self );
}
@end
用法：
NSMutableDictionary *dict = [[NSUserDefaults standardUserDefaults] objectForKey:@"mydictionary"];
dict = [[dict mutableCopyDeepPropertyList] autorelease];