time_based_uuid.m

// related to SO question:
// http://stackoverflow.com/questions/26252432/how-do-i-set-a-auto-increment-key-in-realm/26257616#26257616
// keywords: realm auto increment id primary key autoincrement
//
// two assumptions:
// 1) time always goes forward
// 2) realm and app has shared lifetime:
//    no external realms are imported, if app deleted realm is deleted
@interface TSUUID : NSObject
@property (nonatomic) double baseOffset;
@end

static NSString *const kBaseOffsetKey = @"kBaseOffsetKey";
static double const kBaseOffsetDouble = 0.00000001;

@implementation TSUUID
+ (instancetype)sharedInstance
{
    static id sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[[self class] alloc] init];
        // Do any other initialization stuff here
    });
    return sharedInstance;
}
- (instancetype)init
{
    self = [super init];
    if (self) {
        
    }
    return self;
}
- (double)baseOffset {
    if (!_baseOffset) {
        _baseOffset = CFAbsoluteTimeGetCurrent();
        
        id number = [[NSUserDefaults standardUserDefaults] objectForKey:kBaseOffsetKey];
        if (!number) {
            number = @(_baseOffset);
            [[NSUserDefaults standardUserDefaults] setObject:number forKey:kBaseOffsetKey];
        }
        
        // jump forward in case stored time > current time
        double number_time = [number doubleValue];
        if (number_time > _baseOffset) {
            _baseOffset = number_time;
        }
    }
    return _baseOffset;
}
- (double)generate {
    double time = CFAbsoluteTimeGetCurrent();
    if (time == self.baseOffset) {
        self.baseOffset = self.baseOffset + kBaseOffsetDouble;
        time = self.baseOffset;
    }
    return time;
}
@end