iOS 8 freezes at updating UserDefaults object - ios

Question

I have a queue of objects stored to the NSUserDefaults. When I need to add objects to it, I call the following method:
+ (void)addCodeToQueue:(Code *)code {
// Note: userDefaults is a static, initialized variable
NSDictionary *codeModel = [self generateCodeModelWith:code];
// Read array from UserDefaults, or create one if nil
NSMutableArray *codeQueue = [userDefaults mutableArrayValueForKey:#"CodeQueue"] ? : [[NSMutableArray alloc] init];
// Add code model
[codeQueue addObject:codeModel];
// Add/replace array & sync
[userDefaults setObject:codeQueue forKey:#"CodeQueue"]; // App freezes here if uncommented
[userDefaults synchronize];
}
My app freezes when I call setObject:forKey. If I add a breakpoint to that line, the continue running, it works. If I don't break, it freezes.
This started happening after I updated Xcode to version 8 and started using the new SDK.
Any hints on this?

Answer 1

I had this problem in several apps as well -- it appears that by returning the mutable array via mutableArrayValueForKey, you can get stuck in a mutex lock. For my code, I swapped this out by:
Getting an immutable array as NSArray *arrSource = [defaults arrayForKey:strKey];
Copying the data into a mutable array:
NSMutableArray *arrMutable = [NSMutableArray arrayWithArray:arrSouce];
Setting my values in arrMutable...
Storing the "modified" array back into the defaults:
[defaults setObject:arrMutable forKey:strKey];
... for me, at least, this has fixed the mutex lock issue.

Answer 2

After my experiment, I have found the minimum code required to generate a mutex lock.
The code is as follows.
NSMutableArray *videoArray = [[NSUserDefaults standardUserDefaults] mutableArrayValueForKey:VIDEOKEY];
[videoArray addObject:item];
[[NSUserDefaults standardUserDefaults] setObject:videoArray forKey:VIDEOKEY];
There are 3 points to pay attention to:
If I delete [videoArray addObject:item];, the app will crash.
The mutex lock is occurring in [[NSUserDefaults standardUserDefaults] setObject:videoArray forKey:VIDEOKEY];.
If I make a breakpoint at [videoArray addObject:item];, the mutex lock is still exist. If I make it at [[NSUserDefaults standardUserDefaults] setObject:videoArray forKey:VIDEOKEY];, the mutex lock will not exist.
It can be inferred that the mutex lock is result by [NSKeyValueSlowMutableArray addObject:]; and [NSUserDefaults setObject:forKey:]
By the way, my solution to solve it is as follows:
NSMutableArray *mutableVideoArray;
NSArray *videoArray = [[NSUserDefaults standardUserDefaults] arrayForKey:VIDEOKEY];
if (videoArray)
mutableVideoArray = [videoArray mutableCopy];
else
mutableVideoArray = [NSMutableArray array];
if (![mutableVideoArray containsObject:item])
{
[mutableVideoArray addObject:item];
[[NSUserDefaults standardUserDefaults] setObject:mutableVideoArray forKey:VIDEOKEY];
}