NSMutableArray isn't adding my NSString - objective-c

Question

Im trying to add a NSString to an NSMutableArray and then make the array into NSData and save it with NSUserDefaults. But the array is always nil.
Here is my code:
- (void)viewDidLoad {
[super viewDidLoad];
library = [[ALAssetsLibrary alloc] init];
groups = [[NSMutableArray alloc] init];
userDefaults = [[NSUserDefaults alloc] init];
NSData *data = [userDefaults objectForKey:#"GroupArray"];
groups = [NSKeyedUnarchiver unarchiveObjectWithData:data];
NSLog(#"%i", [groups count]);
}
-(IBAction)newFolder {
if (textField.text != nil) {
NSString *string = textField.text.capitalizedString;
[library addAssetsGroupAlbumWithName:string
resultBlock:^(ALAssetsGroup *group) {
NSLog(#"Created a folder named: %#", group);
}
failureBlock:^(NSError *error) {
NSLog(#"An error occured! - %#", error);
}
];
[groups addObject:string];
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:groups];
[userDefaults setObject:data forKey:#"GroupArray"];
NSLog(#"%i", [groups count]);
}
[self.view sendSubviewToBack:subView];
}
When the app starts i get a message in the console that the array is NULL. When I try to add the string the NSLog(#"%i", [groups count]); always return 0.
Why does this happen?

Answer 1

userDefaults = [[NSUserDefaults alloc] init];
NSData *data = [userDefaults objectForKey:#"GroupArray"];
In this case, data will be nil when the code is executed for the first time, since there is yet no "GroupArray" property present.
groups = [NSKeyedUnarchiver unarchiveObjectWithData:data];
This causes groups be become nil as well, because calling unarchiveObjectWithData: with nil as an argument will return nil as well.
And because of all that, in -newFolder, [groups addObject:string] becomes [nil addObject:string]
Calling a method on nil is allowed in Objective-C, so you get no exception there. The return value of any method called on nil is, again, nil or 0.
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:groups];
This causes data to be nil because groups is nil.
So you are always calling:
[userDefaults setObject:data forKey:#"GroupArray"];
with data = nil
Quick fix:
Add if (groups == nil) groups = [NSMutableArray array]; to the beginning of -newFolder

Answer 2

In view didLoad method every time you are initialising new instance of NSUserDefaults and calling
NSData *data = [userDefaults objectForKey:#"GroupArray"]; on the newly created object
which will return nil all the time because in the new instance there wont be any object for key #"GroupArray". Instead replace userDefaults with singleton object [NSUserDefaults standardUserDefaults]
Modify your code as shown below
- (void)viewDidLoad
{
[super viewDidLoad];
library = [[ALAssetsLibrary alloc] init];
NSData *data = [[NSUserDefaults standardUserDefaults] objectForKey:#"GroupArray"];
groups = [NSKeyedUnarchiver unarchiveObjectWithData:data];
if (!groups)
{
groups = [[NSMutableArray alloc] init];
}
NSLog(#"group %#", groups);
NSLog(#"%i", [groups count]);
}
Your newFolder method
-(IBAction)newFolder
{
if (textField.text != nil)
{
NSString *string = textField.text.capitalizedString;
[library addAssetsGroupAlbumWithName:string
resultBlock:^(ALAssetsGroup *group) {
NSLog(#"Created a folder named: %#", group);
}
failureBlock:^(NSError *error) {
NSLog(#"An error occured! - %#", error);
}
];
[groups addObject:string];
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:groups];
[[NSUserDefaults standardUserDefaults] setObject:data forKey:#"GroupArray"];
[[NSUserDefaults standardUserDefaults] synchronize];
NSLog(#"%i", [groups count]);
}
[self.view sendSubviewToBack:subView];
}
This should work. As it worked for me.