Translating swift to objective C "for in loop" NSString (I think) in NSDictionary

I'm currently stuck translating the code below for my apps database in firebase:

Swift :

var databaseRef = FIRDatabase.database().reference() 
var userDict = NSDictionary?() 
var userNamesArray = [String]()
 var userImagesArray = [String]() 


override func viewDidLoad() {
 super.viewDidLoad() 


self.databaseRef.child(@"users").observeEventType(.Value, withBlock : {
 (snapshot in 


self.userDict = snapshot.value as? NSDictionary



for(userId,details) in self.userDict!{ 
let img = details.objectForKey(@"profile_pic_small" as! String 
let name = details.objectForKey(@"Name" as! String 
let firstName = name.componentsSeperatedByString(" ")[0] 
self.userImagesArray.append(img) 
self.userNamesArray.append(firstName) 
self.collectionView?.reloadData()
 } 
})

The code below is my attempt at translating but i'm definetly missing something:

- (void)viewDidLoad {
    [super viewDidLoad];
    _userNamesArray = [NSMutableArray new];
    _userImagesArray = [NSMutableArray new];

    self.databaseRef = [[FIRDatabase database] reference];


    [[_databaseRef child:@"users"] observeEventType:FIRDataEventTypeValue withBlock:^(FIRDataSnapshot * _Nonnull snapshot) {
     
        _usersDict = snapshot.value;
     
        NSString *details;
        for (details in _usersDict) {
            NSString *img = (NSString *)[details objectForKey:@"profile_pic_small"]; 
            NSString *name =(NSString *)details objectForKey:@"Name"];
            NSString *firstName; firstName = [[name componentsSeparatedByString:@" "] objectAtIndex:0];

               [_userImagesArray addObject:img];
            [_userNamesArray addObject:name];
            [self.collectionView reloadData];
         
        }
     
     
     
     
    }];
}

Any help would be appreciated.